The first record of albanerpetontid amphibians (Amphibia: Albanerpetontidae) from East Asia
The first record of albanerpetontid amphibians (Amphibia: Albanerpetontidae) from East Asia
Ryoko Matsumoto 0 1
Susan E. Evans 1
☯ These authors contributed equally to this work. 1
0 Department of Zoology, Kanagawa Prefectural Museum of Natural History , Odawara, Kanagawa Prefecture , Japan , 2 Department of Cell and Developmental Biology, University College London , London, England
1 Editor: Thierry Smith, Royal Belgian Institute of Natural Sciences , BELGIUM
Albanerpetontids are an enigmatic fossil amphibian group known from deposits of Middle Jurassic to Pliocene age. The oldest and youngest records are from Europe, but the group appeared in North America in the late Early Cretaceous and radiated there during the Late Cretaceous. Until now, the Asian record has been limited to fragmentary specimens from the Late Cretaceous of Uzbekistan. This led to speculation that albanerpetontids migrated into eastern Asia from North America in the Albian to Cenomanian interval via the Beringian land bridge. However, here we describe albanerpetontid specimens from the Lower Cretaceous Kuwajima Formation of Japan, a record that predates their first known occurrence in North America. One specimen, an association of skull and postcranial bones from a single small individual, permits the diagnosis of a new taxon. High Resolution X-ray Computed Microtomography has revealed previously unrecorded features of albanerpetontid skull morphology in three dimensions, including the presence of a supraoccipital and epipterygoids, neither of which occurs in any known lissamphibian. The placement of this new taxon within the current phylogenetic framework for Albanerpetontidae is complicated by a limited overlap of comparable elements, most notably the non-preservation of the premaxillae in the Japanese taxon. Nonetheless, phylogenetic analysis places the new taxon closer to Albanerpeton than to Anoualerpeton, Celtedens, or Wesserpeton, although Bootstrap support values are weak. The results also question the monophyly of Albanerpeton as currently defined.
Data Availability Statement: The data is within the
paper in the form of description and images. The
data used in the phylogenetic analysis can be found
in S1 Supporting information file.
Funding: The first author received funding from the
Linnean Society (GB), named as the Anne Sleep
award, which funded travel to the UK for
collaboration (to RM). The second author received
no specific funding for this work.
Competing interests: The authors have declared
that no competing interests exist.
Albanerpetontidae form a distinct and highly derived extinct amphibian clade with a long
fossil record. Albanerpetontids share the double occipital condyle of lissamphibians but are
characterised by a unique set of skeletal features including: a complex `mortise and tenon'
interdentary joint; non-pedicellate, slightly tricuspid, teeth; a sculptured median (fused)
frontal; and an amniote-like `atlas-axis' involving three anterior cervical components.
Albanerpetontids have been variously considered to be caudates or stem-caudates [1±5],
stem-batrachians [6±9], the sister group of Gymnophionomorpha [10±11]; or stem-lissamphibians
[5,11±13]. This uncertainty is due partly to differing opinions on the origins and monophyly
of Lissamphibia , but also to the fragmentary nature of most albanerpetontid material.
Where complete specimens exist, from the Early Cretaceous Spanish locality of Las Hoyas [6,
8, 15], they are two-dimensionally compressed, leaving many skull features difficult to
interpret. Some three-dimensional skull associations have been described [13, 16], but this material
(Albanerpeton pannonicum) is fragmentary and comparatively recent (Pliocene), so not
necessarily representative of primitive albanerpetontid morphology.
Albanerpetontidae are predominantly Laurasian in distribution , the exceptions being
specimens from the Middle Jurassic (Bathonian)  and Early Cretaceous (Berriasian) [19±
20] of Morocco. The longest record for albanerpetontids is in Europe and extends from the
Middle Jurassic (Bathonian) of France  and England [20, 22±23], to the Pliocene of
Hungary  and Italy , albeit with an unexplained hiatus in the Eocene . In North
America, albanerpetontids are first recorded from the uppermost Aptian or lower Albian [26±28]
and then remained a consistent element of North American microvertebrate assemblages
through to the late Palaeocene [17, 26±27, 29±30]. By contrast, the Asian record of the group is
poor and, until now, was restricted to rare and fragmentary material from the Late Cretaceous
(Cenomanian/Coniacian) of Uzbekistan, Central Asia [17, 31±33]. The relative ages of the first
Asian and American records have influenced discussions as to the direction, timing, and route
of possible albanerpetontid dispersal between the two continents [26, 32±34].
Here we extend the record of albanerpetontids in Asia with the description of a new taxon
represented by an associated specimen and two isolated dentaries from the Early Cretaceous
(Barremian) of Japan. This is the earliest confirmed record from Asia and the first from
eastern Asia. Moreover, the three-dimensional preservation of individual elements, as revealed
through High Resolution X-ray Computed Microtomography (μCT), provides important new
information on albanerpetontid morphology. Given that most recent phylogenetic analyses
find albanerpetontids to be related to crown Lissamphibia, a fuller understanding of their
morphology has the potential to inform the debate on lissamphibian origins.
The Mesozoic (Middle Jurassic-Lower Cretaceous) deposits of the Tetori Group are widely
distributed in the western part of central Japan, and present a gradual transition from marine
to freshwater conditions [35±38]. The Tetori Group is traditionally subdivided into three
ascending units: the Kuzuryu Subgroup, dominated by marine deposits; the Itoshiro
Subgroup, containing marine and terrestrial deposits; and the Akaiwa Subgroup, consisting
mainly of terrestrial sediments [36±37].
