The osteology of Periptychus carinidens: A robust, ungulate-like placental mammal (Mammalia: Periptychidae) from the Paleocene of North America

PLOS ONE, Jul 2018

Periptychus is the archetypal genus of Periptychidae, a clade of prolific Paleocene ‘condylarth’ mammals from North America that were among the first placental mammals to radiate after the end-Cretaceous extinction, remarkable for their distinctive dental anatomy. A comprehensive understanding of the anatomy of Periptychus has been hindered by a lack of cranial and postcranial material and only cursory description of existing material. We comprehensively describe the cranial, dental and postcranial anatomy of Periptychus carinidens based on new fossil material from the early Paleocene (Torrejonian) of New Mexico, USA. The cranial anatomy of Periptychus is broadly concurrent with the inferred plesiomorphic eutherian condition, albeit more robust in overall construction. The rostrum is moderately elongate with no constriction, the facial region is broad, and the braincase is small with a well-exposed mastoid on the posterolateral corner and tall sagittal and nuchal crests. The dentition of Periptychus is characterized by strongly crenulated enamel, enlarged upper and lower premolars with a tall centralised paracone/protoconid. The postcranial skeleton of Periptychus is that of a robust, medium-sized (~20 Kg) stout-limbed animal that was incipiently mediportal and adopted a plantigrade stance. The structure of the fore- and hindlimb of Periptychus corresponds to that of a typically terrestrial mammal, while morphological features of the forelimb such as the low tubercles of the humerus, long and prominent deltopectoral crest, pronounced medial epicondyle, and hemispherical capitulum indicate some scansorial and/or fossorial ability. Most striking is the strongly dorsoplantarly compressed astragalus of Periptychus, which in combination with the distal crus and calcaneal morphology indicates a moderately mobile cruropedal joint. The anatomy of Periptychus is unique and lacks any extant analogue; it combines a basic early placental body plan with numerous unique specializations in its dental, cranial and postcranial anatomy that exemplify the ability of mammals to adapt and evolve following catastrophic environmental upheaval.

A PDF file should load here. If you do not see its contents the file may be temporarily unavailable at the journal website or you do not have a PDF plug-in installed and enabled in your browser.

Alternatively, you can download the file locally and open with any standalone PDF reader:

http://journals.plos.org/plosone/article/file?id=10.1371/journal.pone.0200132&type=printable

The osteology of Periptychus carinidens: A robust, ungulate-like placental mammal (Mammalia: Periptychidae) from the Paleocene of North America

July The osteology of Periptychus carinidens: A robust, ungulate-like placental mammal (Mammalia: Periptychidae) from the Paleocene of North America Sarah L. Shelley 0 1 2 Thomas E. Williamson 0 2 Stephen L. Brusatte 0 1 2 0 Current address: Edward O'Neil Research Center, Carnegie Museum of Natural History , Pittsburgh, Pennsylvania , United States of America 1 School of GeoSciences, University of Edinburgh , Edinburgh, Scotland , United Kingdom , 2 New Mexico Museum of Natural History and Science, Albuquerque, New Mexico , United States of America 2 Editor: Thierry Smith, Royal Belgian Institute of Natural Sciences , BELGIUM Periptychus is the archetypal genus of Periptychidae, a clade of prolific Paleocene `condylarth' mammals from North America that were among the first placental mammals to radiate after the end-Cretaceous extinction, remarkable for their distinctive dental anatomy. A comprehensive understanding of the anatomy of Periptychus has been hindered by a lack of cranial and postcranial material and only cursory description of existing material. We comprehensively describe the cranial, dental and postcranial anatomy of Periptychus carinidens based on new fossil material from the early Paleocene (Torrejonian) of New Mexico, USA. The cranial anatomy of Periptychus is broadly concurrent with the inferred plesiomorphic eutherian condition, albeit more robust in overall construction. The rostrum is moderately elongate with no constriction, the facial region is broad, and the braincase is small with a well-exposed mastoid on the posterolateral corner and tall sagittal and nuchal crests. The dentition of Periptychus is characterized by strongly crenulated enamel, enlarged upper and lower premolars with a tall centralised paracone/protoconid. The postcranial skeleton of Periptychus is that of a robust, medium-sized (~20 Kg) stout-limbed animal that was incipiently mediportal and adopted a plantigrade stance. The structure of the fore- and hindlimb of Periptychus corresponds to that of a typically terrestrial mammal, while morphological features of the forelimb such as the low tubercles of the humerus, long and prominent deltopectoral crest, pronounced medial epicondyle, and hemispherical capitulum indicate some scansorial and/or fossorial ability. Most striking is the strongly dorsoplantarly compressed astragalus of Periptychus, which in combination with the distal crus and calcaneal morphology indicates a moderately mobile cruropedal joint. The anatomy of Periptychus is unique and lacks any extant analogue; it combines a basic early placental body plan with numerous unique specializations in its dental, cranial and postcranial anatomy that exemplify the ability of mammals to adapt and evolve following catastrophic environmental upheaval. - Data Availability Statement: Data are within the paper and its Supporting Information files. Funding: SLS was funded by a Natural Environment Research Council PhD studentship (www.nerc.ac.uk/) administered through the School of Geosciences, University of Edinburgh. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. TEW was supported by National Science Foundation EAR 0207750 (www.nsf.gov/). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Both TEW and SLB were supported by National Science Foundation EAR 1325544 and DEB 1654952 (www.nsf.gov/). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. SLB is also supported by an European Research Council Starting Grant (PalM) (erc.europa.eu/funding/ starting-grants). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. SLB and SLS were supported by a Marie Curie Career Integration Grant (CIG 630652) (ec.europa.eu/ research/mariecurieactions/funded-projects/ careerintegration-grants_en). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. SLS, TEW and SLB were also supported by NSF DEB 1654949 (www.nsf.gov/). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist. Introduction The diversification of mammals following the end-Cretaceous mass extinction was a critical event in evolutionary history. The proliferation of eutherian mammalsÐplacentals that give birth to live to well-developed young, and their closest fossil relativesÐduring this time produced numerous clades of `archaic' mammals which did not survive beyond the Paleogene and whose phylogenetic affinities with extant mammals remain contentious, but which promise to help disentangle the early history of mammals if their anatomy and relationships can be better understood. One such group are the Periptychidae, a clade of morphologically robust, ungulate-like, `condylarths' known from North America. The Periptychidae were among the placental mammals to appear after the end-Cretaceous extinction. Thus, these species are key for understanding how mammals were affected by the extinction and blossomed afterwards. Periptychus is the archetypal genus of Periptychidae and easily recognised by its distinctive teeth, fossils of which are common in the Paleocene deposits of western North America. Periptychus was first described nearly 140 years ago; following his initial findings, Cope ([1], p.801) wrote, ªIts discovery I consider to be an important event in the history of palaeontological scienceº. Cope did little expand on this statement much beyond a brief description of the material at hand and by hypothesising on the unusual appearance of the animal. Subsequent authors have supplemented our knowledge of Periptychus, but a detailed description of this enigmatic taxon has been left wanting. Little is currently known about the cranial and, particularly, the postcranial anatomy of this taxon, which has inhibited a detailed understanding of its paleobiology and evolution. An overview of the literature reveals a surprisingly long and convoluted taxonomic history for Periptychus but little discussion regarding its paleobiology; in fleeting sentences it has been described as a medium-sized, terrestrial/generalist, `archaicungulate', showing dental specialisations towards an herbivorous to durophagus diet [2,3] and with some superficial postcranial attributes similar to extant tayassuids [4], vombatids, and Orycteropus [5]. These descriptions have not been recently updated to place them in context of our modern understanding of mammalian anatomy and evolution. The necessity of a revision of Periptychus is also buoyed by the discovery of a wealth of new fossil material from the lower Paleocene (Torrejonian) deposits in the San Juan Basin of New Mexico, much of it collected over the past two decades by teams led by TEW. By combining these new fossils with restudy of historic collections, we here present a comprehensive anatomical re-description of Periptychus carinidens. Institutional Abbreviations AMNH American Museum of Natural History, New York, NY, USA; MNHN MuseÂum national d'Histoire naturelle, Paris, Collection de PaleÂontologie, Paris, France; NMMNH New Mexico Museum of Natural History and Science, Albuquerque, NM, USA; SPSM St Paul Science Museum, MN, USA; TMM Texas Memorial Museum, Austin, TX, USA; UNM Department of Geology, University of New Mexico, Albuquerque, NM, USA; USNM National Museum of Natural History, Washington, D. C., USA. Historical background The taxonomy and systematics of Periptychus, the Periptychidae and `Condylarthra' have a long and complex history. `Condylarths' have long been recognised as the ancestral stock from which ungulate mammals, including extant artiodactyls and perissodactyls, arose [3,6]. However, over time, `Condylarthra' has come to include an assortment of ungulate-grade Paleogene 2 / 139 mammals which do not appear to form a natural group. Periptychidae is an anatomically distinctive `condylarth' subgroup; they are seemingly easy to recognise and were pivotal in the establishment of `Condylarthra'. The historical literature pertaining to Periptychus and the Periptychidae is convoluted, which is perhaps surprising given how seemingly distinctive periptychids are. Periptychus first appears in Torrejonian aged deposits and is immediately recognisable by its distinctive dentition with enlarged premolars and highly crenulated enamel. Carsioptychus, a medium-sized periptychid from the older Puercan deposits of North America, is morphologically similar to Periptychus. The relative abundance of dental specimens has allowed workers to observe subtle variations in the dental morphology of these taxa, which some workers have asserted are important enough to warrant species-level recognition (e.g. [5,7±10]). Because of the dental similarities between Periptychus and Carsioptychus and morphological variation within each genus, there has been, and continues to be, debate over whether Carsioptychus should be regarded as a genus distinct from Periptychus, and over the validity of the numerous species referred to one genus or the other. Periptychus was first diagnosed by Cope based on a dentary fragment [11]. Not realising that the specimen represented a juvenile individual, Cope named the taxon Periptychus carinidens and assigned it to Creodonta. As an aside here, Cope erroneously named Periptychus carinidens as a new genus and species twice, in two separate publications [11,12]. In the same year, Cope also described `Catathlaeus rhabdodon’, based on a partial maxilla preserving the P1-M3 [12]. `Catathlaeus rhabdodon’ was provisionally allied with Phenacodontidae given its highly bunodont molar dentition. In 1882, Cope revised `Condylarthra' with the discovery of Periptychus postcrania and several new periptychids from the `Puerco beds' of New Mexico [13]. The new associated dental and postcranial material allowed Cope to recognise his error and synonymize `Catathlaeus rhabdodon’ to Periptychus as `Periptychus rhabdodon’. Cope further observed that the tarsal structure of Periptychus bore very little resemblance to that of Phenacodus, so he erected Periptychidae as a family within `Condylarthra', which he considered a suborder of Taxeopoda at the time [13]. Cope advocated that `Periptychus rhabdodon’ represents a separate, larger and more robust species than Periptychus carinidens [4]. Matthew noted that intermediate forms exist between P. carinidens and `P. rhabdodon’ and that had Cope not misdiagnosed P. carinidens based on deciduous teeth, the issue over the validity of `P. rhabdodon' would probably be a moot point [5]. Nevertheless, Matthew retained `P. rhabdodon’ and P. carinidens as separate valid species. In 1959, Simpson measured and compared an assortment of 37 Periptychus dental specimens and found no bimodality in tooth size, so he formally synonymized `P. rhabdodon’ with P. carinidens and noted that the size variation between the different morphotypes could represent intraspecific variation [8]. `Periptychus superstes’, a Tiffanian species of Periptychus, was first identified and named by Matthew, but Matthew died before publishing his work. Simpson [14] credited Matthew with the diagnosis of this taxon; consequently, the descriptions in Simpson [14] and Matthew's posthumous monograph [5] are effectively the same. Matthew [5,14], asserted the validity of `P. superstes’ based on the p5 being proportionally smaller relative to the molar series (in comparison to P. carinidens) and the m3 possessing a more elongate talonid heel. However, Matthew [5] lowered `P. superstes’ to subspecies rank within `P. rhabdodon’. Matthew commented that `P. superstes’ was likely a progressive form of `P. rhabdodon’, but concluded that it was not known well enough to distinguish it as a separate species [5]. In 1967, Wilson briefly described three Periptychus specimens from Black Peaks Formation, exposed at Tornillo Flat in Big Bend National Park, Texas, which he assigned to Periptychus carinidens [15]. In 1974, Schiebout provided a more detailed description of the specimens and 3 / 139 tentatively referred five new Tiffanian specimens to `Periptychus superstes’ (TMM 41274±1; 40147±4; 40147±17; 40537±59; 41367±8) [16]. Standhardt [17] subsequently also referred TMM 40147±1 to `Periptychus superstes’ (also from Tornillo Flat). Schiebout highlighted similarities between the Big Bend specimens and `P. superstes’, but also concluded that not enough was known about the size variability within Periptychus carinidens and `P. superstes’ to definitively assign the Big Bend specimens to a species [16]. Williamson [10] found the range of size variation of the first lower molar between P. carinidens to overlap with specimens of `P. superstes’ from the San Juan Basin, with the exception of two particularly large specimens from Big Bend (TMM 40147±17 and 40537±59), and consequently synonymized ‘P. superstes’ with P. carinidens. We agree with Williamson [10] in that the morphological features described by Matthew [5,14] for distinguishing `P. superstes’ are present in specimens of P. carinidens, and thus do not warrant species distinction. However, we do recognize that there is variation in proportions between the premolar and molar dentition in Periptychus specimens, which may mark an important ecological, temporal, and/or geographical transition. ‘Periptychus gilmorei’ was first described by Gazin [18] from the North Horn Formation, Dragon Canyon, Emery County, Utah. Gazin proposed `P. gilmorei’ as an intermediate species between Carsioptychus coarctatus and Periptychus carinidens based on a combination of dental characters present in `P. gilmorei’ that are characteristic of both C. coarctatus or P. carinidens. The Dragon Fauna was initially thought to represent a distinct North American Land Mammal Age (NALMA; the Dragonian) between the Puercan and Torrejonian NALMAs [19]; however, magnetostratigraphy and correlation of the North Horn Formation with the Nacimiento Formation has shown the Dragonian to be equivalent to the early Torrejonian [20±23]. An early Torrejonian age for `P. gilmorei’ supports the hypothesis that it represents a transitional form between Carsioptychus and the bulk of Periptychus specimens from the Nacimiento Formation, in addition to its purported intermediate morphology. The upper premolars of `P. gilmorei’ exhibit a crescentic lingual shoulder as in P. carinidens; however, the lingual shoulder is somewhat anteroposteriorly constricted, bearing some resemblance to C. coarctatus. Gazin [18] also noted that the upper molar dentition of `P. gilmorei’ resembles C. coarctatus in being transversely expanded with a moderately elongate lingual slope. Williamson [10] found that the dentition of `P. gilmorei’ falls within the size and morphological range exhibited by P. carinidens and therefore concluded that `P. gilmorei’ is a junior synonym of P. carinidens. We tentatively agree with Williamson [10], given that specimens of P. carinidens from the San Juan Basin exhibit a range of morphologies which fit the description for `P. gilmorei’ and cover the range of difference between `P. gilmorei’ and P. carinidens. We do not think that intermediate dental morphology of `P. gilmorei’ warrants specific rank, but it does highlight a subtle morphological shift within P. carinidens with an apparent prevalence of a ‘P. gilmorei’ morphotype in the North Horn Formation, Utah. The taxonomy for Carsioptychus