The Kuwajima Formation forms the upper part of the Itoshiro Subgroup in the Tetodori
River District, and it is mainly composed of non-marine sandstones and mudstones. The
albanerpetontid specimens described herein were collected from an outcrop of the Kuwajima
Formation forming the Fossil Cliff ("Kaseki-Kabe") in the Kuwajima district, Hakusan City
(formerly Shiramine Village), Ishikawa Prefecture (Fig 1). Isajii et al.  identified three facies
at this locality: Facies I, a carbonaceous swamp; Facies II, a shallow lake; and Facies III, a
vegetated swamp. Together, these are thought to represent the wide, stable, vegetated floodplain of
a meandering river system, with a humid environment [40±41]. All of these facies have yielded
vertebrate remains, including dinosaurs [42±43], pterosaurs , mammals [45±46],
tritylodonts [47±49], lizards [50±54], fish [55±57], turtles , turtle eggshells , choristoderes
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Fig 1. Map of western Japan showing position of the type locality. Areas marked by hashed lines indicate outcrops of the Tetori Group in this
[60±61], and rare frogs . To date, more than 2500 specimens have been recorded from this
locality. The greatest abundance of terrestrial taxa has been recovered from Facies III, as were
the albanerpetontid specimens described herein.
Most researchers have dated the Kuwajima Formation to the Early Cretaceous, but age
estimates have varied from early Neocomian  to Hauterivian , Valanginian , or
Berriasian-Hauterivian [42, 60], based on biostratigraphic correlations with other Tetori Group
strata. However, zircon U-Pb from a tuff that intruded into the lower part of the Kuwajima
Formation gave a date of 130.7 ± 0.8 (2SE) Myr , and zircon U-Pb ages of 132 ± 0.9 (2 SE)
and 117 ± 0.7 (2 SE) Myr have been reported  for the Okurodani Formation in
neighbouring Gifu Prefecture, which shares faunal components with the Kuwajima Formation (e.g. the
lizard Sakurasaurus and the choristodere Monjurosuchus). The most recent work  dated
the Kuwajima Formation to the Barremian, and this is the age estimate we are using herein.
The deposit may be older than this but is unlikely to be younger.
Material and methods
The construction of a road tunnel through the "Kaseki-Kabe" fossil cliff in 1997 yielded almost
17, 000 m3 of fossiliferous matrix. Much of this was attributed to the plant rich Facies I, but a
sample of 210 m3 of Facies II and Facies III was separated out and retained. The matrix is
resistant to chemicals and each large block is carefully split into smaller pieces, while examining the
exposed surfaces for bone. Any exposed bone is then prepared manually by collections staff
and volunteers, with the resulting specimens also being examined periodically by specialists.
However, this approach does make it almost impossible to trace the component parts of an
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original block once it has been broken into smaller pieces. No permits were required for the
described study, which complied with all relevant regulations.
The available albanerpetontid material comprises two referred dentaries and a small block
of matrix, containing a bone association. All three specimens are held in the Shiramine
Institute of Paleontology, Hakusan Board of Education, Hakusan City, Ishikawa Prefecture, Japan
(SBEI, Shiramine Board of Education). They are accessible to researchers on application to the
curators. Comparative albanerpetontid material from the UK (NHMUK PV R 36956, a
parietal) and Spain (MCCM-LH-15710, a partial skeleton) was on loan from the permanent
collections of the Natural History Museum, London, and the Museo de las Ciencias de
Castilla-LaMancha, Cuenca, Spain, respectively.
The matrix block containing the bone association is very small. Initially, only a small area
of sculptured bone was exposed on one surface, providing no indication that it originally
formed part of a larger specimen, the remains of which cannot be traced. Subsequent manual
preparation of the block revealed not only several skull bones on the surface (Fig 2A), but also
further underlying bones that could not be accessed without damaging neighbouring elements.
Courtesy of the Tokyo Metropolitan Industrial Technology Research Institute, we were able to
scan the whole block using a CT scanner (Toscaner 30000 micro CN) at a slice distance of
0.029 mm (100 kV, 30 μA). The braincase region was then rescanned on the same machine at
a slice thickness of 0.016 mm (100.0 kV, 30 μA) to achieve greater resolution. One dentary
specimen (SBEI 2405) was scanned at the Nagoya Municipal Industrial Research Institute
(Toscanner 30000) at a slice width of 0.008 mm (100 kv, 35 μA). A second dentary (SBEI
2462), recovered later, was scanned at the Tokyo Metropolitan Industrial Technology Research
Fig 2. Shirerpeton isajii gen. et sp. nov., SBEI 2459, holotype block. A, digital photograph showing surface view of the
specimen after manual preparation; B, rendered view of the surface from μCT data showing identifications of exposed elements.
Abbreviations: Br, braincase elements; Fr, frontal; L.La, left lacrimal; L.Mx, left maxilla; L.N, left nasal; L.Pa, left parietal; LPf, left
prefrontal; L.Sm, left septomaxilla; L.Sq, left squamosal; R.La, right lacrimal; R.Pa, right parietal; R.Pf, right prefrontal; R.Sq,
right squamosal;?, unidentified element. Scale bars = 5 mm.
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Fig 3. Shirerpeton isajii gen. et sp. nov., skull reconstruction. A-D, Model construction. 3-D models constructed using printouts of the individually
segmented elements from the μCT data (mirrored as needed: nasal, parietal, possible supratemporal) and fitted into modelling clay. A, dorsal; B, right
lateral; C, left lateral; D, left anterolateral showing the relations of the nasal, lacrimal, and maxilla in the narial margin. The tip of the rostrum is roughly
reconstructed in modelling clay. E, outline reconstruction of the skull in dorsal view, based on the 3-D model in A-D. Note that the suspensorial
elements are omitted as their positions are uncertain. Abbreviations: Fr, frontal; J, jugal; La, lacrimal; Mx, maxilla; N, nasal; Pa, parietal; Pf, prefrontal;
S.O, supraoccipital;? St, possible supratemporal.
Institute at a slice thickness of 0.006 mm (100 kV, 65 μA). Image reconstruction in all cases
used AVISO v. 8 (Fig 2B), although the small size of individual elements rendered
segmentation of features like foramina, tooth tips, and fine edges difficult. Individual elements
segmented out from the scanned block were then 3-D printed (Objet350 Connex) at 20x original
size, mirrored where necessary (to provide pairs), and fitted together manually using
modelling clay to determine bone positions and articulations in three dimensions (Fig 3A±3D). The
physical model generated was then used for the skull reconstruction shown in Fig 3E.