is equally complicated. Carsioptychus coarctatus was first described by Cope [24] as a species of Periptychus (= Periptychus coarctatus). Simpson [14], citing unpublished notes by Matthew, considered `Periptychus coarctatus’ more distinct from Periptychus carinidens than the other purported Periptychus species, and thus established the subgenus `Plagioptychus’ within Periptychus for `Periptychus coarctatus’. In the same publication, Simpson formally raised `Plagioptychus’ to genus level. Unfortunately, the genus `Plagioptychus’ was occupied so Simpson proposed the new generic name Carsioptychus, into which he transferred Carsioptychus coarctatus (= Periptychus coarctatus) and `Carsioptychus matthewi’ (= Plagioptychus matthewi) [25]. Van Valen [9] proposed Carsioptychus be treated as a subgenus of Periptychus (as previously suggested by Matthew [5]) but provided no further explanation for his decision. Archibald et al. [23] asserted that there are enough morphological 4 / 139 differences in premolar cusp development and occlusal tooth profile between Periptychus and Carsioptychus to warrant generic distinction. Given the similarities between Carsioptychus and Periptychus relative to other periptychids, and the facts that the two are known from the same area and the former is older than the latter, it is possible that Carsioptychus is directly ancestral to Periptychus [5,23]. Williamson [10] noted the usefulness of retaining Periptychus and Carsioptychus as separate genera given that the first occurrence of Periptychus is used to define the base of the Torrejonian, but was doubtful over whether there is enough morphological dissimilarity between the taxa to warrant generic distinction. ‘Periptychus brabensis’ was first formally described by Osborn & Earle [26] and credited to Cope (no date). However, subsequent authors have synonymized specimens referred to `P. brabensis’ to both Periptychus carinidens and Carsioptychus coarctatus. Osborn & Earle [26] described `P. brabensis’ based on a mandible specimen (AMNH 849), not the type (AMNH 3782) (note that the type was never formally designated by Cope). They asserted that the upper premolars of `P. brabensis’ are buccolingually wider than the upper molars, and the upper molar conules are seemingly absent, and noted that this species exhibits some intermediary form between ‘C. coarctatus’ and ‘P. rhabdodon’. In 1888, Cope referred twenty individual specimens from the Nacimiento Formation, San Juan Basin, New Mexico to `P. brabensis’. Cope noted the similarities between `P. brabensis’ and `C. coarctatus’ but retained `P. brabensis’ as a separate species [27]. Matthew [5] found that AMNH 849 is a juvenile specimen of P. carinidens preserving the deciduous dentition, and the morphology displayed by specimens referred to `P. brabensis’ by Osborn & Earle [26] fall within the morphological range of P. carinidens. Matthew [5] also noted that specimens referred to ‘P. brabensis’ by Cope [27] exhibit morphologies which fall within the range of Carsioptychus coarctatus. Based on the morphology and Puercan age of the type, `P. brabensis' is a synonym of C. coarctatus; however, Cope never formally designated the type specimen. Specimens assigned to `P. brabensis' by Osborn & Earle [26], based on comparison to AMNH 849, are generally referable to P. carinidens. ‘Plagioptychus (= Carsioptychus) matthewi’ was first described by Simpson [25] based on a partial dentary preserving p2-m3, recovered from the Nacimiento Formation, San Juan Basin, New Mexico. Simpson described ‘Plagioptychus matthewi’ as a larger and more derived species than Carsioptychus coarctatus, but failed to mention any unique characters not found in C. coarctatus. Van Valen [9] subsequently synonymized `P. matthewi’ with Carsioptychus coarctatus without providing any justification (note that Van Valen also considered Carsioptychus a subgenus of Periptychus), although we note there are there no obvious morphological differences between the purported species. Williamson [10] plotted the log of the first lower molar area of Periptychus carinidens and Carsioptychus coarctatus (into which he included `Carsioptychus matthewi’). The results showed no statistical differences between first lower molar area of Carsioptychus coarctatus and `Carsioptychus matthewi’, further supporting the synonymy of `Carsioptychus matthewi’ with Carsioptychus coarctatus. ‘Carsioptychus hamaxitus’ was described by Gazin [28] based on a partial maxilla preserving M1-2 from the Wagonroad fauna, North Horn Formation, Dragon County, Utah. Based on two additional specimens from the same area, which preserve the M2-3 and m2-3, Gazin proposed `Carsioptychus hamaxitus’ as a smaller variant of Carsioptychus coarctatus with some rudimentary Periptychus characteristics, namely a more developed premolar paraconid than Carsioptychus coarctatus. Williamson [10] found development of the premolar paraconid to be variable within specimens of Carsioptychus coarctatus from the Nacimiento Formation in the San Juan Basin, New Mexico, and the range in morphology overlapped with that of specimens of the Wagonroad fauna, justifying the synonymy of `Carsioptychus hamaxitus’ with Carsioptychus coarctatus. Gazin [18] also named a new species of Periptychus, `P. gilmorei’ (see above), from the Dragon Fauna of the North Horn Formation and other authors have noted that other 5 / 139 species from the North Horn Formation are distinct from their Torrejonian congeners [29]. Such findings raise the question of whether North Horn Formation faunas represent a geographically isolated population, which led to the prevalence of different morphotypes compared to their southern and northern counterparts. To summarize the above discussion, and provide a guide to the reader: in this paper, we recognise Periptychus carinidens as a single valid species and agree with Williamson [10] regarding the synonymy of `P. gilmorei’ and `P. superstes’ with Periptychus carinidens, although we note that this conclusion is subject to change if new fossil material documents discrete variation between these different forms. We also choose to retain Carsioptychus at generic rank, as we recognize that there are distinct morphological differences with specimens of Periptychus carinidens and Carsioptychus coarctatus (outlined above and in more detail in the Diagnosis below). Retaining Puercan-aged Carsioptychus as a distinct genus-level taxon also provides useful information in determining character polarity within Periptychinae in phylogenetic analyses; its exclusion or assimilation with Torrejonian-aged Periptychus has produced erroneous results previously (see [30±32]). However, we note that within genera and species there are subtle changes in dental morphology and tooth proportions, and the differentiation of species based on single tooth measurements may not be the best protocol for distinguishing Periptychus from Carsioptychus given the large size variation exhibited by these taxa. Instead, the proportional size differences between the premolar and molar dentition better encapsulates the variation based on the descriptions by previous authors; these measurements require further investigation, which may lead future workers to modify the classification scheme we use here. Geological setting Periptychus is known solely from the early Paleogene deposits of western North America, with fossil specimens recovered from Colorado, Montana, New Mexico, Texas, Utah and Wyoming. The Paleocene climate was warmer than present, with mean annual temperatures in the San Juan Basin of ~12Ê ± 4.4Ê C and mean annual precipitation amounts of ~1,100 mm [33± 35]. Mean annual temperatures in mid-latitude continental interiors were warm, with average winter temperatures likely above freezing and a reduced latitudinal temperature gradient [36± 41]. Periptychus is particularly well represented in the Nacimiento Formation in the San Juan Basin of New Mexico. The Nacimiento Formation contains one of the world's best records of terrestrial vertebrate succession through the early Paleocene (~64.5 to ~61 million years ago) and includes the type faunas for the first two North American Land Mammal Ages of the Cenozoic: the Puercan (excluding Pu1) and Torrejonian [19,23,42±44] Periptychus is one of only a few genera to extend through the entire Torrejonian before going extinct in the Tiffanian, with a total genus duration of approximately three million years. It left no apparent descendants. The Nacimiento Formation facies are primarily comprised of bentonitic mudstones intercalated with fluvial channel and crevasse sandstones, moderately to well-developed paleosols, and carbonaceous shale units representing a non-marine, fluvial to lacustrine depositional environment [10,42]. The floral and faunal composition of the Nacimiento Formation indicates a frost-free environment [45]. The flora was dominated by an array of angiosperms which were relatively diverse locally while remaining comparatively heterogeneous across the region [46]. Stable isotope analysis of carbon and oxygen indicate an ecosystem dominated by C3 vegetation [46]. The predominance of C3 vegetation was prevalent up until the Miocene and indicates a paleoenvironment where sunlight intensity was moderate, temperature was moderate, carbon 6 / 139 dioxide concentration was relatively high and groundwater was abundant [47]. Isotopic analysis of the enamel of Periptychus carinidens, Claenodon ferox, Mioclaenus subtrigonus and Tetraclaenodon puercensis found high carbon values indicative of feeding in comparatively open and relatively drier habitats [46]. Based on these proxies, the paleoenvironment of the San Juan Basin during the early Paleocene can be inferred as being composed of areas with densely vegetated, closed canopy forest interspersed with more open expanses. The abundance of crocodile and turtle fossils is also indicative a warm, humid climate [38,39]. Materials and methods Description and comparison This study is based largely on new Periptychus carinidens specimens collected from the San Juan Basin in New Mexico, curated at the New Mexico Museum of Natural History and Science. Access to precise locality information is restricted to qualified researchers and land management personnel. These new specimens are the primary focus of this study, and allow for a comprehensive re-description of the cranial, dental and postcranial anatomy of Periptychus carinidens. Descriptions are supplemented with information from the collection held at the American Museum of Natural History in New York, USA. Throughout the descriptive text, comparisons are made to numerous medium-sized Paleocene mammals, including other periptychid taxa, known from cranial, dental and postcranial specimens. These include, but are not restricted to: Carsioptychus coarctatus, Ectoconus ditrigonus, Mithrandir gillianus, Arctocyon primaevus, Claenodon ferox, Protungulatum sp., and Pantolambda bathmodon. We have observed the comparison taxa first-hand except for Mithrandir gillianus, for which a postcranial skeleton (NMMNH P-3083) was not available during the period of study. Carsioptychus and Ectoconus are both medium-sized periptychids thought to be closely related to Periptychus [2,6,30]. Ectoconus is known from a near complete skeleton (AMNH 16500) and shares many morphological similarities with Periptychus. Mithrandir gillianus is the only small periptychid known from a partial skeleton (NMMNH P-3083), which was described by Rigby [48] (note that in [48] the specimen is referred to as Gillisonchus and referenced as UNM-B029). We include Mithrandir in our study to assess the morphological features of Periptychus associated with larger body size in relation to a smaller, relatively closely related taxon. Arctocyon primaevus and Claenodon ferox are members of `Arctocyonidae'. The `arctocyonids' are generally considered the ancestral stock from which other `condylarth' groups arose [3], and closely related to the Periptychidae [9], although they have previously been allied with carnivorans due to homoplasic characters of their dentition [49,50]. The morphological similarities between Arctocyon and Claenodon have resulted in these two taxa often being considered synonymous with one another [3,10,51,52]. We include both Arctocyon and Claenodon in our comparisons with Periptychus as these taxa are seemingly morphologically distinct from each other, but of a similar size to Periptychus. This provides useful insight into the functional morphology and ecology of Periptychus. We note that future work is required to assess the validity of Arctocyon and Claenodon as separate genera. Protungulatum is an ungulate-like eutherian mammal known from latest Cretaceous-Paleogene deposits of North America [53]. The phylogenetic position of Protungulatum remains contentious. It has previously been considered a basal member of `Arctocyonidae' and plausible ancestor for Periptychidae [9]. More recent studies have excluded Protungulatum from `Arctocyonidae' [6], considered it as the oldest undisputed species within crown Placentalia [54], and found it as a non-placental stem eutherian [55±57]. Regardless, of its phylogenetic 7 / 139 affinities, Protungulatum provides a useful comparison to Periptychus and is often considered broadly representative of the primitive eutherian condition [54], albeit with some more ungulate-like features. At the very least, Protungulatum is a reasonably well-known taxon documenting a major shift in anatomical specializations between Cretaceous and Paleogene eutherians. We also make detailed comparisons with Pantolambda bathmodon, a medium-sized pantodont also known from the Torrejonian of the San Juan Basin, New Mexico. Previous workers, namely Cope [58], Osborn [59] and Gregory [60], noted the remarkable postcranial similarities between Periptychus and Pantolambda bathmodon, and proposed a close relationship between these two taxa. The vastly disparate dental anatomy of these two taxa likely means that they are not closely related [5,61,62], but their cranial and postcranial similarities warrant full investigation in order to better understand their functional morphology and paleoecology. Measurements are provided throughout the text and in tables (S1 Appendix). Measurements were made using digital callipers, in millimetres, to the nearest two decimal places. Digital measurements were taken with ImageJ 1.6.0 [63]. Dental notation follows that of McKenna [64], more recently used by Wible et al. [55,56] and O'Leary et al. [54], whereby the first and second premolars are P1/p1 and P2/p2, respectively; P3/p3 in basal eutherians is considered to be a retained deciduous tooth (it is not present in Periptychus); the penultimate molar is referred to as P4/p4; and the ultimate premolar is referred to as P5/p5. Tooth nomenclature follows the standard eutherian terminology outlined by Szalay [65] where applicable or otherwise specified. Osteological and myological nomenclature and directional references are draw from a range of studies: generally we follow the terminology and protocols outlined in Miller's Anatomy of the Dog [66] with reference to studies on numerous other Paleogene eutherians [3,5,23,48,52,67±72]. Specimens were imaged in the standard anatomical views unless otherwise specified. Some specimens were dusted with magnesium oxide or ammonium chloride prior to imaging to enhance surface details and contrast (this was not possible for all specimens due to collection restrictions). Body mass was estimated for Periptychus using the long bone scaling equation of Campione and Evans [73] whereby logBM = 2.749 logCH+F− 1.104 (BM, body mass; H, humeral minimum midshaft circumference; F, femoral minimum midshaft circumference; whereby CH+F, sum of humeral and femoral minimum midshaft circumference). Long bone circumference measurements were taken with a vinyl tape measure, which was marked up and then measured using digital callipers to two decimal places. Systematic Paleontology MAMMALIA Linnaeus 1758 [74] EUTHERIA Gill 1872 [75] `CONDYLARTHRA' Cope 1881 [76] PERIPTYCHIDAE Cope 1882 [77] PERIPTYCHINAE Osborn & Earle 1895 [26] Periptychus Cope 1881 [11] Catathlaeus Cope 1881 [12] Type species and only known species: Periptychus carinidens Cope, 1881 [11] Age and locality Torrejonian to Tiffanian (~63.3- ~61.7Ma BP), Early Paleogene. Best known from the Nacimiento Formation, San Juan Basin, New Mexico, USA. Also known from North Horn Formation, Emery County, Utah, USA; Fort Union Hanna formations, Carbon County, Wyoming, 8 / 139 USA; Black Peaks Formation, Big Bend National Park, Texas, USA; Fort Union Formation, Makoshika State Park, Montanta, USA; Animas Formation, San Juan Basin, Colorado, USA. Etymology Cope did not explain the etymology of Periptychus carinidens. The generic name Periptychus derives from Ancient Greek, peri (around/near/surrounding) and the noun ptych, (fold/layer), yielding the word 'periptych' which is Latinised with the suffix -us. Thus, Periptychus translates as `folds around' or `folds surrounding'. The species name carinidens derives from the Latin, (keel/prow) and dens, (tooth); with carinidens translating as `keeled-tooth'. Emended diagnosis Upper premolars ovoid in occlusal view and enlarged relative to molars. Premolar paracone forms tall, erect, bulbous centralized cusp, cristae weak, flanked with crescentic lingual shoulder on P1-P5. Crescentic lingual shoulder formed by protocone and cuspules, which develop posteriorly along tooth row. P5 postcingulum is positioned level with protocone. Upper premolar parastyle forms a small but distinct mesially directed lobe. Lower premolars enlarged relative to molars. Premolar protoconid forms tall, erect, bulbous centralized cusp; flanked by paraconid mesiolingually and metaconid lingually on p2-p5. Talonid present on p1-p5, positioned lingually relative to protoconid and increasing in development posteriorly along tooth row. Upper molars near quadrate in occlusal