The electronic edition of this article conforms to the requirements of the amended
International Code of Zoological Nomenclature, and hence the new names contained herein are
available under that Code from the electronic edition of this article. This published work and
the nomenclatural acts it contains have been registered in ZooBank, the online registration
system for the ICZN. The ZooBank LSIDs (Life Science Identifiers) can be resolved and the
associated information viewed through any standard web browser by appending the LSID to the
prefix ªhttp://zoobank.org/º. The LSID for this publication is: urn:lsid:zoobank.org:pub:
C8490EBE-E927-4CD6-BEA6-BEFE1A71DF77. The electronic edition of this work was
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digital repositories: PubMed Central, LOCKSS, UCL Discovery Publications database.
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Note on nomenclature
Marjanović and Laurin  noted that as erpeton is neuter, a number of original species
names have a grammatically incorrect ending e.g. Anoualerpeton unicus  should be An.
unicum. We have therefore used the corrected species spelling throughout this manuscript.
Shirerpeton gen. nov.
urn: lsid: zoobank.org: act:C3726120-0EF5-4407-8624-05A4C0B28EFD
From the Japanese Shiro, white, partly for Shiramine, the type locality, but also because the
family name, Albanerpetontidae, derives from the original French locality of La
Grive-SaintAlban, with Alba/Alban (Latin) meaning white.
Shirerpeton isajii sp. nov.
Diagnosis. As for type and only species
Shirerpeton isajii sp. nov.
Holotype. Shiramine Board of Education Ishikawa Prefecture, SBEI 2459, a small block
bearing most of a disarticulated but associated skull with some postcranial elements (Fig 2A).
The specimen is housed in the Shiramine Institute of Paleontology, Hakusan Board of
Education, Hakusan City, Ishikawa Prefecture, Japan.
Etymology. Species name honours Dr Shinji Isaji, Chiba Prefecture Museum, Japan, for
his longstanding work on the fossils, geology, and palaeoenvironment of the Kuwajima
Locality and horizon. "Kaseki-Kabe" (fossil cliff), Kuwajima, Hakusan City (formerly
Shiramine Village), Ishikawa Prefecture, Honshu, Japan. Lower Cretaceous, (Barremian),
Kuwajima Formation, Tetori Group. Both the holotype and the referred specimen came from
Facies III, the more terrestrial of the three facies at the type locality.
Differential diagnosis. A genus of albanerpetontid that resembles Albanerpeton spp. and
Wesserpeton evansae , and differs from Anoualerpeton spp.  and Celtedens ibericus ,
in having a frontal that is triangular rather than bell-shaped; differs from all but Albanerpeton
arthridion , in that the frontal body is short but anteriorly quite wide, but differs from A.
arthridion in having a longer, more pointed internasal process; resembles Wesserpeton and A.
arthridion in its very small body size, but the frontal differs from that of Wesserpeton in the
more tapered internasal process, the anterior contact between the ventrolateral crests, and the
fact that the posterior margins of the prefrontal facets lie posterior to the mid-length of the
bone; differs from Wesserpeton, Albanerpeton inexpectatum , Celtedens ibericus , and
Anoualerpeton priscum , in having parietals with proportionally longer postorbital wings,
the dorsal surfaces of which remain completely unsculptured; further differs from A.
inexpectatum in lacking fusion of the prefrontal and lacrimal, having a bifurcate occipital shelf of each
parietal, and in having a nasal that enters the narial margin. In the latter feature it resembles
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A. pannonicum , but differs in that the nasal makes a larger contribution to the narial
margin; differs from both Neogene taxa in the lack of fusion of the basicranial and otic elements,
the discrete supraoccipital, and the long anterodorsal process of the latter bone. The dentary
and maxilla resemble Anoualerpeton spp. and differ from Wesserpeton, Celtedens, unattributed
material from the Cretaceous of Uzbekistan (`Nukusurus'), and Albanerpeton (except A.
]) in showing size heterodonty with large anterior teeth supported by a convex profile of
the labial alveolar margin. However, the maxilla of Shirerpeton is distinguished from that of
Anoualerpeton spp and A. nexuosum by the combination of a pointed (rather than rounded)
anterior premaxillary process and a weakly concave anterior narial margin. The dentary of
Shirerpeton is distinguished from that of Anoualerpeton spp and A. nexuosum by the greater
sinuosity of the labial dental margin (with a concave-convex-concave profile), the positioning
of the prearticular facet behind the tooth row (rather than extending forward beneath it), and
the shallow posterior inclination of the subdental shelf such that the posterior teeth are not
markedly smaller than those in the symphyseal region. Note that direct comparison with the
type species of Celtedens, C. megacephalus , is not possible as the holotype specimen
(Instituto Geologico dell'UniversitaÂ de Napoli, Italy, IGUN M542: the anterior part of a skeleton)
does not show comparable features.
Referred specimens. SBEI 2405, an almost complete right dentary (Fig 4A±4D); and SBEI
2462 (Fig 4E), a second right dentary, both from the type locality.