view. Molar protocone flanked by subequally developed hypocone and protostyle. Hypocone and protostyle exhibit tendency to polybuny. Ectocingulum present but reduced. Paraconule and metaconule present and separate, with distinctive wing-like cristae. Lower molar cusps are well-separated. Paraconid distinct and separate, and only slightly smaller than the metaconid. The cristid obliqua is strong and where it intercepts the protocristid notch may be variably marked by an obliconid. The hypoconid, hypoconulid and entoconid form discrete, well separated cusps, and approach the trigonid cusps in height. The hypoconid and entoconid are subequal in size, the hypoconulid is slightly smaller. Differential diagnosis The dentition of Periptychus differs from Carsioptychus in that the enamel is more strongly crenulated with distinct apicobasally aligned ridges. The upper postcanine dentition of Periptychus is not as transversely expanded as Carsioptychus, with the latter possessing a shallower lingual slope on all postcanine teeth. In Periptychus, the premolar paracone/protocone forms an erect cusp whereas in Carsioptychus the premolar paracone/protocone is distinctly posteriorly pitched. The premolar protocone of Periptychus forms a crescentic shoulder on all upper premolars, whereas in Carsioptychus the lingual shoulder is present on only P4 and P5 and more transversely expanded, with strong anteroposterior constriction and weak to indistinct cuspules. The lower premolar paraconid of Periptychus is more developed on p2-5, whereas the paraconid is only weakly developed on p4-5 and absent on p1-2 in Carsioptychus. The premolar metaconid is absent in Carsioptychus. The premolar trigonid is relatively more developed in Periptychus and positioned on the lingual side of the protoconid. Upper molars of Periptychus are near quadrate in occlusal view whereas they are more rectangular in Carsioptychus due to a longer lingual slope. The protostyle and hypocone of Periptychus are both well developed, subequal in size with a tendency to polybuny; in Carsioptychus, the hypocone is proportionally larger than the protostyle. A molar ectocingulum is present but reduced in Periptychus, and continuous and more prominent in Carsioptychus. The lower molar cusps of Periptychus are more widely separated and more subequal in size. The talonid of Carsioptychus 9 / 139 is proportionally shorter and not as tall as that in Periptychus and lacks a strong cristid obliqua and obliconid. The ectocingulid is stronger in Carsioptychus. Species Periptychus carinidens Cope 1881 p.337 [11] Catathlaeus rhabdodon Cope 1881 p.487 [12] Periptychus rhabdodon Cope 1882 p.465 [13] Periptychus brabensis Osborn & Earle 1895 p.55 [26] Periptychus superstes Matthew (in Simpson) 1935 p.25 [14] Periptychus rhabdodon superstes Matthew 1937 p.121 [5] Periptychus gilmorei Gazin 1938 p.275 [18] Lectotype. AMNH 3620, left partial dentary with dp3; right partial dentary with dp4 (Fig 1). Lectotype locality. Nacimiento Formation, San Juan Basin, NM, USA. Designation of lectotype. Herein we formally designate AMNH 3620 as the lectotype of Periptychus carinidens in accordance of Article 74 of the International Code on Zoological Nomenclature Code, specifically Article 74.6. Cope (1881) did not designate a holotype specimen in his original description of Periptychus carinidens. A left partial dentary with dp3 and right partial dentary with dp4, identified as Periptychus carinidens and described by Cope (1881), was first formally identified by a specimen number (AMNH 3620) in Matthew [5]. Matthew's [5] inference that this specimen represents the type specimen is deemed to constitute lectotype fixation (International Commission on Zoological Nomenclature Article 74.6.1). Hypodigm. AMNH 3637, right mandibular fragment with dp2-5, m1-3; AMNH 15937, right dentary with dp2-5, m1 and a partially erupted m2 (Fig 2). Diagnosis. Same as for genus. Comments. We treat `Periptychus gilmorei’ and `Periptychus superstes’ as junior synonyms of Periptychus carinidens following Williamson [10], contra Archibald [2]. The morphological Fig 1. Lectotype of Periptychus carinidens (AMNH 3620). Left partial dentary with dp4 in (A) occlusal view; (B) buccal view; (C) lingual view. Right partial dentary with dp5 in (D) buccal view; (E) lingual view; (F) occlusal view. Scale bar: 30mm. 10 / 139 Fig 2. Hypodigm specimens of Periptychus carinidens. Left dentary with dp2-5, m1-3 (AMNH 3637) in (A) occlusal view; (B) buccal view; (C) lingual view. Right dentary with dp2-5, m1 and a partially erupted m2 (AMNH 15937) in (D) buccal view; (E) lingual view; (F) occlusal view. Scale bar: 30mm. features which have been previously been stated as diagnostic of `P. gilmorei’ [18,28] and `P. superstes’ [5,14] are widely observed in specimens assigned to P. carinidens. A more convincing rationale for distinguishing between the purported species is the proportional sizes differences between the premolar and molar teeth. However, the size difference between Puercan `P. gilmorei’ and older Torrejonian (To1) P. carinidens, and `P. superstes’ and younger Torrejonian (To3) P. carinidens is less than the difference between older Torrejonian (To1) and younger Torrejonian (To3) P. carinidens, and therefore is not robust enough to warrant specific recognition. Comparative description Skull The skull of Periptychus is reasonably well known from numerous specimens preserving various features of the cranial anatomy; however, a single complete skull remains unknown. The following observations are based on a selection of Periptychus specimens covering most of the cranial anatomy, including the auditory region. The alisphenoid and infraorbital regions of Periptychus remain poorly known at present. Comparative taxa used in the following description of the skull bones include: Carsioptychus coarctatus (AMNH 27601, figured in [5]), Ectoconus ditrigonus (AMNH 16500, figured in [5]), Arctocyon primaevus (MNHN.F.CR700 figured in [52]), Claenodon ferox (NMMNH P8627 figured in [78]) and Pantolambda bathmodon (AMNH 16663, figured in [5]). Detailed comparisons of the auditory region are made to Protungulatum (AMNH 118359, figured in [79]), Arctocyon primaevus (MNHN BR. L9) and Pantolambda bathmodon (AMNH 16663, figured in [69]). Comparisons of the dentition are primarily made to Carsioptychus coarctatus, Ectoconus ditrigonus, and other periptychid taxa where necessary. The overall construction and morphology of the skull of Periptychus is typical of a medium sized `condylarth', albeit somewhat stouter in form (Figs 3 and 4). The rostrum of Periptychus is moderately elongate, dorsoventrally deep and tapers anteriorly with no rostral constriction. The rostral morphology of Periptychus is broadly similar to that in Carsioptychus and Ectoconus, all of which possess a dorsoventrally deep and anteriorly tapering snout. The rostrum of Periptychus is not as elongate as that of Arctocyon, but is longer than the comparatively short rostrum of Pantolambda. Further to this, both Arctocyon and Pantolambda exhibit a degree of rostral constriction between the upper canine and ultimate upper premolar. In lateral view, the dorsal surface of skull of Periptychus is relatively flat in comparison to the more domed morphology exhibited by Pantolambda. The anterior portion of the skull of Periptychus (rostrum and frontal region) gently slopes anteroventrally and is mediolaterally broad across the facial region. The posterior portion (braincase) is somewhat taller than the anterior skull in lateral aspect, due to the moderate expansion of the sagittal crest rather than an expanded braincase. The zygomatic arches of Periptychus are broad and laterally spreading. In dorsal view, the zygomatic arches exhibit a sub-rectilinear profile. The anterior portion of the arch rapidly spreads away from the facial region before extending posteriorly along a parasagittal plane. Posteriorly, the arch forms a rounded angle, anterior of the glenoid fossa. The dorsal profile of the zygomatic arches of Periptychus is more rectilinear than that observed in Carsioptychus and Ectoconus, where the arches are more convex in profile. The arch morphology in Periptychus is not as angled as that in Arctocyon and Pantolambda. In Arctocyon and Pantolambda, the posterior angle of the zygomatic arch is positioned anterior of the glenoid fossa as in Periptychus, but is considerably sharper, forming a near right angle. The braincase of Periptychus is small and low, with well-developed sagittal and nuchal crests (Figs 5 and 6). In dorsal view, the braincase is mediolaterally broadest at the level of the temporomandibular joint. Anteriorly from this point, the braincase tapers and is strongly constricted where it contacts the frontal region. Posteriorly, the braincase tapers slightly but remains relatively broad due, in part, to the relatively large exposure of the mastoid on the lateroventral surface of the braincase. The braincase of Periptychus is proportionally mediolaterally broader but not as anteroposteriorly elongate as that in Arctocyon. The braincase morphology is notably different to that observed in Pantolambda where the braincase continues to increase in width posteriorly beyond the temporomandibular joint. 12 / 139 Fig 3. Skull of Periptychus carinidens (NMMNH P-19482). (A) lateral view; (B) dorsal view, (C) ventral view. Abbreviations:; Fr, frontal; Ju, jugal; La, lacrimal; Mx, maxilla; Na, nasal; Pa, parietal; Pl, palatine; Sq, squamosal. Scale bar: 30mm. Nasal The delicate paired nasal bones of Periptychus are not well known. The following description is based on NMMNH P-19482, a portion of the skull roof including the nasal bones. In this 13 / 139 Fig 4. Skull of Periptychus carinidens (NMMNH P-36631). (A) ventral view, (B) dorsal view. Abbreviations: Bo, basioccipital; Bs, basisphenoid; Fr, frontal; Ju, jugal; mp, mastoid protuberance; Mx, maxilla; Pa, parietal; Occ, occipital; Sq, squamosal. Scale bar: 30mm. specimen, the nasals are incomplete, the lateral edges are highly damaged and the anterior nasal aperture is absent. Comparisons will be made to Arctocyon primaevus (MNHN.F.CR700, figured in [52] and MNHN BR L9), Claenodon ferox (NMMNH P-8627 figured in [78]), and Pantolambda bathmodon (AMNH 16663, figured in [5]). The paired nasal bones of Periptychus are anteroposteriorly long and together form a broad, flat roof over the rostrum. Based on NMMNH P-19482 (Fig 3), the anteroposterior length of the nasal is equal to approximately 50% of the anteroposterior length of the skull (note that on this specimen the anterior tip of the nasal is missing and the posterior end of the sagittal crest is damaged). The nasals of Periptychus are proportionally slender and elongate, forming a flat roof over the rostrum rather than a transversely arched roof as seen in Pantolambda, Arctocyon and Claenodon. The anterior-most portion of the nasals is unknown for Periptychus, and it is not possible to determine the nature of the contact with the premaxilla or the morphology of the nasal aperture from the specimens available. 14 / 139 Fig 5. Braincase of Periptychus carinidens (NMMNH P-65619). (A) dorsal view; (B) lateral (right) view; (C) ventral view; (D) posterior view. Abbreviations: fm, foramen magnum; pgp, glenoid process; ma, mastoid; mp, mastoid protuberance; nc, nuchal crest; oc, occipital condyle; pa, parietal; prz, posterior root of the zygomatic arch; sc, sagittal crest; sq, squamosal. Scale bar: 30mm. Posteriorly, the nasals extend past the anterior edge of the anterior root of the zygomatic arch to terminate at a point well within the transverse level of the orbit and approximately level with M3. Based on the shape of the bone present on NMMNH P-19482, the nasals are highly expanded posteriorly and appear to form a broad contact with the paired frontals. The frontals of NMMNH P-19482 overlie the nasals and are broken along a transverse line that could be interpreted as the nasofrontal suture. However, the ventral surface of the specimen shows the nasal bones tapering to point posteriorly along the sagittal axis of the skull that extends well under the frontals. As such, there are several plausible interpretations for the nasofrontal suture in Periptychus. It is possible that the nasals projected posteriorly into the frontal region on the dorsal surface of the skull, as observed in Arctocyon, so that in NMMNH P-19482 the frontal has been displaced over the nasal and the maxillary processes of the frontals are missing. Alternatively, the nasals may have formed a mediolaterally broad nasofrontal contact on the dorsal surface of the skull with a posterior projection of the nasals on the ventral surface of the frontals. It is also plausible that the condition of Periptychus is an intermediary between the two aforementioned states, in which the nasals form a small dorsal projection into the frontal with a larger ventral extension underlying the frontal. Comparisons with Ectoconus help elucidate the nasal morphology in Periptychus. In Ectoconus, the dorsal surface of the nasal shows a small, mediolaterally narrow posterior projection into the frontal region, which tapers to a posterior point which approximates the transverse plane of the mesial surface of the ultimate upper molar. Given the similarity of preserved 15 / 139 Fig 6. Partial left braincase of Periptychus carinidens (NMMNH P-30684). (A) lateral view; (B) ventral view; (C) posterior view. Abbreviations: frtsa, foramen for the rami temporalis of the stapedial artery; gf, glenoid fossa; gp, glenoid process; mp, mastoid process; ocp, occipital condyle. Scale bar: 30mm. portions of the nasal, Periptychus likely possessed a similar morphology to that observed in Ectoconus. In Claenodon and Arctocyon, the nasals form a more distinct posterior projection into the frontal region on the dorsal surface of the skull than that observed in Ectoconus. The condition in Pantolambda is harder to describe as the nasals have been displaced over the frontals, which artificially shortens the anteroposterior length of the frontal region. Regardless, the nasals are anteroposteriorly short, barely extending past the plane of the anterior root of the zygomatic arch, with a broadly rounded posterior border. In dorsal aspect, the midsection of the nasals of Periptychus is mediolaterally constricted and forms a concave suture with the maxillae. The anterior portion of the nasals are not as broad as the posterior region, with the posterior region expanding abruptly from above P2 to the frontal region. The nasal foramina are not observable along the nasofrontal suture on NMMNH P-19482. The constriction of the nasals in Periptychus is not as extreme as in Arctocyon, but is more marked than the very slight constriction seen in Claenodon and Pantolambda. Posteriorly, the nasal is excluded from contacting the lacrimal by the maxilla and frontals. In Periptychus, the ventral surface of the nasal exhibits a dorsal nasal meatus. Two grooved nasal fossae are separated by a distinct nasal septum along their entire length, and laterally delimited by a curved ridge for the attachment of the nasoturbinals. The nasal fossae are mediolaterally broad anteriorly and taper to a posterior point which underlies the frontals. It is not clear if the lateral surfaces of the nasals are damaged and extended to the same posterior point as the dorsal nasal meatus or diverged laterally prior to the posterior projection of the meatus. On each side, posterolateral to the dorsal nasal meatus, there are two open ended-ovoid fossae. The nasals appear to contribute to the anterior border of the fossae, with the frontal 16 / 139 contributing to the posterior border. Based on their posterodorsal position, we tentatively infer these depressions to be a pair of fossae for the frontotubinals. Premaxilla. The premaxilla of Periptychus is very poorly known. Of the specimens observed, only three preserve portions of the premaxilla: NMMNH P-19482, a subcomplete skull, (Fig 3); NMMNH P-35194, a highly concreted specimen preserving a small portion of the right premaxilla above the third incisor; and AMNH 3665, a subcomplete skull which preserves a similar portion of the premaxilla as NMMNH P-35194, but lacks an associated incisor. Comparisons are with Claenodon ferox (NMMNH P-8627 figured in [78]); Arctocyon primaevus (MNHN.F.CR700, figured in [52] and MNHN BR L9) and Pantolambda bathmodon (AMNH 16663, figured in [5]). The upper canine and upper third incisor of NMMNH P-35194 are subequal in size and separated by a narrow diastema. This condition to appears to be unique to Periptychus compared to Ectoconus, Arctocyon, Claenodon and Pantolambda, which exhibit a much greater size disparity between the upper canine and upper third incisor, with a much larger diastema between the teeth. The third incisor of Periptychus is situated within the premaxilla whilst the canine is situated within the maxilla and possesses a deep root (subequal in size to the crown). Given the proximity of the teeth, there is no room for the posterior migration of the premaxilla, suggesting a deep, dorsoventrally aligned contact between the premaxilla and maxilla which restricted the posterodorsal process of the premaxilla to the dorsal-most edge of the rostrum. Maxilla. The following maxilla description is based on: NMMNH P-19482, a subcomplete skull preserving the lateral and palatal components of the left and right maxillae separately (Fig 3); NMMNH P-36631, a fragmentary skull preserving parts of the left and right maxilla above the dentition (Fig 4); and NMMNH P-35194, a concreted specimen preserving the upper dentition in the maxilla. Comparisons will be made to Carsioptychus coarctatus (AMNH 27601 figured in [5]); Claenodon ferox (NMMNH P-8627 figured in [78]); Arctocyon primaevus (MNHN.F.CR700, figured in [52] and MNHN BR L9) and Pantolambda bathmodon (AMNH 16663, figured in [5]). The surface texture of the maxilla of Periptychus is unusual and features a distinct network of