The holotype of Shirerpeton isajii, SBEI 2459 (Fig 2), is a small block of grey mudstone bearing
three-dimensionally preserved but partly disarticulated skull and postcranial elements. From
the anatomical relationships of the bones, lack of duplication, and equivalent size, the remains
clearly pertain to a single individual. The reconstructed midline skull length was 8-10mm,
with an estimated snout-pelvis length (SPL) of roughly 45 mm (based Gardner , with SPL
approximately 10x frontal length). As exposed on the surface of the block (Fig 2A), the most
distinctive elements are a fused frontal bearing the raised polygonal sculpture typical of
albanerpetontids, and paired sculptured parietals. Despite the small size of the bones, the sculpture
is of strong relief. Displaced to the left of the frontal are several other bones including the left
and right lacrimal, the left prefrontal, and the left maxilla preserved in partial dorsal view. The
right prefrontal, crosses the anterior end of the frontal. Anterior to the prefrontal is the left
nasal, inverted but complete. Posterior to the frontal are the left and right parietals. An
anteriorly directed, rod-like element adjacent to the edge of the right parietal is interpreted as the
displaced squamosal of that side. There are further partially exposed elements posterior to the
parietals. These were difficult to interpret in surface view but were subsequently found to be
vertebrae and parts of the braincase. The surface view has been supplemented by the μCT data
which has revealed important details of the exposed bones as well as those of elements that are
fully or partially embedded within the matrix (Figs 5±7). These additional bones include the
right maxilla, both septomaxillae, quadrates, epipterygoids, and jugals, as well as many parts of
the endocranium, several vertebrae, a few limb elements, and several additional elements,
some of which have a distinctive structure but have yet to be identified. Unfortunately there is
no trace of the premaxillae on the holotype block, nor the lower jaws, and these parts of the
block were probably removed inadvertently during trimming of a larger block.
Skull. There are few published accounts of the albanerpetontid nasal [3,8], leading to a
degree of uncertainty about its presence, size, and relations in most taxa. Venczel and Gardner
 described the first three-dimensionally preserved albanerpetontid nasal in the Pliocene
Albanerpeton pannonicum, revealing that it was larger than previously estimated .
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Fig 4. Shirerpeton isajii gen. et sp. nov., referred right dentaries. A-D, SBEI 2405, in A, labial view as
preserved, digital photograph; B-C, specimen as segmented from μCT slice data in B, labial, and C, lingual
views; D, lingual view, interpretative drawing. E, SBEI 2462, labial view as preserved, digital photograph.
Abbreviations:? an, possible angular facet; mk.f, Meckelian fossa; pra, facet for prearticular; sds, subdental
shelf; sym, symphyseal prongs. Scale bars = 1 mm.
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Fig 5. Shirerpeton isajii gen. et sp. nov., SBEI 2459, holotype block in dorsal view showing elements
segmented from μCT slice data. Abbreviations: Ept, epipterygoid; Ex.O, exoccipital plate; Fr, frontal; L.J,
left jugal; L.La, left lacrimal; Lm.B, limb element; L.Mx, left maxilla; L.N, left nasal; L.Pa, left parietal; LPf, left
prefrontal; L.Q, left quadrate; L.Sm, left septomaxilla; L.Sq, left squamosal;? Opis, possible opisthotic;? Ot,
otic associated element;? Pal, possible palatal element;? Pro, possible prootic; R.J, right jugal; R.La, right
lacrimal; R.Mx, right maxilla; R.Pa, right parietal; R.Pf, right prefrontal; R.Q, right quadrate; R.Sq, right
squamosal; Sph, sphenoid;? St, possible supratemporal;?, unidentified elements. Scale bar = 1 mm. Note
that the vertebrae are not labelled in this figure but are described and figured later in the text.
Mechanical preparation of SBEI 2459 exposed a single complete nasal in ventral view, and
further details have been revealed by the μCT data (Fig 8). Based on its morphology and fit with
the frontal, we interpret the bone as a left nasal. It is roughly rhomboidal. The posterior tip is
tapered whereas the anterior tip is blunt and thickened. The facetted posteromedial edge fitted
against the internasal process of the frontal. The complementary edges (of nasal and frontal)
are of similar length, suggesting that the nasals met only, at most, for a short distance at their
anteromedial angle. The posterolateral edge of the nasal bears a narrower surface that
contacted the medial edges of the prefrontal and the lacrimal. Our reconstruction demonstrates
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Fig 6. Shirerpeton isajii gen. et sp. nov., SBEI 2459, holotype block in ventral view showing elements
segmented from μCT slice data. Abbreviations: Ex.O, exoccipital plate; Fr, frontal; L.J, left jugal; L.La, left
lacrimal; Lm.B, limb element; L.Mx, left maxilla; L.N, left nasal; L.Pa, left parietal; L.Pf, left prefrontal; L.Q, left
quadrate; L.Sm, left septomaxilla; L.Sq, left squamosal;? St, possible left supratemporal;? Opis, possible
opisthotic;? Ot, otic associated element;? Pal, possible palatal element;? Pro, possible prootic; R.J, right
jugal; R.La, right lacrimal; R.Mx, right maxilla; R.Pa, right parietal; R.Q, right quadrate; R.Sq, right squamosal;
S.O, supraoccipital;?Sk1, unidentified skull element; Sph, sphenoid;? St, possible supratemporal;?,
unidentified elements. Scale bar = 1 mm. Note that the vertebrae are not labelled in this figure but are
described and figured later in the text.
that the nasal-lacrimal contact excluded the prefrontal from the narial margin. Of the two
anterior nasal borders, the thin unfacetted anterolateral edge clearly entered the narial
opening. The anteromedial edge is thicker, inflected ventrally, and slightly buttressed. It supports a
dorsal facet for the nasal process of the premaxilla (Pm.ft). The thickened edge is squared-off
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Fig 7. Shirerpeton isajii gen. et sp. nov., SBEI 2459, holotype block. Lateral view of skull elements as
segmented from μCT slice data. Abbreviations: Ex.O, exoccipital plate; Fr, frontal; L.La, left lacrimal; Lm.B,
limb element; L.Mx, left maxilla; L.N, left nasal; L.Pa, left parietal; LPf, left prefrontal; Q, quadrates; L.Sm, left
septomaxilla;? Opis, possible opisthotic;? Ot, otic associated element;? Pal, possible palatine;? Pro, possible
prootic; R.La, right lacrimal; R.Pa, right parietal; R.Pf, right prefrontal; R.Sq, right squamosal; S.O,
supraoccipital; Sph+ Ept, sphenoid and epipterygoid;? St, possible supratemporal;?, unidentified elements.
Scale bar = 1 mm. Note that the vertebrae are not labelled in this figure but are described and figured later in
and bears several rugosities. In other albanerpetontid taxa, the naso-premaxillary contact is
usually described as being an abutment and/or a variably developed suture in which the
premaxilla overlaps the anterior margin of the nasal . The Japanese specimen appears to
combine these features, with the premaxilla overlapping the nasal, and the tip of the nasal abutting
against a ridge or tuberosity on the underside of the premaxillary nasal process. However, a
second anteroventral facet on the Japanese nasal suggests the articulation may have been more
complex, perhaps with a pocket facet on the premaxilla (?Pm.ft). Without the premaxilla,
however, we cannot speculate further.
The μCT scans revealed the presence of a very small element lying adjacent to the
anteromedial edge of the nasal. This has a smooth, rounded, hemispherical external surface and an
internal surface divided between a deep concavity and a rugose portion that tapers to a point.
It appears to be a single complete element rather than a broken part of a larger bone (Fig 9).
From its position in relation to the nasal, and its convex-concave shape, a septomaxilla is the
most plausible identification. This element, associated with the nasal capsule, is present in at
least some representatives of both extinct and extant amphibian lineages [68, 69], and therefore
its presence in albanerpetontids would not be unexpected.
The frontal is well preserved. In surface view the internasal process is obscured by the
overlying right prefrontal (Figs 2 and 5), but this has been digitally removed. As reconstructed
from μCT data (Fig 10), the frontal has a midline length of c. 4.5 mm and a posterior width of
c. 3.7 mm, giving a midline length to posterior width proportion of 1.21. The internasal process
is relatively long and tapering (length = 1.26x basal width), with the edge recessed in its
posterior half by the nasal facet. Further anteriorly, the naso-frontal articulation appears to have
been more of an abutment but allowance must be made for possible artefacts introduced
during the segmentation of these very small bones. Small anterolateral processes separate the nasal
facets from the prefrontal facets. However, these processes do not appear to have reached the
level of the dorsal surface, so that the prefrontal and nasal would have been in contact in a
dorsal view of the skull. The anterior end of the orbital margin (demarcated by the posterior end
of the slot for reception of the prefrontal) lies posterior to the mid-point of the frontal long
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Fig 8. Shirerpeton isajii gen. et sp. nov., SBEI 2459, nasal. Left nasal as segmented from μCT slice data in A, dorsal; B, ventral; C, medial;
D, lateral; and E, anterior views. Abbreviations: Fr.ft, frontal facet; La.ft, lacrimal facet; N.bd, narial border; Pf.ft, prefrontal facet; Pm.ft,
anterodorsal facet for nasal process of premaxilla;? Pm.ft, possible anteroventral facet for the premaxilla. Scale bar = 1 mm.
axis. The lateral edges of the bone are only slightly concave, but the dorsal surface of the bone
(Fig 10A) is slightly narrower than the ventral one (Fig 10B), so that a narrow gutter runs
along the edge from the posterior margin of the prefrontal facet to the posterolateral tip of
the frontal (Fig 10C and 10D). Here there is a small dorsal slot facet for the parietal (more
completely preserved on the left than the right). The posterior margin of the frontal (Fig 10A,
10B and 10E) is weakly W-shaped, with shallow bilateral emarginations flanking a short
The ventral surface of the frontal has been rendered digitally (Fig 10B). The surfaces of the
ventrolateral crests are weakly concave. The crests are widest at the posterior margin of the
prefrontal facet and then narrow slightly toward the frontoparietal suture. In the posterior
two-thirds of the bone, the crests are separated by a deep concavity, but this is closed off
further anteriorly where the ventrolateral crests are joined in the midline. There is a weak
ventromedian crest. Seen in lateral view, the prefrontal facets are deep and extend along the full
length of the lateral surface of the anterolateral processes. As noted above, the nasal facets are
shallower. At the posterodorsal corner of the frontal (preserved more completely on the left
than the right, Fig 10A), the frontal bears a small facet for the parietal. This is continuous with
a larger ventral facet that extends medial to the ventrolateral crest. The posterolateral corner of
the frontal thus slots into a recess in the anterior face of the parietal.
Albanerpetontid parietals have rarely been described and, to date, the only nearly complete
bones that have been figured are those of the Miocene Albanerpeton inexpectatum  and the
Early Cretaceous Celtedens ibericus [6, 8]. The parietals are well preserved in the Tetori
specimen. The left bone is roughly in situ relative to the frontal (Figs 5, 6 and 11), but the right has
been rotated clockwise so that its anterior margin faces laterally (Fig 2). Most of the important
details of the dorsal surface can be seen on the original block (Fig 11), but the description is
supplemented by images from μCT scans which also allow visualisation of the ventral surface
(Figs 12 and 13).
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Fig 9. Shirerpeton isajii gen. et sp. nov., SBEI 2459, septomaxilla. Left septomaxilla as segmented
from μCT slice data in A, dorsal; B, ventral; C, posterior; D, anterior; E, lateral; and F, medial views. Scale
bar = 0.5 mm.
Each parietal is roughly triangular. The lateral margin of the parietal is curved and extends
from the lateral tip of the postorbital process to the tip of the occipital shelf. The anterior
margin is smooth along the tapering postorbital process, then irregular where it bears the frontal
facet. The μCT scan images reveal a deep pocket (Fr.ft) that received the posterolateral tip of
the frontal (Fig 12C). Anteroventral and anterolateral flanges match the facets on the
corresponding surfaces of the frontal, contributing to a firm articulation. The central part of the
frontoparietal suture and the straight interparietal suture appear weaker with only small
interdigitations. However, the medial edge of the occipital shelf bears an incised slot facet for the
anterior ramus of the supraoccipital (Figs 12B, 12C and 13B±13D).