fine pits just above the tooth row; this texturing is particularly concentrated above the premolars and around the infraorbital foramen. The pits are numerous and variable in size and shape: some are circular while others are more ovoid, but not exceeding 0.7 mm in diameter. In life, they most likely housed a network of capillaries supplying blood to the external maxilla and possibly innervated the vibrissae. The paired maxillae form the lateral walls of the rostrum. They extend posteriorly and contribute to the anterior root of the zygomatic arches. Ventrally they form the lateral and anterior components of the hard palate and house the upper canine-postcanine dentition. Anteriorly, the maxilla contacts the premaxilla. Given that the anterior-most maxilla and premaxilla of Periptychus are poorly known, very little can be deduced about the anterior rostrum and nasal aperture. Based on the size and depth of the known maxillae and the position of the anterior dentition, the maxilla-premaxilla contact must have been deep and dorsoventrally orientated. Dorsally the lateral walls of the maxillae overlay the nasals. The external suture between the maxilla and nasal extends anteroposteriorly to form a concave arc, with no distinct projections. The vertical walls of the maxillae are deep and form the lateral walls of the rostrum. Dorsoventrally, the maxillae walls are slightly convex so that with the nasals they form a rounded rostrum in anterior aspect. Anteroposteriorly, the maxillae are long and positioned subparallel to one another; there is no constriction along the rostrum. The lack of rostral constriction in Periptychus is evident in all specimens and is notably different from the conditions in Ectoconus, Claenodon, Arctocyon and Pantolambda in which the maxillae constrict above the second 17 / 139 upper premolar. This condition is most exaggerated in Claenodon and Arctocyon, but still evident in Ectoconus and is more subdued in Pantolambda. Curiously, the maxillae of Carsioptychus exhibit a lateral expansion above the ultimate and penultimate upper premolars that is echoed in the arc of the postcanine dentition. This lateral expansion is somewhat apparent in Periptychus but is reduced to a very subtle expansion. The anterior opening of the infraorbital canal is demarcated by a single infraorbital foramen. The foramen is positioned on the lateral wall of the maxilla directly above the midpoint of the penultimate premolar at the same level as (but anterior to) the ventral edge of the anterior root of the zygomatic arch. It forms an anteriorly directed opening for the infraorbital canal, which transmitted the infraorbital artery and infraorbital nerve (CN V2). The position of the infraorbital foramen in Periptychus is comparable to Arctocyon and Claenodon but differs from Ectoconus and Pantolambda, both of which exhibit a more anteriorly placed opening above the second upper premolar, but which is positioned closer to the anterior root of the zygomatic arch. In Periptychus, the anterior and posterior infraorbital foramina are smaller than the lacrimal foramen and proportionally much smaller than the infraorbital foramina of any of the comparison taxa. Furthermore, in Ectoconus, Arctocyon, and Pantolambda the anterior infraorbital foramen opens into a shallow, anteriorly directed ovoid groove, which is not observable on Periptychus (it is also indistinct on Claenodon, but this may be due to damage). In NMMNH P-36631 the infraorbital canal is exposed and it almost twice the width as that of NMMNH P-19482. The length of the infraorbital canal in all observed specimens is far greater than one upper molar length; in NMMNH P-19482 it is approximately 2.5 times the length of M1 (which is equivalent to the length of the ultimate and penultimate premolars. The posterodorsal region of the maxilla is preserved in NMMNH P-19482 and AMNH 3665, but the inferred life position of the maxilla relative to other bones in this vicinity is somewhat tentative. In NMMNH P-19482 the maxillae are displaced ventrally over the nasals, both lacrimals are incomplete and the nature of the contact between frontals and nasals is somewhat ambiguous; in AMNH 3665 the rostrum has been heavily reconstructed. Based on the size and configuration of the known bones from both specimens, we tentatively infer that posterodorsally the maxilla contacts the frontal at the base of the rostrum and in doing so excludes the nasals from contacting the lacrimal on the dorsal skull surface. The maxilla contacts the lacrimal on the edge of the orbit, with the suture between the maxilla and lacrimal following the contour of the orbital rim and inhibiting the facial process of the lacrimal from making any contribution to the rostral region of the skull. The maxillae contribute a large portion of bone to the anterior root of the zygomatic arches. In lateral aspect, the maxilla does not extend far onto the zygomatic arch; it contacts the jugal at a point just dorsal of the lacrimal foramen, forming a concave contact. The maxilla and jugal do not interdigitate on the lateral surface of the zygomatic arch, unlike in Arctocyon where the maxilla bifurcates into a short process on the ventral edge of the zygomatic and a long process in between the short and long processes of the jugal. In Periptychus the long process of the jugal occupies the entire lateral surface of the zygomatic arch, displacing the reduced long and short processes of the maxilla to the ventral surface of the anterior root of the zygomatic arch. The maxilla extends underneath the jugal to form an anteroposteriorly deep orbital floor. The orbital floor of Periptychus forms a large, horizontal shelf in the anterior corner of the orbit and is completely composed of maxillary bone. On the ventral surface, the orbital floor houses the entire molar row. In dorsal aspect, the maxillary bone of the orbital floor forms a sweeping convex suture which abuts the jugal laterally on the medial wall of the zygomatic arch; it extends into the anterior corner of the orbit and onto the anterior wall of the orbit, where it contacts the transversely concave ventral border of the lacrimal above the posterior 18 / 139 infraorbital foramen. The posterior infraorbital foramen marks the orbital opening of the infraorbital canal; it forms a small posteriorly directed opening within the maxilla at the anterior corner of the orbit. The medial contacts of the maxilla are more ambiguous. A sharp groove is positioned near the medial limit of the maxillary component of the orbital floor, but the medial wall of the orbit is poorly preserved, making observations on the contact between the maxilla, lacrimal, palatine and frontal difficult. The labial roots of the ultimate upper molar roots are just visible on the dorsal surface of the orbital floor and provide some contour to the surface of the bone. The bone texture on the dorsal surface of the orbital floor displays a distinct pitting texture where the inferior oblique muscle attached. The pitting is evident in Carsioptychus and Ectoconus, but to a lesser extent than in Periptychus. The condition in Periptychus is most like that in Ectoconus, both of which possess a larger orbital floor than Carsioptychus. The orbital floor of Arctocyon is not as large as Periptychus, and in Claenodon it is highly reduced. Pantolambda possesses a highly expanded orbital floor which occupies almost 50% of the orbital space in ventral view, with the single lingual root of the ultimate upper molar (M3) visible on the dorsal surface of the orbital floor. On the ventral surface of the zygomatic arch of Periptychus, the contact between the maxilla and jugal shallowly interdigitates. The maxilla on the ventral surface of the anterior root of the zygomatic arch is expansive, with the jugal restricted to forming a small short process which projects slightly into the maxillary region. On the medial surface of the zygomatic arch the maxilla contribution to the orbital floor closely abuts the anterior jugal; it extends from the orbital floor to the dorsal rim of the zygomatic arch. Posteriorly, the zygomatic process of the maxilla extends along the medial surface of the jugal. Based on NMMNH P-36631 it appears that the zygomatic process of the maxilla extends posteriorly along the medial wall of the zygomatic arch to a point just posterior to where the squamous process of the zygomatic terminates dorsally over the jugal. The ventral surface of the anterior root of the zygomatic arch forms an anteroposteriorly broad shelf which extends as far as the mesial border of the ultimate upper premolar (P5). The ventral surface exhibits two well-defined ovoid fossae, which we interpret as particularly welldeveloped attachment sites for the zygomaticus major and minor muscles. The posterior border of the ventral surface of the anterior root of the zygomatic arch is demarcated by a thick ridge which is continuous with the ventral edge of the zygomatic arch. A small tubercle is positioned on the ridge level with the second upper molar (M2); this is part of the attachment area of the masseter. The ventral surface of the maxilla on the anterior root of the zygomatic arch is very well developed in Periptychus. None of the comparison taxa exhibit such a well-defined shelf, although the zygomatic arches of Pantolambda exhibit a similar level of mediolateral expansion (but the ventral surface is predominantly occupied by the dentition). On the ventral surface of the skull the maxillae contribute to the hard palate, forming the anterior and lateral components of the palate in conjunction with the premaxilla anteriorly and the palatines posteromedially. The maxilla contribution surrounds the lateral edges of the palatines and supports the canine and post-canine dentition. The alveolar processes of the maxillae house the dentition and are dorsoventrally deep, giving the palate a highly arched profile in anterior view. Posteriorly the alveolar processes project past the posterior edge of the palatines to form a pronounced maxillary tuberosity above the ultimate upper molar. A maxillary tuberosity housing the ultimate upper molar is present in all comparison taxa. In Periptychus the dorsal surface of the tuberosity contributes to the medial half of the orbital floor with the portion of the orbital floor lateral to the tuberosity forming a notch, the apex of which is level with the distal edge of the penultimate upper molar. In Carsioptychus the lateral notch is even deeper, extending to the level of the ectoflexus of the penultimate upper molar; in Ectoconus, Arctocyon and Claenodon the morphology of the notch 19 / 139 is comparable to Periptychus. Pantolambda exhibits an unusual condition in this respect: the maxillary tuberosity is very well developed and the lateral notch is distinct, but the tuberosity does not extend past the mesial border of the ultimate molar. In Periptychus the maxillary tuberosity is medially defined by the minor palatine notch. The notch is variably developed in different specimens. In NMMNH P-19482 the notch is present and reasonably distinct, so it forms a rounded apex that is directed anteromedially. In NMMNH P-36631, however, the medial notch is indistinct. In Carsioptychus, Ectoconus, Arctocyon and Claenodon the medial notch is highly reduced so that the maxillary tuberosity is not well defined from the palatine; in Pantolambda the notch is relatively well developed so that the notch is transversely aligned and its apex is directed anteriorly. In Periptychus and all the comparison taxa except Pantolambda, the maxillary tuberosity surrounds the ultimate upper molar, forming a rim around its distal edge. In Pantolambda the ultimate upper molar is positioned on the very edge of the tuberosity. The development of the maxillary tuberosity and the associated lateral and medial notches of Periptychus are likely related to the mesial migration of the molars during ontogeny [69]. A similar morphology is observed in the South American pantodont, Alcidedorbignya inopinata [69], and as such, comparisons of the development of the maxillary tuberosity between taxa are here made with caution. The palatine process of the maxilla surrounds the horizontal processes of the paired palatine bones. The hard palate of Periptychus is best observed on NMMNH P-19482. Here, the palatines overlay the maxillae but the maxilla-palatine suture is not well defined. The maxillary contribution to the posterior hard palate is restricted and provides more dorsoventral height to the palate than mediolateral width, as opposed to the condition in Pantolambda where the maxilla provides a mediolaterally broad shelf between the dentition and lateral edges of the horizontal process of the palatine. Posteriorly the maxilla-palatine suture in Periptychus is marked by a short, deep sulcus on the palatine, which extends around the mesial and lingual border of the ultimate upper molar. On the left side of NMMNH P-19482 the maxilla-palatine suture is visible and extends dorsoventrally underneath the alveolar line of the molar row, and then at the level of the first upper molar protostyle (= pericone) the suture turns medially towards the inferred position of the major palatine foramen. On the same specimen, we infer a small broken notch on the left side of the palate at the level of the hypocone on the first upper molar as the greater palatine foramen. The maxilla-palatine suture is less well-defined on the left side but clearly shows an anterior projection of the palatine lateral to the major palatine foramen. A shallow sulcus extends anteriorly from the major palatine foramen to a point on the palate level with the medial border of the ultimate upper premolar. The incisive foramen is not preserved on any of the specimens observed and it is not possible to deduce the presence or absence of a maxillary fossa. On the ventral surface of the palate the nasal septum marks the midline suture between the paired maxillae and palatines. The mesial roots of the last three premolars form prominent bony protuberances along the lateral edge of the dorsal surface of the hard palate. Palatine. The delicate paired palatine bones are not well known for Periptychus. The following description is based on NMMNH P-19482, which preserves fragments of the palatines within the hard palate (Fig 3); NMMNH P-36631, which preserves fragments of the posterior parts of the palatines (Fig 4); and AMNH 3669, which also preserves fragments of the posterior palatine. The horizontal processes of the palatines form the posterior quarter of the hard palate. They form a mediolaterally broad plate of thin bone which overlays the maxillae. The maxilla-palatine suture extends along the palate adjacent to the alveolar line of the molars and turns medial at the point level with the protostyle on the first upper molar. Posteriorly, the contact between the maxilla and palatine is marked by the minor palatine canal which runs from the 20 / 139 Fig 41. Left pes of Periptychus carinidens (AMNH 17075) in dorsal view. Abbreviations: as, astragalus; ca, calcaneum; cu, cuboid, ec, ectocuneiform; en, entocuneiform; m, mesocuneiform; n, navicular; t, tibiale; I-V refer to digit number; denotes reconstructed elements. Scale bar: 30mm. Note that this specimen is reconstructed and mounted on a resin base. Photographs of the individual bones are illustrated and photographed in [5]. 118 / 139 support to the astragalus. When considering the relative positions of the cruropedal elements it becomes evident that the astragalus must have articulated with the crus at an oblique angle, so that the mediolateral transverse axis of the astragalar body is angled at approximately 45Ê, with the surface of the trochlea facing medially. Astragalus. The astragalus forms the lower component of the cruropedal joint. Proximally it articulates with the distal epiphyses of the tibia and fibula, plantarly it articulates with the calcaneum and distally it articulates with the navicular and cuboid. No obvious muscles attached to the astragalus. The astragalus of Periptychus is distinctive: it is a robust, dorsoplantarly compressed bone with a quadrate body and a short and broad neck and head (Figs 42 and 43). The proportions are most like those of Ectoconus and Pantolambda, in that the mediolateral width of the astragalus is equal to slightly wider than the anteroposterior length (width/length = ~1). This differs from the arctocyonids, where the astragalus is typically longer than it is wide (Protungulatum 0.84, Arctocyon 0.91). The trochlea of Periptychus is short: the anteroposterior length of the trochlea relative to the anteroposterior length of the astragalus is 0.60, similar to Pantolambda (0.61). This is relatively short in comparison to Ectoconus (0.68) and Arctocyon (0.68), but longer than Protungulatum (0.57). The astragalar body of Periptychus is subquadrate in dorsal view and dominated by the lateral tibial articular facet, with a well-developed posteromedial expansion of the astragalar body. The lateral tibial facet forms a shallow trochlea with low-lying lateral and medial keels. The lateral keel is marginally taller than the medial keel, and strongly convex with a prominent fibula facet. The medial keel is positioned lower and is more rounded. The posteromedial border of the astragalar body is strongly expanded, forming a rounded projection from underneath the trochlea. In dorsal aspect, the anteroposterior long axis of the trochlea and its associated groove are offset at a 20Ê oblique angle (posterolateral to anteromedial) relative to the anteroposterior long axis of the pes. In medial view, the trochlea of Periptychus covers an arc of approximately 140Ê compared to only 120Ê in Protungulatum [71]. The trochlea is consistent in mediolateral width along its entire anteroposterior length. Anteriorly, the trochlea is clearly delimited from the astragalar neck by a subtle crease. The condition exhibited by Periptychus (which is also seen in Ectoconus and Hemithlaeus) differs from that in Protungulatum, Arctocyon and Claenodon, in which the medial half of the anterior border of the trochlea extends anteriorly onto the neck of the astragalus to form a shelf (beyond the anterior border of the lateral half of the trochlea). The condition in Pantolambda is proportionally like Periptychus, but morphologically distinct; the head is not strongly differentiated from the body of the astragalus, with the medial border of the trochlea and medial tibial facet extending anteriorly to form a near continuous surface with the head. The astragalus of Periptychus is perforated by a large astragalar foramen (= canal), positioned on the posterodorsal surface of the tibial trochlea. Plantarly, the foramen opens into a deep and broad, obliquely orientated sulcus for the tendons of the flexor digitorum fibularis. The sulcus continues around onto the plantar surface of the body of astragalus. The relative size and position of the astragalar foramen and associated sulcus of Periptychus are broadly like the morphology exhibited by Ectoconus, Hemithlaeus and Pantolambda. The condition exhibited by Periptychus is proportionally enlarged in comparison to Protungulatum and Arctocyon. In Periptychus, the lateral side of the tibial trochlea does not extend posteroplantarly beyond the proximal border of the astragalar foramen, whereas the medial side of the tibial trochlea extends slightly further posterodorsally to reach the plantad border of the foramen. This implies that the tibia may have partially covered the contents of the astragalar foramen during extreme plantar flexion. This condition is also observed in all the comparison taxa. 119 / 139 Fig 42. Left astragalus of Periptychus carinidens (NMMNH P-47693). (A) dorsal view; (B) plantar view; (C) anterior view; (D) posterior view; (E) medial view, (F) lateral view. Scale bar: 30mm. The border between the lateral and medial tibial articular facets forms the medial rim of the trochlea. In Periptychus, this border is poorly demarcated, forming a rounded continuous rim that, in medial view, is not as sharp or strongly convex as the lateral rim of the trochlea. In posterior aspect, the lateral and medial tibial facets meet at an obtuse (~130Ê) angle, so that the medial tibial facet forms a proximomedially directed surface. The surface of the medial tibial facet is smooth and gently convex. The posterior portion of the facet is dorsoplantarly deeper than the anterior portion. The posterior border of the facet is demarcated from the posteromedial projection of the astragalar body by a shallow sulcus, where part of the astragalotibial ligament, which forms part of the deltoid ligament, inserted. This morphology is clearly defined in Periptychus, but is not as strongly defined as in Ectoconus, Protungulatum or Pantolambda, where the sulcus forms a distinct crease (curiously, this feature is lacking in Arctocyon, but present in Claenodon). The lateral component of the astragalar body is formed by the fibular articular facet and the lateral process of the astragalus. In Periptychus, the fibular articular facet is prominent, producing a large, laterally projecting conical protuberance on the lateral wall of the body of the 120 / 139 Fig 43. Annotated line drawing of the left astragalus of Periptychus carinidens. (A) dorsal view; (B) plantar view; (C) anterior view; (D) posterior view; (E) medial view, (F) lateral view. Abbreviations: ab, astragalar body; ah, astragalar head; an, astragalar neck; cuf, cuboid facet; dfac, dorsal foramen of the astragalar canal; ef, ectal facet; fafl, fossa for the astragalofibular ligament; fif, fibular facet; gatl, groove for the astragalotibial ligament; ltf, lateral tibial facet; mtf, medial tibial facet; naf, navicular facet; sacf, supplementary astragolocalcaneal facet; sf, sustentacular facet; tf, tibiale facet; vfac?, ventral foramen of the astragalar canal (opening not discernible). Scale bar: 30mm. astragalus, with the apex of the cone formed by the lateral process of astragalus. The fibular articular facet of Periptychus (and the other periptychids) is proportionally and morphological like the condition seen in Protungulatum: in these taxa, the lateral process and fibular facet are laterally prominent, forming a conical protuberance. Medially, the fibular facet of Periptychus is clearly delimited from the tibial articular facet by the lateral keel of the trochlea, defining an angle of 115Ê, so that the surface of the fibular articular facet faces proximolaterally and is slightly convex in dorsal view. The lateral keel of the trochlea is sharpest anteriorly, becoming more rounded posteriorly. The condition of Periptychus differs from that of Arctocyon and Claenodon, where the fibular facet is not as laterally prominent, but instead projects more plantarly at a ~90Ê angle with the tibial trochlea. In Periptychus, a small circular depression is present on the anterior portion of the fibular articular facet, which acted to stabilize the fibula and 121 / 139 demarcates the fibular facet from the lateral process of the astragalus. A shallow, but relatively broad sulcus is seen on the posterior border of the fibular facet of Periptychus, which is where the posterior astragalofibular ligament would have inserted. This sulcus is consistently observed in the comparison taxa, but is variable in its morphology. Ectoconus, Pantolambda and Protungulatum exhibit a morphologically similar, but proportionally deeper sulcus than in Periptychus. In Arctocyon, the sulcus is particularly well developed, forming a dorsoplantarly deep, triangular shaped pit. In Periptychus, the plantar surface of the astragalar body articulates with the calcaneum via two points of contact: the ectal (calcaneoastragalar) and the sustentacular facet. the ectal facet extends obliquely along the posterolateral border of the ventral surface of the astragalus, with the surface of the facet facing plantarly. In plantar aspect, the facet is transversely elongate and relatively anteroposteriorly deep, tapering to a sharp point laterally, which comprises the welldeveloped lateral process of the astragalus. The ectal facet is more laterally prominent in Periptychus (and the other periptychids) than in Protungulatum, Arctocyon and Claenodon. In Periptychus, the plantar tuberosity forms a small, rounded eminence on the posteromedial border of the ectal facet. This feature is also present and more strongly developed, projecting further plantarly, in Arctocyon and Claenodon. The sustentacular facet of Periptychus is positioned towards the center of the astragalus on the plantar surface. It is separated from the ectal facet by a deep, but narrow sulcus astragali. A deep pit at the medial end of the sulcus suggests that the astragalar canal opened in this region, but an open canal is not evident on any of the specimens observed. A similar pit is seen in Ectoconus and Protungulatum and an intact astragalar canal is present in Arctocyon, opening at the same position. The sustentacular facet of Periptychus is large relative to the size of the astragalus: the maximum mediolateral width of the sustentacular facet is equivalent to 60% of the mediolateral width of the astragalar head. This is substantially larger than in Protungulatum (35%) and Arctocyon (40%). In Periptychus, the sustentacular facet is circular, subtly convex along its longitudinal axis and positioned at a 45Ê oblique angle (posteromedial-anterolateral) relative to the long axis of the astragalus. The anterior end of the facet is near continuous with the articular surface of the head of the astragalus. The posterior end of the facet is not as continuous with the body of the astragalus as the anterior end is with the head, but is not distinctly separate from it either. The sustentacular facets of Ectoconus and Hemithlaeus closely resemble one another and exhibit a broadly similar morphology to Periptychus. However, they differ in that they both possess a more ovoid sustentacular facet (transversely narrower than Periptychus) that is not posteriorly continuous with the body of the astragalus. Protungulatum and Arctocyon exhibit a different sustentacular morphology due, in part, to the expansion of the astragalar neck. As such, the facet is longer, and in Arctocyon the anterior portion of the facet is strongly continuous with the astragalar head. Further to this, in Arctocyon and Claenodon the facet is shallowly concave, particularly along its transverse plane. The sulcus astragali in both Protungulatum and Arctocyon is much broader than in Periptychus. The astragalar neck of Periptychus is short, contributing to only ~18% of the anteroposterior length of the astragalus, and mediolaterally broad, corresponding to 55% of the mediolateral width of the astragalar body. Despite its robust proportions, the neck is still well defined from the astragalar body and head, and the anterior border of the trochlea is clearly delimited. In dorsal aspect, the astragalar neck is medially offset relative to the astragalar body by an angle of approximately 45Ê. The dorsal surface of the astragalar neck is excavated relative to the body and head of the astragalus, forming a distinct shelf that apparently would have buttressed the tibia during extreme dorsiflexion of the pes. A small, shallow pit adjacent to the anteromedial corner of the astragalar trochlea appears to fit the anterior tubercle on the distal epiphysis of the tibia. Periptychus does not possess a 122 / 139 cotylar fossa or check facet [ 105,106 ]. A deep fossa is present on the medial side of the astragalus, forming a dorsoplantarly compressed, ovoid pit that is anteriorly continuous with the astragalar neck. The pit does not appear to articulate with the medial malleolus in any way and most likely served as an insertion point for a portion of the tibioastragalar component of the deltoid ligament. A homologous morphology is evident in Protungulatum, but the fossa is shallower and broadly open anteriorly. Ectoconus exhibits a more developed morphology than Periptychus, with a deep fossa that strongly undercuts the tibial trochlea. The astragalar head of Periptychus is mediolaterally broad and dorsoplantarly compressed. The surface of the head is smooth and broadly convex. In anterior aspect, the lateral portion of the articular surface of the head is more extensive than the medial portion, and is positioned slightly more dorsally, so that the mediolateral transverse axis of the astragalar head is tilted relative to the mediolateral transverse axis of the body. The navicular facet dominates the surface of the head, forming a large, near convex facet. A very shallow groove is observed towards the plantar-most surface of the navicular facet, with a small protuberance marking the medial edge of the groove. The surface of the navicular facet medial to this groove faces medially, interrupting the arc of the articular facet surface. A similar grooved morphology is also seen in Ectoconus, Mithrandir and Arctocyon, but not in Protungulatum. Laterally on the astragalar head, the navicular facet of Periptychus is poorly demarcated from the cuboid facet, as the two meet at an almost uninterrupted articular surface (marked only by a slight, rounded protuberance). This differs from the condition in Ectoconus, where the transition between the navicular facet and cuboid facet is evidenced by a shallow ridge and a discontinuity in the arc of the articular surface. In Periptychus, the cuboid facet is orientated obliquely (proximoplantarolateral to distodorsomedial) relative to the proximodistal long axis of the astragalus. The surface of the facet is convex and faces plantarolaterally. In Ectoconus, the surface of the cuboid facet is marked by a shallow groove on its plantar edge. This morphology is absent in Periptychus. The medial side of the astragalar head of Periptychus exhibits a distinct, distomedially facing articular surface, which extends from the astragalar head onto the neck. A similar facet is observed in all the comparison taxa (albeit reduced in Protungulatum); however, in Ectoconus, Arctocyon and Claenodon the facet faces more medially than distomedially. Previous workers have associated this facet with the presence of an additional tarsal ossicle, the so-called `tibiale', most notably in Claenodon [ 107 ] and also mentioned by Matthew [5] with regards to Ectoconus and Pantolambda (the tibiale of Periptychus is discussed below). Calcaneum. In Periptychus, the calcaneum is the largest tarsal bone in the pes and it forms a prominent, robust heel (Figs 44 and 45). The tuber of the calcaneum is moderately elongate, representing 50% of the anteroposterior length of the bone. The tuber of Periptychus is proportionally shorter in comparison to that of Ectoconus, where it represents 60% of the total length of the calcaneum. Interestingly, Protungulatum, Arctocyon and Pantolambda all possess a tuber that accounts for close to 50% of the total length of the calcaneum. The posterior apex of the calcaneal tuber of Periptychus is bulbous and enlarged relative to the rest of the structure. The roughened surface of the apex served for the insertion of the tendon of the gastrocnemius and soleus posteriorly, the plantaris medially and the plantar aponeurosis plantarly. The apex of the tuber of Periptychus is similar in morphology, but not as robust as that of Ectoconus, and both these taxa exhibit a proportionally more swollen apex than Protungulatum, Arctocyon and Pantolambda. The calcaneal body of Periptychus, defined as the part of the calcaneum that seats the articular facets, is relatively expanded, equating to half the overall length of the bone. The facets are widely separated and allowed for the superposition of the astragalus directly over the 123 / 139 Fig 44. Left calcaneum of Periptychus carinidens (NMMNH P-48429). (A) dorsal view; (B) lateral view; (C) plantar view; (D) medial view; (E) distal view, (F) anterior view. Scale bar: 30mm. calcaneum. In Periptychus, the astragalus is more directly superimposed over the calcaneum than in Arctocyon and Claenodon, where the astragalus is more medially positioned relative to the calcaneum. The ectal (= astragalocalcaneal) and sustentacular facets are positioned towards the anterior end of the calcaneum, but are not immediately adjacent to the cuboid facet and are separated by a deep, but relatively narrow calcaneal sulcus. The calcaneal sulcus corresponds to an Fig 45. Annotated line drawing of the left calcaneum of Periptychus carinidens. (A) dorsal view; (B) lateral view; (C) plantar view; (D) medial view; (E) distal view, (F) anterior view. Abbreviations: cf, cuboid facet; ef, ectal facet; fif, fibular facet; gfhl, groove for the flexor hallucis longus tendon, pp, peroneal process; pt, plantar tubercle; sf, sustentacular facet; su, sustentaculum; tc, tuber calcanei. Scale bars: 30mm. 124 / 139 equally deep groove on the astragalus, and together the two structures form a large tarsal sinus that transmitted the interosseous astragalocalcaneal ligament, blood vessels and nerves. In Protungulatum, Arctocyon and Claenodon the calcaneal sulcus is proportionally broader than in Periptychus and any of the other periptychid taxa observed. In Periptychus, the ectal facet is positioned at a posteromedial-anterolateral oblique angle on the dorsal surface of the body of the calcaneum. The facet is roughly ovoid in shape, elongate along its subanteroposterior axis. The articular surface is highly convex along its longitudinal plane and faces broadly anterodorsomedially. The articular surface of Periptychus forms a continuous, arched surface, whereas in Arctocyon (but not Claenodon) the surface of the ectal facet is subtly angled, and therefore is subdivided into a more medially directed posterior portion and a more anteriorly directed anterior portion. In Periptychus, the posterior border of the facet does not reach the medial border of the calcaneum, and the anterior border is positioned well posterior of the cuboid facet. In Arctocyon and Claenodon, the posterior border of the facet extends to reach the medial border of the calcaneum. In Protungulatum, the facet extends more medially than in the periptychids, but does not reach the medial border. In Periptychus, the ectal facet is laterally buttressed by the calcaneal fibular facet. The fibular facet forms a slim band along the dorsal edge of the ectal facet. The former facet is narrower posteriorly than anteriorly, with a convex articular surface. The border between the ectal and fibular facets is poorly delimited, forming a simple rounded ridge. The fibular facet morphology of Periptychus is also seen in the other periptychid taxa observed, with Periptychus possessing a proportionally broader facet than Ectoconus. The fibular facet in periptychids is highly reduced in comparison to Protungulatum, Arctocyon and Claenodon, where the facet forms a broad band, the surface of which is grooved in Arctocyon and positioned slightly ventral to the ectal facet. The sustentaculum of Periptychus is transversely expanded, forming a prominent medial protrusion that projects at a 90Ê angle to the anteroposterior long axis of the calcaneum. The sustentacular facet is positioned on the dorsal side of the sustentaculum. It is ovoid to subquadrate in shape, shallowly concave and faces anterodorsally. In anterior aspect, the sustentacular facet is positioned slightly anterior to the ectal facet and closer to the anterior border of the calcaneum and the cuboid facet. A small accessory facet connects the sustentacular facet to the cuboid facet. The sustentacular facet of Periptychus (and the other periptychid taxa observed) is relatively more transversely expanded than that of Protungulatum, Arctocyon and Claenodon. In Periptychus, a broad groove extends along the plantar surface of the sustentacular facet, which transmitted the flexor hallucis longis. The medial border of the groove is demarcated by a large protuberance, which is also seen in