The dorsal surface of the parietal (Figs 12A and 13A) is subdivided into three areas: a
sculptured triangular anteromedial surface that forms the posterior part of the skull table; an
unsculptured lateral postorbital wing; and an unsculptured occipital shelf to which neck
muscles presumably attached. On the right parietal the shelf is extensive, as in A. inexpectatum, but
it differs in being bifurcated. This does not appear to be an artefact of breakage (although a
vertebral transverse process projects between the two rami) or crushing, as the edges bordering
the emargination appear intact, and the completely preserved posteromedial processes on both
parietals match one another in shape and size. On the left parietal the more lateral of the
posterior processes has broken off at its base, as is clear from a comparison of the ventral surfaces of
the right and left bones. On both elements, a sharp anteromedial to posterolateral transverse
crest is seen to divide the main, medial, part of the bone from the recessed postorbital wing
(Figs 12B and 13B). A second anteroposterior crest runs smoothly from the midpoint of the
transverse crest to the tip of the posterolateral process. On the left parietal, this second crest is
truncated posteriorly, marking where the posterolateral process has broken away. On the right
parietal, the area lateral to the second crest is seen to bear a lateral recess, with a more oblique
anterolateral recess towards its anterior end. Estes and Hoffstetter's image of the parietal in
Albanerpeton inexpectatum ( plate 8) shows a similar lateral recess, which the authors
interpreted as accommodating the dorsal surface of the otic capsule. However, the larger lateral
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Fig 10. Shirerpeton isajii gen. et sp. nov., SBEI 2459, frontal. Frontal as segmented from μCT slice data in A,
dorsal; B, ventral; C, right lateral; D, left lateral; and E, posterior views. Abbreviations: alp, anterolateral process; inp,
internasal process; N.ft, nasal facet; Pa.ft, parietal facet; Pf.ft, prefrontal facet; vlc, ventrolateral crest; vmc,
ventromedian crest. Scale bar = 1 mm.
recess in Shirerpeton more closely resembles a facet for a dermal element, possibly a
supratemporal (see below), whereas the more anterior recess matches the size and position of the dorsal
end of an element that we interpret as the epipterygoid (see below). The strongly concave
anteroventral surface of the postorbital wing presumably provided an area of origin for jaw
adductor muscles (possibly the deep part of the internal adductor).
As reconstructed, therefore, the occipital shelves appear to have been deeply emarginated in
the Japanese taxon. With the supraoccipital in articulation between the posteromedial parietal
processes, this creates large ovoid openings in the skull roof on each side of the midline, medial
to the otic capsules (Fig 3E). If genuine (i.e. not an artefact), these openings could have been
covered by a thick sheet of fascia or an additional dermal element such as a postparietal.
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Fig 11. Shirerpeton isajii gen. et sp. nov, SBEI 2459, digital photograph showing details of skull
roofing elements. A, frontal and left parietal; B, right parietal. Abbreviations: Fr, frontal; Fr,ft, frontal facet; L.
La, left lacrimal; L.Mx, left maxilla; L.Pa, left parietal; L.Pf, left prefrontal; plp, posterolateral process; pmp,
posteromedial process; PoW, postorbital wing; R.Pf, right prefrontal;? St, possible supratemporal; V,
vertebra. Scale bars = 1 mm.
Both maxillae are preserved, with the left roughly in situ and the right lying under the right
parietal (Figs 5 and 6). The maxilla has the shape of a shallow scalene triangle (Fig 14A and
14B), with a tapering premaxillary process, a low rounded facial process, and a long tapering
jugal process (Fig 14A±14F and 14H). The anterior, narial, margin of the facial process is
almost straight (rather than strongly concave as in some other taxa). Allowing for artefacts in
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Fig 12. Shirerpeton isajii gen. et sp. nov., SBEI 2459, right parietal. Right parietal as segmented
from μCT slice data in A, dorsal; B, ventral; C, oblique anteroventral; D, anterior views. Abbreviations: al.re,
anterolateral recess; ap.c, anteroposterior crest; Fr.ft, frontal facet; l.re, lateral recess; Occ.s, occipital shelf;
plp, posterolateral process; pmp, posteromedial process; PoW, postorbital wing; SO.ft, supraoccipital facet;
tr.c, transverse crest; Scale bar = 1 mm.
segmentation and some damage, the labial surface of the maxilla appears unsculptured, but at
least two neurovascular foramina open on this surface. The premaxillary process is bifurcated
(Fig 14C±14E and 14G). The medial ramus bears a ventral flange that is almost vertical in
orientation on the left bone (Fig 14G), but ventromedial on the right. It appears to bear a facet,
either for the premaxilla or the vomer. The posterodorsal edge of the facial process is recessed
both labially and lingually where it was straddled by the lacrimal bone (Fig 14B±14E and 14H).
Further posteriorly, the dorsomedial surface bears a slightly flattened surface that supported
the jugal. Lacrimal and jugal facets meet, indicating that the maxilla was excluded from the
ventral orbital margin, and this is confirmed when the bones are rearticulated (Fig 3A±3D). A
medial view of the right maxilla (Fig 14H) also shows a distinct embayment in the lingual edge
of the maxillary shelf posterior to the medial premaxillary process. We interpret this as the
lateral margin of the choana. It is less obvious on the left due to damage. Posterior to the
embayment, the edge of the shelf is straighter and bears a facet for a palatal element. A large medial
foramen perforates the posterior base of the right facial process and probably carried
neurovascular structures into the bone. There is a second, more posterior, excavation but we cannot
determine whether this also opened as a foramen. Anteriorly, the maxillary tooth row reaches
the bifurcation of the premaxillary process, a point anterior to the narial margin of the facial
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Fig 13. Shirerpeton isajii gen. et sp. nov., SBEI 2459, left parietal. Left parietal as segmented from μCT
slice data in A, dorsal; B, ventral; C, anteroventral; D, ventromedial; and E, anterior views. Abbreviations: al.
re, anterolateral recess; ap.c, anteroposterior crest; br.ed, broken edge; Fr.ft, frontal facet; occ.s, occipital
shelf; pmp, posteromedial process; PoW, postorbital wing; So.ft, supraoccipital facet; tr.c, transverse crest.