Ectoconus, but not in Protungulatum or Arctocyon. In Periptychus, the surface formed by the sustentacular and ectal facets, when viewed anteriorly, forms a relatively open configuration (~110Ê). This morphology is manifested in the axis of rotation between the astragalus and calcaneum. Movement between these two bones would have occurred around an axis set at a 45Ê oblique angle to the long axis of the calcaneum, with a small component of dorsoplantar movement due to the way the astragalus sits on the calcaneum (and considering the position of the tibia and fibula, so that astragalar trochlea faces dorsomedially). The sustentacular and ectal facets of Periptychus form a more open configuration than in Protungulatum (90Ê), suggesting that Protungulatum was capable of less dorsoplantar movement between the astragalus and calcaneum than Periptychus. The cuboid facet of Periptychus is positioned on the anterior surface of the calcaneum. The facet forms an irregular oval in anterior aspect, which is strongly expanded laterally, but also retains some dorsoplantar depth. The articular surface is shallowly concave (more strongly concave along the transverse axis than the dorsoplantar axis) and set at an oblique angle (25Ê) relative mediolateral axis of the calcaneal body. The facet faces anteromedially and the 125 / 139 dorsoplantar axis is orientated vertically. The transverse axis of the cuboid facet of Periptychus is not as obliquely orientated as in Pantolambda, where the facet is set at a 45Ê angle. The cuboid facet of Periptychus is broadly like that in the other `condylarth' taxa, but it is proportionally more transversely expanded than in Protungulatum and Arctocyon (particularly the lateral portion). The cuboid facet of Periptychus is not as transversely expanded as in Ectoconus, although it is more expanded in the dorsoplantar direction. The peroneal process of Periptychus forms a large, well-defined protuberance on the distal end of the calcaneum, with a small but sharp crest extending proximally along the calcaneal tuber. In dorsal view, the process is positioned close to the cuboid facet anteriorly, but it does not extend beyond the anterior border of the cuboid facet. Dorsally and plantarly, the peroneal process is flanked by shallow grooves for the passage of the peroneus brevis dorsally and the peroneus longus plantarly. The peroneal process of Periptychus is more anteroposteriorly elongate than the process in Protungulatum, but not as laterally prominent. Furthermore, the condition exhibited by Periptychus, although similar in morphology, is not as well developed as the condition in Ectoconus, where the peroneal process is massive, forming a laterally prominent shelf which extends the length of the calcaneum. In Periptychus, a large anterior plantar tubercle is positioned on the anteroplantar border of the calcaneum for the attachment of the plantar calcaneocuboid ligament. In anterior view, the plantar tubercle is dorsoplantarly compressed against the body of the calcaneum. A narrow, transverse sulcus separates the anterior border of the plantar tubercle from the cuboid facet. Laterally, the anterior plantar tubercle is delimited from the peroneal process by a deep sulcus for the passage of the abductor digiti minimi muscle. The plantar tubercle of Periptychus is not as dorsoplantarly deep as in Protungulatum and Arctocyon. Cuboid. The cuboid of Periptychus is known from only two specimens: AMNH 17075 (Fig 41) and 3636 (Fig 46). AMNH 3636 is more intact than 17075, which is mounted in articulation, obscuring the plantar and articular surfaces from view. Neither specimen, however, preserves the entire cuboid. The cuboid of Periptychus is quadrate and massively proportioned. In dorsal aspect, the mediolateral width of the cuboid is subequal to its anteroposterior depth, giving it a broad (rather than elongate) profile. The proportions of the cuboid of Periptychus are broadly similar to those of Ectoconus (in which the cuboid is mediolaterally broader than anteroposteriorly long), both of which feature a more massive construction than that exhibited by Arctocyon and Claenodon (where the cuboid is more anteroposteriorly elongate than mediolaterally broad). Proximally, the cuboid of Periptychus articulates with the calcaneum and the head of the astragalus in an alternating sequence. The calcaneal facet accounts for approximately 60% of the proximal surface of the cuboid and forms a large convex facet. In proximal view, the profile of the facet is somewhat irregular and is not well preserved in either specimen observed. There is a prominent apex towards the medial side of the facet, with a large lateral flank, which is well expanded onto the dorsal surface of the cuboid. The morphology of the cuboid calcaneal facet of Periptychus differs considerably from that seen in Arctocyon, where the facet is much less convex and is not as dorsally expanded, with the articular surface facing proximolaterally. Claenodon exhibits a broadly similar morphology to Arctocyon, but possesses a larger dorsal expansion of the calcaneal facet, albeit not as expanded as that of Periptychus. The condition in Periptychus is generally most like Ectoconus in terms of overall proportions, but Ectoconus does not exhibit a large dorsal expansion of the cuboid facet. Medially, the calcaneal facet of Periptychus is poorly demarcated from the astragalar facet. The astragalar facet is relatively large and accounts for the remaining 40% of the proximal surface of the cuboid. The surface of the facet is strongly concave, with well-defined proximal and medial surfaces. In Periptychus, the contact between the cuboid and astragalus appears to be 126 / 139 Fig 46. Tarsal elements of Periptychus carinidens (AMNH 3636). A-D: left cuboid: (A) dorsal view; (B) plantar view; (C) proximal view; (D) distal view. E-H: left navicular: (E) dorsal view; (F) plantar view; (G) proximal view; (H) distal view. I-L: left ectocuneiform: (I) medial view; (J) lateral view; (K) proximal view; (L) distal view. Scale bar: 30mm. more extensive than that in Ectoconus, with the astragalar facet accounting for approximately 20% of the proximal surface of the cuboid, forming a smaller, proximomedially facing, shallowly concave facet. The medial surface of the cuboid of Periptychus features two facets: a proximal facet that articulates with the navicular and a distal facet that articulates with the ectocuneiform. Note that the navicular and ectocuneiform facets are poorly preserved on the specimen observed (AMNH 3636), so the following observations are somewhat tentative. The navicular facet is proximodistally short and slightly convex along its dorsoplantar and proximodistal planes. In medial aspect, the facet is orientated at an oblique (dorsoproximal to plantodistal) angle relative to the dorsoplantar axis of the cuboid. The proximal border of the facet borders the astragalar facet and the distal border appears to border the ectocuneiform facet along its entire dorsoplantar length. This is broadly like the condition in Ectoconus, although in Ectoconus the facet is not set an oblique angle relative to the dorsoplantar axis of the cuboid. The morphology exhibited by Periptychus and Ectoconus differs from the condition in Arctocyon, where the navicular facet is orientated at a proximodistal to dorsoproximal angle and the distal border of the facet borders the ectocuneiform facet along its entire dorsoplantar length. 127 / 139 In Periptychus, the ectocuneiform facet on the cuboid is slightly larger than the navicular facet. It is dorsoplantarly more elongate than the navicular facet, but the surface is flat and not as convex. In medial aspect, the facet is parallel to the navicular facet and set at a dorsoproximal-plantodistal oblique angle relative to the dorsoplantar axis of the cuboid. The navicular facet of Periptychus appears to be proportionally expanded (both dorsoplantarly and proximodistally) on the medial surface of the cuboid in comparison to the navicular facet of Arctocyon, which is more dorsoplantarly elongate. Distal to the two medial facets for the navicular and ectocuneiform, the medial surface of the cuboid of Periptychus is slightly excavated to form a dorsoplantar sulcus. The plantar surface of the cuboid of Periptychus features a mediolaterally broad, hookshaped plantar process, which overhangs a deep transverse sulcus for the tendon of the peroneus longus. The sulcus continues onto the lateral face of the cuboid. The anterior surface of the cuboid is formed by a large, concave facet that articulates with metatarsals IV and V. The border between the individual facets for the metatarsals is poorly demarcated. The facet is subtriangular in profile, broadest dorsally, and tapers to a round apex plantarly. Navicular. The following description is based on AMNH 3636 (Fig 46) and 17075 (Fig 41), both of which include a complete navicular for Periptychus. The navicular articulates proximally with the astragalus, distally with the three cuneiform bones, and laterally with the cuboid. Furthermore, a possible medial articulation with an eighth tarsal ossicle, the so called `tibiale', will be discussed. There is no contact between the navicular and calcaneum. In dorsal aspect, the navicular is anteroposteriorly shallow and forms a partial cup around the distal portion of the head of the astragalus. Periptychus (and Ectoconus) possesses an anteroposteriorly shallower navicular in comparison to Arctocyon and Claenodon, but not as shortened as the condition in Pantolambda. In quantitative terms, the lateral edge of the navicular in Periptychus, Ectoconus, Arctocyon and Claenodon equates to approximately half the anteroposterior length of the cuboid (note that in the periptychids the transverse tarsal elements are proportionally shorter in relation to the other tarsal elements when compared to the arctocyonid taxa). In contrast to this, in Pantolambda the navicular is distinctly thin and is equivalent to less than one quarter of the anteroposterior length of the cuboid. In Periptychus, the medial process of the navicular is reduced, leaving the medial surface of the astragalar head exposed when the pes is held in the neutral position. The medial process of Periptychus is highly reduced in comparison to Ectoconus and to a lesser extent Arctocyon, where the process extends proximally to more completely cup the astragalar head. The proportions of the navicular of Periptychus more closely resemble the somewhat reduced condition exhibited by Claenodon, which possesses a smaller medial process than Arctocyon. The proportions of the proximal navicular surface of Periptychus are somewhat similar to Pantolambda; however, in Pantolambda the medial process of the navicular is a separate and unfused bone [5]. Matthew goes further to equate the unfused medial process of the navicular with the `tibiale'. However, a `tibiale' has been found in taxa which also possess a medial process, for example Claenodon [ 107 ]. In proximal aspect, the astragalar facet of Periptychus is mediolaterally broader than dorsoplantarly deep. The highly reduced medial process of Periptychus is not well distinguished from the distal astragalar facet, whereas in Arctocyon, and to a lesser extent in Ectoconus, a narrow `neck' separates the proximal and medial facets. The proximal navicular profile exhibited by Periptychus is dorsopalmarly reduced in comparison to Arctocyon, where the astragalar facet is more rounded in profile. The plantar tuberosity of Periptychus is mediolaterally robust, but is not as ventrally prominent as that of Arctocyon. The distal surface of the plantar tuberosity of Periptychus is deeply grooved for the passage of the tendon of the peroneus longus. 128 / 139 The medial edge of the navicular possesses a small, trapezoidal shaped cuboid facet, which articulates with the navicular facet on the medial surface of the cuboid. The articular surface of the facet is very slightly concave along its dorsoplantar axis. The navicular cuboid facet of Periptychus differs from the condition in Arctocyon, where the facet is dorsoplantarly sigmoidal, forming a tight articulation with the cuboid; a similar condition is observed in Claenodon [ 107 ]. In proximal aspect, the navicular and cuboid of Periptychus form a continuous surface with a sigmoidal profile, with both the navicular and cuboid forming a tight articulation around the lateral portion of the head of the astragalus. The distal surface of the body of the navicular is convex and subdivided into three smooth surfaces, which articulate with the cuneiforms. The facet for the ectocuneiform is mediolaterally broad. The articular surface is slightly concave and orientated distolaterally. The facet for the mesocuneiform is roughly subequal in mediolateral width, but dorsoplantarly deeper. Its articular surface is convex and faces distally. The ectocuneiform facet is the smallest of the three cuneiform facets on the navicular. The articular surface is convex, faces distomedially and extends on to the medial process of the navicular. The extension of the ectocuneiform facet onto the medial navicular process in Periptychus differs from the condition in both Ectoconus and Arctocyon, where the ectocuneiform facet is restricted to the body of the navicular. The extension of the ectocuneiform facet onto the medial process of the navicular constitutes a `well-consolidated' navicular, as described by Matthew ([5] p.143). Tibiale. The tibiale is a sesamoid bone of the navicular, embedded within the tendon of the tibialis posterior muscle [105]. Note that the tibiale, as thus defined, is not homologous with the reptilian tibiale. The tibiale is known for Periptychus based on AMNH 17075 (Fig 41). Matthew figured the tibiale on AMNH 17075, but did not make any reference to it within the text [5]. The tibiale of AMNH 17075 is a small, flat, thin piece of bone positioned proximal to the medial process of the navicular, covering the exposed medial surface of the astragalar head which is not covered by the navicular. There are several considerations of the tarsal anatomy of Periptychus that help elucidate the anatomy and function of the tibiale sesamoid. Previous workers have inferred the existence of a tibiale based on the presence of a medial facet on the astragalar head, as exhibited by Claenodon, a taxon for which the tibiale is putatively known [ 5,7,59,107 ]. Such a facet is present and well developed in Periptychus, extending distally onto the astragalar neck. A similar facet is seen in Ectoconus, Mithrandir, Protungulatum, Arctocyon and Pantolambda. However, there are several other anatomical details to consider when interpreting this facet. First, the medial astragalar facet is associated with the plantar calcaeneonavicular ligament (the `spring ligament') in plesiadapid primates [71]. The plantar calcaneonavicular ligament attaches the sustentaculum to the navicular and serves to support the head of the astragalus against the navicular. It is not explicitly clear how such a configuration would affect the positioning and function of a tibiale. More problematically, this also raises the possibility that a facet in this position is not always indicative of a bony tibiale, but may in some cases be a ligament pit. Secondly, several of the taxa with an observed medial astragalar facet that have been proposed to have possessed a tibiale (namely Ectoconus [5] and Mithrandir [48]) also possess an enlarged medial process of the navicular, which would completely omit a tibiale from contacting the head of the astragalus (dismissing the tarsal configuration proposed above). Where a tibiale is present alongside an enlarged medial navicular process, the primary function of the tibiale could be to solely increase the moment arm of the tibialis posterior. Both Periptychus and Claenodon possess a reduced medial navicular process, which would allow for the articulation of the tibiale with the astragalus. Therefore, it is plausible that both a tibiale and an expanded medial navicular process represent different conditions converging on the same 129 / 139 function: to support the astragalar head during inversion. It is also likely that the unfused medial navicular process described for Pantolambda [5] is actually a tibiale. Ectocuneiform. The ectocuneiform of Periptychus is the largest of the three tarsal cuneiforms. The following description is based on AMNH 3636 (Fig 46) and 17075 (Fig 41), both of which include a complete ectocuneiform. The body of the ectocuneiform is a robust, dorsoplantarly elongate, quadrate bone with a prominent plantar process. Proximally the ectocuneiform articulates with the navicular, laterally it articulates with the cuboid, medially with the mesocuneiform and distally with the second and third metatarsals. The ectocuneiform of Periptychus is not as proportionally robust as that of Ectoconus, in which the bone is proximodistally shorter but mediolaterally broader. The proximal surface of the ectocuneiform of Periptychus possesses a weakly convex, subrectangular navicular facet. The convex morphology of this facet in Periptychus differs from the condition in Ectoconus, where the articular surface is near flat, and Arctocyon where the articular surface is shallowly concave. A large hook-shaped plantar process dominates the ventral surface of the ectocuneiform of Periptychus, overhanging a transverse sulcus for the tendon of the peroneus longus distally. The lateral surface of the ectocuneiform of Periptychus features a dorsoplantarly elongate, rectangular cuboid facet. The distal portion of the lateral surface of the ectocuneiform is excavated to form a dorsoplantar sulcus, which corresponds to a similar morphology on the cuboid. The medial surface of the ectocuneiform features a proximal facet for the mesocuneiform. This facet is dorsoplantarly elongate and trapezoidal in shape. A second, smaller, subquadrate facet is positioned on the dorsal-most edge of the distal half of the medial surface of the ectocuneiform. This facet is shallowly concave and articulated with the proximal epiphysis of the second metatarsal. A second facet for metatarsal II extends plantarly from the first and is not as clearly delimited from the body of the ectocuneiform. The condition in Periptychus is broadly similar to that of Ectoconus, as in both taxa the facets on the ectocuneiform for the second metatarsal are in close proximity with one another, forming a near continuous surface. This condition differs from that seen in Arctocyon, in which the two facets are clearly delimited from one another. The distal surface of the ectocuneiform of Periptychus features a large, subtriangular concave facet for the proximal epiphysis of the third metatarsal. Mesocuneiform. The mesocuneiform of Periptychus is known from AMNH 17075; the specimen is mounted in articulation, limiting observation of this element to its dorsal surface only (Fig 41). The mesocuneiform is the smallest of the cuneiforms and the smallest bone in the tarsus. Proximally it articulates with the navicular, laterally with the ectocuneiform, medially with the entocuneiform and distally with the second metatarsal. In dorsal aspect, the mesocuneiform is quadrate in profile and proximodistally shortened relative to the ectocuneiform and entocuneiform, so that the second metatarsal extends proximally into the tarsal region, with its proximal epiphysis buttressed by the ectocuneiform and entocuneiform. Entocuneiform. This entocuneiform of Periptychus is only known from AMNH 17075, and like the mesocuneiform, can only be observed in dorsal view (Fig 41). The entocuneiform is mediolaterally broad and proximodistally long, but dorsoplantarly shallow. Proximally the entocuneiform exhibits a relatively mediolaterally broad contact with the navicular, so that the entocuneiform contacts the medial process of navicular unlike the condition in Ectoconus. The proximal border of the entocuneiform remains distal to the proximal border of the navicular. On its medial edge, the entocuneiform articulates with the mesocuneiform proximally and the second metatarsal distally. Distally the entocuneiform exhibits a mediolaterally broad, but dorsoplantarly shallow contact with the first metatarsal. 130 / 139 Metatarsals. The metatarsals of Periptychus are generally similar to the metacarpals: they are robust and mediolaterally broad, but remain well spaced from each other, and the proximal and distal epiphysis of each metatarsal is mediolaterally broad relative to the diaphysis, which is distinctly dorsoplantarly flattened (Fig 41). The second, third, fourth and fifth metatarsals are known from an associated, near complete pes of Periptychus (AMNH 17075). Because this specimen is mounted in articulation, observation of the plantar surface is not possible. The third metatarsal is the longest, the second metatarsal is 12% shorter than the third, the fourth metatarsal is slightly shorter than the third (4%) and the fifth is considerably reduced, 23% shorter than the fourth metacarpal. Both Ectoconus and Pantolambda show a similar trend, with the third metatarsal being the longest and the subsequent metatarsals becoming shorter. In Ectoconus, the second metatarsal is 16% shorter than the third, the fourth metatarsal is 2% shorter than the third metatarsal and the fifth metatarsal is 16% shorter than the fourth. In Pantolambda the second metatarsal is 9% shorter than the third, the fourth metatarsal is 3% shorter than the third metatarsal and the fifth metatarsal is 23% shorter than the fourth. The robusticity of the metatarsals can be quantified using the ratio of the mediolateral width of the distal epiphysis of the third metatarsal divided by its proximodistal length [69]. In Periptychus (AMNH 17075), the ratio is 0.39; this is slightly higher than Pantolambda (0.34, AMNH 16663) and much higher than Ectoconus (0.29, AMNH 16500); Arctocyon (0.27, MNHN.F.CR42), and Claenodon (0.27, AMNH 3268). The relative robusticity of the metatarsals of Periptychus, Ectoconus and Pantolambda do not fit the same trend seen with the metacarpals. The third metacarpal of Periptychus is proportionally as robust as Pantolambda, but less robust than Ectoconus. Further to this, the third metatarsal of Periptychus is longer than the associated third metacarpal (based on AMNH 17075). Both Ectoconus and Pantolambda exhibit similar metatarsal/metacarpal proportions, whereby the third metatarsal is longer than its associated third metacarpal (based on AMNH 16500 and 16663 respectively). The metatarsals of Periptychus are disproportionately robust in comparison to the metacarpals, whereas, the metacarpals of Ectoconus are disproportionately robust in comparison to the metatarsals. The second metatarsal of Periptychus is broadly like the third in its overall morphology, albeit slightly shorter in length. Proximally it articulates with all three cuneiform bones, with a larger proximal facet contacting the mesocuneiform and smaller medial and lateral facets contacting the entocuneiform and ectocuneiform, respectively. The diaphysis is not as dorsopalmarly flattened as the third metacarpal. The distal epiphysis is broadly similar to that of the third metatarsal; however, the distal articular surface is asymmetrical, rather than symmetrical, due to the reduction of its medial portion. The third metatarsal is asymmetrical along its proximodistal long axis due to the expansion of the proximolateral portion of the proximal epiphysis and medial expansion of the distal epiphysis. Proximally, the third metacarpal articulates with the ectocuneiform via a large, convex facet. The transverse axis of the facet is set at an oblique angle, so that lateral surface of the facet is positioned proximally relative to the medial surface. The lateral surface of the proximal epiphysis exhibits a large flat articular facet which abuts the fourth metatarsal, but does not overlap it. The medial surface of the proximal epiphysis barely contacts the proximal epiphysis of the second metatarsal, due to the proximal placement of the second metatarsal and the proximodistal shortening of the mesocuneiform. The proximal epiphysis of the third metatarsal is dorsoplantarly deeper than the distal epiphysis, but not as mediolaterally broad. The dorsal surface of the proximal epiphysis is lightly damaged, but there is little evidence of the two tuberosities for the insertion of the tarso-metatarsal ligaments, like those seen in Ectoconus. Such tuberosities are also absent in Claenodon. 131 / 139 The diaphysis of the third metatarsal of Periptychus is mediolaterally broad and dorsopalmarly compressed, with a smooth surface. The broad dorsal surface provided a large attachment area for the dorsal interossei. The mediolateral width of the diaphysis is near constant along its length, with some mediolateral broadening towards the distal epiphysis. The distal epiphysis is mediolaterally expanded so that it is broader than the proximal epiphysis, but not as dorsopalmarly deep. The distal surface of the bone forms a saddle-shaped articular surface with a dorsoplantar convexity that is broadly symmetrical. There is little evidence of a median keel on the dorsal articular surface, which is likely restricted to the plantar surface. The medial edge of the distal epiphysis is expanded and marked by a small, but prominent, tuberosity. A similar medial protuberance is present in Ectoconus, but is not as developed as in Periptychus; there is little evidence of a medial protuberance in Claenodon. In Periptychus, the distal articular surface extends well onto the dorsal surface. The proximal border of the distal articular surface is not demarcated by a fossa like that seen in the metacarpals of Periptychus, and both the metacarpals and metatarsals of Claenodon and Pantolambda. The fourth metatarsal is generally like the second in its overall morphology, and in how it differs from the third metatarsal. Proximally, it articulates with the medial half of the distal surface of the cuboid. Medially, it contacts the third metatarsal via a small medially orientated facet, and laterally it contacts the fifth metatarsal via a laterally orientated facet. There is no evidence of overlap between the third, fourth and fifth metatarsals. The distal epiphysis displays the reverse morphology of the second metatarsal. The distal articular surface is asymmetrical due to the reduction of its lateral portion, to mirror the morphology of the second metatarsal. The fifth metatarsal exhibits a somewhat different morphology to the second, third and fourth due to the expansion of its proximal epiphysis. Proximally it articulates with the lateral half of the distal surface of the cuboid, projecting distolaterally from the body of the pes. It does not contact the lateral surface of the cuboid, differentiating it from Claenodon. A large lateral tuberosity projects from the proximal epiphysis and provided a large grooved insertion site for the peroneus brevis. A large, medial facet on the proximal epiphysis contacts the fourth metatarsal. The distal epiphysis is mediolaterally broad, as is the case with the other metatarsals; however, the articular surface is not as dorsally expanded and appears to be hemispherically convex to surround the head of the bone rather than forming a dorsoplantarly convex saddle-shaped articular area. Tarsal phalanges. The tarsal phalanges of Periptychus are essentially morphologically identical to the manual phalanges, albeit somewhat larger. The robustness of the tarsal phalanges can be quantified using the ratio of the mediolateral width of the proximal epiphysis of the fourth phalanx divided by its proximodistal length [69]. Ideally, the ratio should be based on the third proximal phalanx, but only incomplete specimens are not known for Periptychus so we are using the fourth proximal phalanx, as it closely approximates the third in size and is known for all the comparison taxa. The ratio for Periptychus (based on AMNH 17075) is 0.76 compared to 0.66 for Ectoconus (AMNH 16500), 0.75 for Pantolambda (AMNH 16663) and 0.51 for Claenodon (AMNH 16543). As such, we can infer that the proximal phalanges of Periptychus are only slightly more robust than those of Pantolambda (1%), but 13% more robust than those of Ectoconus and 33% more robust than those of Claenodon. Conclusion New specimens of the Paleocene periptychid, Periptychus carinidens, are described here. These include some of the most complete specimens known for the species and provide new anatomical information on this abundant taxon, which was among the first eutherian mammals to evolve moderately large body size and distinct adaptations (particularly related to diet) after 132 / 139 the end-Cretaceous mass extinction. These specimens also provide new data with which to examine periptychid and Paleocene mammal phylogeny and paleobiology. Periptychus is an unusual taxon in that it unites a suite of dental, cranial and postcranial specializations with an otherwise relatively generalized skeleton. The overall shape of the skull of Periptychus is broadly concurrent with the inferred plesiomorphic eutherian condition [54,69,108], albeit more robust in its overall construction. Derived dental specializations included crenulated enamel, enlarged upper and lower premolars with a tall centralised paracone/protoconid and lingual shoulder. The relatively small canines and broad bunodont postcanine teeth, combined with a greatly expanded mandibular angle, raised mandibular condyle and broad zygomatic arches indicate Periptychus was herbivorous. The enlarged premolars are highly suggestive of an animal adapted towards durophagy and adept at crushing tough foodstuffs. The postcranial skeleton of Periptychus is that of a robust, stout-limbed animal that was incipiently mediportal (adapted to moving slowly over land but also having some characteristics conducive to quick movements when needed) and adopted a plantigrade stance. Features of note in the forelimb of Periptychus include: a shortened humerus relative to the ulna and radius; a hemispherical humeral head with large but low tuberosities; a broad and elongate deltopectoral region; a reduced insertion for teres major on the humerus; expanded lateral and medial epicondyles; an open humeroradial joint, a relatively straight ulna with little posterior bowing of the diaphysis or anterior bowing of the olecranon process; a broad carpus with enlarged centrale; and relatively short digital bones terminating in hoof-like unguals. Key features of the hindlimb of Periptychus include: a relatively unspecialized innominate with a widely open acetabulum; robust femoral trochanters including a third trochanter; the greater trochanter of the femur is tall but does not extend beyond the femoral head; a dorsoplantarly compressed astragalus which in articulation is wedged between the tibia and fibula permitting the fibula to contact the calcaneum; and a retained tibiale. In describing the anatomy of Periptychus, it is apparent that it closely resembles other medium-sized Paleocene mammals, given their array of shared `primitive' characteristics compared to the vast range of morphologies and adaptations exhibited by extant mammals. Consequently, the task of distinguishing between `primitive' Paleocene mammals gets distorted by our bias from observing features in extant mammals, which often serve to define what a constitutes an animal adapted for a certain lifestyle. The skeleton of Periptychus bears numerous resemblances to the other Paleocene taxa observed during the course of this study; however, there are subtle distinctions between the Paleocene taxa indicative of adaptations that are not easily comparable to extant mammals. This suggests that, far from being just a generalised ancestral stock for extant orders, Paleocene mammals were experimenting with their own unique morphologies. To this end, how can we describe Periptychus? In the broadest sense, it is a medium-sized obligate terrestrial generalist, albeit a versatile one, adept at moving through and over obstacles on the forest floor with adaptations of the limbs which do not preclude some scansorial and fossorial ability, and a simple but effectively modified dentition adapted to a plant-based durophagus diet (Fig 47). During this study, we studied numerous other medium-sized Paleocene mammals and could not help postulating on their paleobiology, albeit not to the same depth as Periptychus. In relation to Periptychus, Ectoconus, a considerably larger (~100Kg body mass) appears to be more fossorially adapted with dental adaptations indicative of an herbivorous diet. Arctocyon was likely more scansorial with dental adaptations towards an omnivorous to carnivorous diet [52]. Pantolambda exhibits some traits which suggest it to be more fossorially adapted than Periptychus but not to the same degree as Ectoconus, but it also lacks several key fossorial adaptions (no indication of a fossorially adapted triceps, and relatively reduced 133 / 139 Fig 47. Skeletal reconstruction of Periptychus carinidens. manual elements). Further more detailed study is required, but during the course of study our observations of Pantolambda have led us to hypothesise it might have been semi-aquatic, an ecology which has also been inferred for some larger, Eocene pantodont species [3]. From a wider perspective, the anatomy of Periptychus is broadly concurrent with what has been inferred as representative as the primitive eutherian condition. The dentition retains a primitive formula, occlusal pattern and cusp configuration albeit with numerous, modifications, but these are simple in their alteration and easily discerned. The skull is largely like other contemporaneous Paleogene taxa, although more robust in construction. The basicranial anatomy provides some interesting information, with Periptychus exhibiting a petrosal that is generally comparable the anatomy observed in taxa such as Pantolambda, Deltatherium and Eoconodon, but is somewhat divergent from the morphology exhibited by Protungulatum and Arctocyon, potentially suggesting a deeper division between periptychids and `arctocyonids' than has previously hypothesised (see [3,6,9]). Throughout its evolutionary history Periptychus was apparently a highly successful taxon as evidenced by its abundant dental fossil record, and was one of the few periptychids to persist through the Torrejonian and into the Tiffanian. Consider also, that while periptychids were abundant during the Puercan, they spent most of their evolutionary history exhibiting high turnover rates, which makes the persistence and widespread abundance of Periptychus even more notable. Consequently, Periptychus±and to a broader extent, periptychids±are prime exemplars by which to tackle the taxonomic and systematic conundrum that is `Condylarthra'. Supporting information S1 Appendix. Periptychus carinidens raw anatomical measurements. This file is formatted as an excel file and includes raw measurements for the Periptychus specimens described in this paper. Individual bones are listed in separate tabs. (XLSX) 134 / 139 Acknowledgments We are grateful to Judy Galkin for assistance while visiting the AMNH collections, Dr. Christine Argot for assistance to the MNHN collections and Dr. Andrew Kitchener for assistance while visiting the National Museum of Scotland zoology collections. Thanks also go to Dr. John Wible for his helpful comments regarding mammalian cranial anatomy. SLS is grateful to Davide Foffa and the various members of the Edinburgh PalaeoLab for their assistance and feedback during the writing of this paper. We thank all the peopleÐstudents, volunteers, academics, and many others far too numerous to listÐwho have worked with us in the field over the years to collect Periptychus specimens in the San Juan Basin, and the Bureau of Land Management for permits and logistical support. We thank Dr. Christine Janis for her comments on a previous version of this manuscript. We would also like to thank the editor, Dr. Thierry Smith, Dr. Michelle Spaulding and three other anonymous reviewers for providing helpful comments and suggestions for improving this publication. Investigation: Sarah L. Shelley. Supervision: Thomas E. Williamson, Stephen L. Brusatte. Writing ± original draft: Sarah L. Shelley. Writing ± review & editing: Sarah L. Shelley, Thomas E. Williamson, Stephen L. Brusatte. 