Scale bar = 1 mm.
process. Allowing for empty positions, the tooth row seems to accommodate 18±22 teeth,
those below the facial process being longer than those at the anterior and posterior ends of the
tooth row. This heterodonty is also reflected by the downward curvature of the ventrolateral
edge of the maxilla which is deepest level with the apex of the facial process (Fig 14A and 14B).
At its anterior end, the right maxilla is in contact with a fragment of another bone. This could
be part of a right septomaxilla or premaxilla (Fig 14H).
The left prefrontal lies lateral to the frontal (Figs 2 and 5±7). It is a slender bone with narrow
medial shelf facets for the nasal and frontal, and a lateral, or ventrolateral, lacrimal facet visible
on its exposed margin (Fig 15). The medial and lateral facets converge anteriorly and, at the tip
of the bone, are separated only by a low ridge. Aligning this element to the edge of the frontal
indicates that the posterior part of the medial facet met the frontal, but the more horizontal
anteromedial part was overlapped by the nasal. The nasal and lacrimal met anterior to the
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Fig 14. Shirerpeton isajii gen. et sp. nov., SBEI 2459, maxilla. A-G, left maxilla as segmented from μCT
slice data, in A, lingual; B, labial; C, dorsal; D, ventral; E, oblique anterior; F, oblique posterior; G, anterior
views. H, right maxilla in lingual view. Abbreviations: a.ft, anterior facet; fa.p, facial process; J.ft, jugal facet; J.
p, jugal process; La.ft, lacrimal facet; m.e, medial emargination; Mx.s, maxillary shelf; n.m, narial margin; p.ft,
posterior facet; Pm.p, premaxillary process;? Sm/Pm, bone fragment, part of premaxilla or septomaxilla; v.fl,
ventral flange; Scale bar = 1 mm.
prefrontal and excluded it from the narial margin (Fig 3E). The right prefrontal lies across the
anterior tip of the frontal and has the same morphology as the left.
The dorsal edge of the left lacrimal, bearing prefrontal and nasal facets, is exposed lateral to
the maxilla and left prefrontal. The remainder of the bone is buried almost vertically in matrix
(Figs 5±7). The right lacrimal is fully exposed in lateral view on the matrix block (Fig 2). Both
bones are biradiate with tapering ventral and anterodorsal edges processes that lie almost at
right angles to one another. Between them, the long, curved, posterior border framed the
orbit, whereas the shorter, almost vertical, anterior edge contributed to the narial border. The
extracted left lacrimal (Fig 16) resembles the right but reveals rather more detail. Just anterior
to the orbital margin, the ventrolateral surface is perforated by two lacrimal duct foramina (Fig
16B). These run anteromedially into a short canal that opens on to the medial surface through
a single foramen (Fig 16A and 16C). The lateral foramina are visible only as indentations on
the exposed right bone, but they lie in the same position as those on the lateral surface of the
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Fig 15. Shirerpeton isajii gen. et sp. nov., SBEI 2459, prefrontal. Left prefrontal as segmented from μCT slice data in A,
dorsal; B, ventral; C, lateral; and D, medial views. Abbreviations: Fr.ft, frontal facet; La.ft, lacrimal facet; N.ft, nasal facet. Scale
bar = 1 mm.
Fig 16. Shirerpeton isajii gen. et sp. nov., SBEI 2459, lacrimal. Left lacrimal as segmented from μCT slice data in A, medial; B, lateral; C,
anterodorsomedial; D, anterior; E, posterior; F, dorsal; and G, ventral views. Abbreviations: J.ft, jugal facet; La.f, lacrimal duct foramen; l.lm, lateral
lamina; m.lm, medial lamina; Mx.ft, maxillary facet; N.bd, narial border; N.ft, nasal facet; Or.bd, orbital border; Pf.ft, prefrontal facet; Scale bar = 1 mm.
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The medial surface of the lacrimal is exposed most clearly on the left bone, with dorsal and
ventral facets separated by a short body (Fig 16A and 16C). The body is divided into a thin,
medially concave anterior part that walled part of the nasal chamber (Fig 16D) and a thickened
posterior orbital margin (Fig 16E). The single lacrimal duct foramen opens into the posterior
edge of the nasal concavity. The dorsomedial border of the bone bears a large tapering facet for
the prefrontal. Anterior to this facet, the smooth bone surface changes slightly in its orientation
and may represent the point at which the lacrimal contacted the nasal (Fig 16A, 16C and 16F).
The ventral margin of the bone bears a deep groove divided into anterior and posterior
articular surfaces for the maxilla and jugal respectively. The ventral groove is flanked by medial and
lateral laminae (Fig 16G). However, whereas the medial lamina is deepest anteriorly, the lateral
lamina is deeper posteriorly. This arrangement ensures that the lacrimal, maxilla, and jugal are
locked. The ventral groove accommodates the nasal process of the maxilla anteriorly, leaving
this process exposed in lateral view but braced medially by the medial lacrimal lamina, and the
jugal posteriorly, with the lateral lacrimal lamina bracing the jugo-maxillary contact. Overall,
the lacrimal of the Japanese albanerpetontid is broadly similar to that figured for both
Albanerpeton inexpectatum  and A. pannonicum , although the detailed shape (lateral recess,
dorsal and ventral process dimensions) is different in each taxon. The reported fusion of the
prefrontal to the lacrimal in A. inexpectatum [3, 7] obscures the dorsal relationship between
Unlike extant lissamphibians, albanerpetontids are known to have retained a jugal [6, 8,16]
but dorsal reconstructions of the skull show it as a sliver of bone running between the maxilla
and suspensorium. McGowan  reconstructed the jugal as overlapping the lateral surface of
the squamosal, but this has not been confirmed from three-dimensional material of the
squamosal. Venczel and Gardner  described several partial jugals of Albanerpeton pannonicum
in association with the maxilla and lacrimal. Their most complete specimen is shown as
extending only a short distance beyond the end of the maxillary tooth row, with its slightly
expanded posterior edge bearing both medial and lateral facets. This would require that the tip
slotted into a recess on the squamosal or was wedged between the squamosal and quadrate.