1. Cope ED. The Condylarthra (Continued). Am Nat. 84;18: 892±906. 2. Archibald JD. Archaic ungulates (ªCondylarthraº). In: Janis CM, Scott KM, Jacobs LL, editors. Evolution of Tertiary Mammals of North America: Volume 1, Terrestrial Carnivores, Ungulates, and Ungulate Like Mammals. Cambridge University Press; 1998. pp. 292±331. 3. Rose KD. The beginning of the age of mammals. Johns Hopkins Press; 2006. 5. Matthew WD. Paleocene faunas of the San Juan Basin, New Mexico. Trans Am Philos Soc. 1937; 30: 1±510. 6. Prothero DR, Manning EM, Fischer M. The phylogeny of the ungulates. In: Benton MJ, editor. The phylogeny and classification of the tetrapods. Oxford: Clarendon Press; 1988. pp. 201±234. 7. Matthew WD. A revision of the Puerco fauna. Bull Am Mus Nat Hist. 1897; 9: 259±323. 8. Simpson GG. Fossil mammals from the type area of the Puerco and Nacimiento strata, Paleocene of New Mexico. Am Mus Novit. 1959; 1±22. 9. Van Valen LM. The beginning of the age of mammals. Evol Theory. 1978; 4: 45±80. Williamson TE. The Beginning of the Age of Mammals in the San Juan Basin, New Mexico: Biostratigraphy and Evolution of Paleocene Mammals of the Nacimiento Formation. New Mex Mus Nat Hist Sci Bull. 1996; 8: 1±141. 11. Cope ED. Geology and PalaeontologyÐMammalia of the Lower Eocene Beds. Am Nat. 1881; 15: 337±341. 12. Cope ED. On some Mammalia of the Lowest Eocene beds of New Mexico. Proc Am Philos Soc. 1881; 19: 484±495. 13. Cope ED. Synopsis of the Vertebrata of the Puerco Eocene Epoch. Proc Am Philos Soc. 1882; 20: 461±471. 14. Simpson GG. The Tiffany Fauna, Upper Paleocene: Primates, Carnivora, Condylarthra, and Amblypoda. Am Mus Novit. 1935; 817: 1±28. Wilson JA. Early Tertiary Mammals. In: Maxwell RA, Lonsdale JT, Hazzard RT, Wilson JA, editors. Geology of Big Bend National Park, Brewster County, Texas. The University of Texas; 1967. pp. 157±169. 135 / 139 16. Schiebout JA. Vertebrate Paleontology and Paleoecology of Paleocene Black Peaks Formation, Big Bend National Park, Texas. Texas Meml Mus Bull. 1974; 24: 1±88. 17. Standhardt BR. Vertebrate paleontology of the Cretaceous/Tertiary transition of Big Bend National Park, Texas. Louisiana State University. 1986. 18. Gazin CL. A Paleocene mammalian fauna from central Utah. J Washingt Acad Sci 1938; 28: 271±277. Wood HE, Chaney RW, Clark J, Colbert EH, Jepsen GL, Reeside JB, et al. Nomenclature and correlation of the North American continental Tertiary Geol Soc Am Bull. 1941; 52: 1±48. 20. Tomida Y, Butler RF. Dragonian mammals and Paleocene magnetic polarity stratigraphy, North Horn Formation, central Utah. Am J Sci 1980; 280: 787±811. 21. Tomida Y. "Dragonian" fossils from the San Juan Basin and the status of the "Dragonian" land mammalºage". In: Lucas SG, Rigby KJ, Kues BS, editors. Advances in San Juan Basin Paleontology. 1981. pp. 222±241. 22. Robison SF. Paleocene (Puercan-Torrejonian) mammalian faunas of the North Horn Formation, central Utah. Brigham Young University. 1986. 23. Archibald JD, Clemens WA, Gingerich PD, Krause DW, Lindsay EH, Rose KD. First North American land mammal ages of the Cenozoic Era. In: Woodburne MO, editor. Cenozoic Mammals of North America. Berkeley, California: University of California Press; 1987. pp. 24±76. 24. Cope ED. On some fossils of the Puerco Formation. Proc Acad Nat Sci. 1883; 35: 168±170. Simpson GG. Additions to the Puerco fauna, lower Paleocene. Am Mus Novit. 1936; 849: 1±12. 26. Osborn HF, Earle C. Fossil mammals of the Puerco beds: Collection of 1892. Bull Am Mus Nat Hist. 1895; 7: 1±71. 27. Cope ED. Synopsis of the Vertebrate Fauna of the Puerco Series. Trans Am Philos Soc. 1888; 16: 298±361. 28. Gazin CL. The mammalian faunas of the Paleocene of central Utah, with notes on the geology. Proc US Natl Mus. 1941. 29. Cifelli RL, Czaplewski NJ, Rose KD. Additions to knowledge of Paleocene mammals from the North Horn Formation, central Utah. Gt Basin Nat. 1995; 55: 304±314. 30. Archibald JD. The Importance of Phylogenetic Analysis for the Assessment of Species Turnover: A Case History of Paleocene Mammals in North America. Paleobiology. 1993; 19: 1±27. 31. Hunter JP. Parallel evolution in the Periptychinae (Condylarthra). J Vert Paleontol SVP Meeting Abstracts. 1995. p. 36A. Wilson GP. A phylogenetic analysis of Periptychidae (Mammalia: Condylarthra: Periptychidae). J Vert Paleontol SVP Meeting Abstracts. 1999;19: p. 85a. 33. Head JJ, Bloch JI, Hastings AK, Bourque JR, Cadena EA, Herrera FA, et al. Giant boid snake from the Palaeocene neotropics reveals hotter past equatorial temperatures. Nature. 2009; 457: 715±717. https://doi.org/10.1038/nature07671 PMID: 19194448 34. Jaramillo C, Ochoa D, Contreras L, Pagani M, Carvajal-Ortiz H, Pratt LM, et al. Effects of Rapid Global Warming at the Paleocene-Eocene Boundary on Neotropical Vegetation. Science. 2010; 330: 957± 961. https://doi.org/10.1126/science.1193833 PMID: 21071667 35. Hobbs K, Fawcett PJ. Paleocene climate change in the San Juan Basin, New Mexico: A paleosol perspective. AGU Abstracts. 2012. p. 2022. 36. Fricke HC, Wing SL. Oxygen isotope and paleobotanical estimates of temperature and δ18O-latitude gradients over North America during the early Eocene. Am J Sci. 2004; 304: 612±635. 37. Greenwood DR, Wing SL. Eocene continental climates and latitudinal temperature gradients. Geology. 1995; 23: 1044±1048. 38. Markwick PJ. "Equability," continentality, and Tertiary "climate": The crocodilian perspective. Geology. 1994; 22: 613±616. 39. Markwick PJ. Fossil crocodilians as indicators of Late Cretaceous and Cenozoic climates: implications for using palaeontological data in reconstructing palaeoclimate. Palaeogeogr Palaeoclimatol Palaeoecol. 1998; 137: 205±271. 40. Wing SL, Harrington GJ, Smith FA, Bloch JI, Boyer DM, Freeman KH. Transient floral change and rapid global warming at the Paleocene-Eocene boundary. Science. 2005; 310: 993±996. https://doi. org/10.1126/science.1116913 PMID: 16284173 41. Zachos J, Pagani M, Sloan L, Thomas E, Billups K. Trends, rhythms, and aberrations in global climate 65 Ma to present. Science. 2001; 292: 686±693. https://doi.org/10.1126/science.1059412 PMID: 11326091 136 / 139 Williamson TE, Lucas SG. Stratigraphy and mammalian biostratigraphy of the Paleocene Nacimiento Formation, southern San Juan Basin, New Mexico. New Mex Mus Nat Hist Sci Bull. 1992; 43: 265± 296. 43. Lofgren DL, McKenna MC, Honey J, Nydam R, Wheaton C, Yokote B, et al. New records of eutherian mammals from the Goler Formation (Tiffanian, Paleocene) of California and their biostratigraphic and paleobiogeographic implications. Am Mus Novit. 2014; 1±57. 44. Lofgren DL, Lillegraven JA, Clemens WA, Gingerich PD, Williamson TE. Paleocene Biochronology: The Puercan Through Clarkforkian Land Mammal Ages. In: Woodburne MO, editor. Late Cretaceous and Cenozoic Mammals of North America: Biostratigraphy and Geochronology. New York: Columbia University Press; 2004. pp. 43±105. 45. Tidwell WD, Ash SR, Parker LR. Cretaceous and Tertiary floras of the San Juan Basin. In: Lucas SG, Rigby JKJ, Kues B, editors. Advances in San Juan Basin Paleontology. Albuquerque: University of New Mexico Press; 1981. pp. 307±332. 46. Secord R, Williamson TE, Brusatte SL, Peppe DJ. Stable isotope paleoecology of a diverse late Torrejonian (early Paleocene) mammalian fauna from the San Juan Basin, New Mexico. J Vert Paleontol SVP Meeting Abstracts. 2015. 47. Hogan CM. Respiration: Encyclopedia of Earth. McGinley M, Cleaveland CJ, editors. Washington, D. C.: National Council for Science and the Environment; 2011. 48. Rigby JK. A skeleton of Gillisonchus gillianus (Mammalia; Condylarthra) from the early Paleocene (Puercan) Ojo Alamo Sandstone, San Juan Basin, New Mexico, with comments on the local stratigraphy of Betonnie Tsosie Wash. In: Lucas SG, Rigby JKJ, Kues B, editors. Advances in San Juan Basin Paleontology. Albuquerque: University of New Mexico Press; 1981. pp. 89±126. 49. Matthew WD. The Carnivora and Insectivora of the Bridger basin, middle Eocene. Mem Am Mus Nat Hist. 1909; 9: 289±576. 50. Simpson GG. The principles of classification and a classification of mammals. Bull Am Mus Nat Hist. 1945; 85: 1±350. 51. Russell DE. Les mammifères PaleÂocènes d'Europe. MeÂmoires du MuseÂe Natl d'Histoire Nat SeÂrie C. EÂ ditions du MuseÂum; 1964; 13: 1±324. 52. Argot C. Postcranial Analysis of a Carnivoran-Like Archaic Ungulate: The Case of Arctocyon primaevus (Arctocyonidae, Mammalia) from the Late Paleocene of France. J Mamm Evol. 2013; 20: 83±114. 53. Archibald JD, Zhang Y, Harper T, Cifelli RL. Protungulatum, Confirmed Cretaceous Occurrence of an Otherwise Paleocene Eutherian (Placental?) Mammal. J Mamm Evol. 2011; 18: 153±161. 54. O'Leary MA, Bloch JI, Flynn JJ, Gaudin TJ, Giallombardo A, Giannini NP, et al. The Placental Mammal Ancestor and the Post-K-Pg Radiation of Placentals. Science. 2013; 339: 662±667. https://doi.org/10. 1126/science.1229237 PMID: 23393258 Wible JR, Rougier GW, Novacek MJ, Asher RJ. Cretaceous eutherians and Laurasian origin for placental mammals near the K/T boundary. Nature. 2007; 447: 1003±1006. https://doi.org/10.1038/ nature05854 PMID: 17581585 Wible JR, Rougier GW, Novacek MJ, Asher RJ. The eutherian mammal Maelestes gobiensis from the Late Cretaceous of Mongolia and the phylogeny of Cretaceous Eutheria. Bull Am Mus Nat Hist. 2009; 1±123. 57. Halliday TJD, Upchurch P, Goswami A. Resolving the relationships of Paleocene placental mammals. Biol Rev. 2017; 92: 521±550. https://doi.org/10.1111/brv.12242 PMID: 28075073 58. Cope ED. Geology and PaleontologyÐThe position of the Periptychidae. Am Nat. 1897; 31: 335±336. 59. Osborn HF. Evolution of the Amblypoda. Part 1, Taligrada and Pantodonta. Bull Am Mus Nat Hist. 1898; 10: 169±218. 60. Gregory WK. The orders of mammals. Bull Am Mus Nat Hist. 1910; 27: 1±524. 61. McKenna MC, Bell SK. Classification of mammals: above the species level. New York: Columbia University Press; 1997. 62. Lucas SG. Pantodonts, tillodonts, uintatheres, and pyrotheres are not ungulates. Mammal Phylogeny. 1993; 2: 182±194. 63. Schneider CA, Rasband WS, Eliceiri KW. NIH Image to ImageJ: 25 years of image analysis. Nat Methods. 2012; 9: 671±675. PMID: 22930834 64. McKenna MC. Toward a Phylogenetic Classification of the Mammalia. In: Luckett WP, Szalay FS, editors. Phylogeny of the Primates. Boston, MA: Springer US; 1975. pp. 21±46. 65. Szalay FS. The Beginnings of Primates. Evolution. 1968; 22: 19±36. https://doi.org/10.1111/j.15585646.1968.tb03445.x PMID: 28564983 66. Evans HE, De Lahunta A. Miller's Anatomy of the Dog. Elsevier Health Sciences; 2013. 137 / 139 67. Gould FDH, Rose KD. Gnathic and postcranial skeleton of the largest known arctocyonid ªcondylarthº Arctocyon mumak (Mammalia, Procreodi) and ecomorphological diversity in Procreodi. J Vert Paleontol. 2014; 34: 1180±1202. 68. Kondrashov PE, Lucas SG. Nearly complete skeleton of Tetraclaenodon (Mammalia, Phenacodontidae) from the early Paleocene of New Mexico: morpho-functional analysis. J Paleontol. 2012; 86: 25± 43. 69. Muizon C de, Billet G, Argot C, Ladevèze S, Goussard F. Alcidedorbignya inopinata, a basal pantodont (Placentalia, Mammalia) from the early Palaeocene of Bolivia: anatomy, phylogeny and palaeobiology. Geodiversitas. 2015; 37: 397±634. 70. Szalay FS, Lucas SG. The Postcranial Morphology of Paleocene Chiacus and Mixodectes and the Phylogenetic Relationships of Archontan Mammals. New Mex Mus Nat Hist Sci Bull. 1996; 7: 1±47. 71. Szalay FS, Decker RL, Jenkins FA. Origins, evolution, and function of the tarsus in Late Cretaceous Eutheria and Paleocene primates. In: Jenkins FAJ, editor. Primate Locomotion. Elsevier; 1974. pp. 223±259. Williamson TE, Lucas SG. Meniscotherium (Mammalia,ºCondylarthraº) from the Paleocene-Eocene of Western North America. New Mex Mus Nat Hist Sci Bull. 1992; 1: 1±75. 73. Campione NE, Evans DC. A universal scaling relationship between body mass and proximal limb bone dimensions in quadrupedal terrestrial tetrapods. BMC Biol. 2012; 10: 60. https://doi.org/10.1186/ 1741-7007-10-60 PMID: 22781121 74. Linnaeus C v. Systema Naturae, edition X, vol. 1 (Systema naturae per regna tria naturae, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata). Holmiae Salvii. 1758. 75. Gill T. Arrangement of the families of mammals. With analytical tables. 1872. Cope ED. Geology and PalaentologyÐA New Type of Perissodactyla. Am Nat. 1881; 15: 1017±1023. Cope ED. Geology and PalaeontologyÐThe Periptychidae. Am Nat. 1882; 16: 832±833. 78. Kondrashov PE, Lucas SG. Arctocyon (Mammalia, Arctocyonidae) from the Paleocene of North America. Bull New Mex Mus Nat Hist Sci. 2004; 26: 11±20. 79. O'Leary MA. An Anatomical and Phylogenetic Study of the Osteology of the Petrosal of Extant and Extinct Artiodactylans (Mammalia) and Relatives. Bull Am Mus Nat Hist. 2010; 335: 1±206. Wible JR, Rougier GW. Cranial Anatomy of Kryptobaatar dashzevegi (Mammalia, Multituberculata), and Its Bearing on the Evolution of Mammalian Characters. Bull Am Mus Nat Hist. 2000; 247: 1±120. Wible JR. On the Cranial Osteology of the Hispaniolan Solenodon, Solenodon paradoxus Brandt, 1833 (Mammalia, Lipotyphla, Solenodontidae). Ann Carnegie Mus. 2008; 77: 321±385. 82. Novacek MJ. The skull of leptictid insectivorans and the higher-level classification of eutherian mammals. Bull Am Mus Nat Hist. 1986; 183: 1±111. 83. Cope ED. On the foramina perforating the posterior part of the squamosal bone of the Mammalia. Proc Am Philos Soc. 1880; 18: 452±461. 84. Luo Z-X, Gingerich PD. Terrestrial Mesonychia to aquatic Cetacea: Transformation of the basicranium and evolution of hearling in whales. Univ Michigan Pap Paleontol. 1999; 31: i±viii, 1±98. 85. Wible JR. The internal carotid artery in early eutherians. Acta Palaeontol Pol. 1983; 28: 281±293. 86. MacIntyre GT. The trisulcate petrosal pattern of mammals. In: Dobzhansky T, Hecht MK, Steere WC, editors. Evolutionary biology. Boston, MA: Springer; 1972. pp. 275±303. Wible JR. The ontogeny and phylogeny of the mammalian cranial arterial pattern (internal carotid artery). Unpubl PhD thesis, Duke Univ NC. 1984. Wible JR. The eutherian stapedial artery: character analysis and implications for superordinal relationships. Zool J Linn Soc. 1987; 91: 107±135. 89. Cifelli RL. The petrosal structure of Hyopsodus with respect to that of some other ungulates, and its phylogenetic implications. J Paleontol. 1982; 56: 795±805. 90. McDowell SB. The Greater Antillean insectivores. Bull Am Mus Nat Hist. 1958; 115: 113±214. 91. Geisler JH, Luo Z-X. Relationships of Cetacea to Terrestrial Ungulates and the Evolution of Cranial Vasculature in Cete. In: Thewissen JGM, editor. The Emergence of Whales. Boston, MA: Springer US; 1998. pp. 163±212. 92. MacPhee RDE. Auditory regions of primates and eutherian insectivores. Contrib Primatol. 1981; 18: 1±284. 93. Turnbull WD. Mammalian masticatory apparatus. Fieldiana Geol. 1970; 18: 149±356. 138 / 139 Slatter DH. Textbook of small animal surgery. Elsevier Health Sciences; 2003. MacPhee RDE. Morphology, adaptations, and relationships of Plesiorycteropus: and a diagnosis of a new order of eutherian mammals. Bull Am Mus Nat Hist. 1994; 220: 1±214. Wible JR, Hopson JA. Basicranial evidence for early mammal phylogeny. In: Szalay FS, Novacek MJ, McKenna MC, editors. Mammal Phylogeny. Springer; 1993. pp. 45±62. Cope ED. The Condylarthra. Am Nat . 1884 ; 18 : 790 ± 805 . 94. Sloan RE , Van Valen LM. Cretaceous mammals from Montana . Science . 1965 ; 148 : 220 ± 227 . https:// doi.org/10.1126/science.148.3667.220 PMID: 17780082 95. Radinsky LB . The adaptive radiation of the phenacodontid condylarths and the origin of the Perissodactyla . Evolution. 1966 ; 20 : 408 ± 417 . https://doi.org/10.1111/j.1558- 5646 . 1966 .tb03375. x PMID: 28562971 96. Simons EL . The Paleocene Pantodonta . Trans Am Philos Soc . 1960 ; 50 : 3± 99 97. Rigby JK . Swain Quarry of the Fort Union Formation, middle Paleocene (Torrejonian), Carbon County, Wyoming: geologic setting and mammalian fauna . Evol Monogr . 1980 ; 3 : 1± 178 . 98. Ziegler AC . A theory of the evolution of therian dental formulas and replacement patterns . Q Rev Biol . 1971 ; 46 : 226 ± 249 . 99. Luo Z-X , Kielan-Jaworowska Z , Cifelli RL . Evolution of dental replacement in mammals . Bull Carnegie Mus Nat Hist . 2004 ; 36 : 159 ± 175 . 100. Szalay FS , Dagosto M. Locomotor adaptations as reflected on the humerus of Paleogene primates . Folia Primatol . 1980 ; 34 : 1± 45 . https://doi.org/10.1159/000155946 PMID: 7002751 101. Gebo DL , Rose KD . Skeletal morphology and locomotor adaptation in Prolimnocyon atavus, an early Eocene hyaenodontid creodont . J Vert Paleontol . 1993 ; 13 : 125 ± 144 . 102. Szalay FS . Evolutionary history of the marsupials and an analysis of osteological characters . Cambridge University Press; 2006 . 103. Evans P. The postural function of the iliotibial tract . Ann R Coll Surg Engl . 1979 ; 61 : 271 . PMID: 475270 106. Zack SP , Penkrot TA , Krause DW , Maas MC . A new apheliscine'condylarth'mammal from the Late Paleocene of Montana and Alberta and the phylogeny of'hyopsodontids' . Acta Palaeontol Pol . 2005 ; 50 : 809 ± 830 . 107. Schaeffer B. Notes on the origin and function of the artiodactyl tarsus . Am Mus Novit . 1947 ; 1356 : 1± 24 .


This is a preview of a remote PDF: http://journals.plos.org/plosone/article/file?id=10.1371/journal.pone.0200132&type=printable

Sarah L. Shelley, Thomas E. Williamson, Stephen L. Brusatte. The osteology of Periptychus carinidens: A robust, ungulate-like placental mammal (Mammalia: Periptychidae) from the Paleocene of North America, PLOS ONE, 2018, DOI: 10.1371/journal.pone.0200132