However, in the Tetori specimen, the scans have revealed complete left and right jugals, the
former lying under the frontal and the latter lying within the matrix in association with the
right maxilla (Figs 2, 5, 6 and 17). The bone has the form of a curved bar, divided into a shallow
anterior section and a vertical posterior blade. The anterior portion is tapered at its tip and
consists of narrow lateral and medial flanges that straddled the dorsal surface of the maxilla.
Dorsally, the anterior end bears a short facet for the lacrimal which overlapped both maxilla
and jugal at this point. The posterior half of the bone is deeper in its mid section, but also
tapers to a point posterodorsally, as seen on the left bone. Precisely how the jugal articulated
with the suspensorium, if at all, is uncertain, as we could find no evidence of an articular
surface on the posteroventral tip of the bone (as recorded for A. pannonicum ). From the
reconstruction, it appears likely that the posterodorsal tip of the jugal approached the lateral
tip of the parietal postorbital wing. However, there are no obvious facets on the postorbital
wing or the tip of the jugal, and the latter may have been connected to the skull roof by a
A rod-like element lying to the right of the frontal and right parietal (Figs 5±7) is
interpreted as the right squamosal. It broadly resembles the squamosal preserved in the Las Hoyas
Celtedens ibericus skull MCCM-LH15710 (Fig 18). As exposed on the Tetori block, the bone
has the appearance of a slightly tapering blade but the μCT scans reveal a somewhat
compressed cylinder, open along its medial side, with the posteroventral lamina slightly narrower
than the anterodorsal one (Fig 19). The lateral margin is narrower than the open medial one
and appears rugose but this edge is exposed on the block and the surface may be damaged. The
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Fig 34. Shirerpeton isajii gen. et sp. nov., SBEI 2459, rib-like elements. Rib-like elements adjacent to vertebrae as
segmented from μCT slice data in five views (original orientation uncertain). The dark grey elements are probably ribs
or hyoid elements. The pale grey element is unidentified. Scale bar = 0.5 mm.
16±17, 20, 32±34] has led to a detailed understanding of many aspects of albanerpetontid
morphology and a relatively stable phylogenetic framework for known taxa.
Recently used data matrices [16, 34] coded 31 characters. Unfortunately, a majority of these
(58%) relate to the premaxilla which is commonly preserved in microvertebrate deposits but is
not preserved in the Japanese specimen. From the structure of the anterior margin of the nasal
we infer that the nasal abutted the premaxilla medially but overlapped part of the premaxilla
Fig 35. Shirerpeton isajii gen. et sp. nov., SBEI 2459, limb element. Limb element as segmented
from μCT slice data in six views (original orientation uncertain). Scale bar = 1 mm.
37 / 58
laterally. This does not match any of the states described  for character 4. Given that we
lack the premaxilla of the Japanese albanerpetontid, we have coded this as unknown rather
than create an additional autapomorphic state. Of the remaining, non-premaxillary,
characters, we were able to score all but one for Shirerpeton. Shirerpeton has a long premaxillary
process of the maxilla (ch.15:0) (Fig 36C), lacks a dorsally projecting process on the dentary
behind the tooth row (ch.16:0), and has the maxillary tooth row extending anterior to the level
of the facial process (ch.20:0), all regarded as primitive states . We were unable to score the
presence or absence of labial sculpture on the maxilla with confidence as the irregularities on
the surface of the maxilla may be artefacts of damage or reconstruction (ch.17:?), but sculpture
seems to be absent on the dentary. However, Shirerpeton shares the presence of a convexity of
the maxillary and dentary alveolar margin (ch.18:1) with Anoualerpeton and Albanerpeton
nexuosum, although it differs from both in having the convexity preceded by a concavity that
renders the dentary very shallow anteriorly. It also resembles Anoualerpeton spp. and
Albanerpeton nexuosum in having larger teeth anteriorly (ch.19:1). With an estimated snout-vent
length of around 45 mm, Shirerpeton also shows the derived condition (ch.25:1), as do A.
arthridion and Wesserpeton. On frontal characters (Fig 37E), Shirerpeton shows the derived
state for frontal shape (ch.21:1), having a frontal that is approximately triangular rather than
bell-shaped; a proportion of frontal length to posterior width that lies at the borderline
between long (ch.22:0) and moderate (ch.22:1); a long derived internasal process (ch.23:1); a
derived condition of the ventrolateral crests (ch.24:1); a derived condition for the anterior
limit of the orbital margin (ch.28:1), located posterior to the midpoint of the anteroposterior
long axis; a derived tapered form for the internasal process (ch.29:1); and a weak median keel
along the ventral surface (ch.31:1). However, character 29 in the matrix of Sweetman and
Gardner  stems from the original description of Celtedens ibericus as having a `bulbous'
nasal process [8, 74], a description based on interpretation of the articulated, but split, holotype
specimen. An attributed specimen (MCCM-LH-15710) from the type locality, Las Hoyas, has
the anterior end of the frontal disarticulated and clearly preserved (Fig 37B). It shows the nasal
process to be a rounded taper rather than bulbous, and there is also greater separation between
nasal and prefrontal facets than originally proposed. This raises doubts as to the original
Given that the most recent data matrices for albanerpetontids are heavily dependent on
premaxillary characters [16, 34], it is important to begin to identify additional features that allow a
more comprehensive characterisation of the skeleton as a whole. As a start, we have added
three parietal characters. The first (ch.32) relates to the length of the postorbital wing
compared to the frontoparietal suture width. The second (ch.33) relates to the extent to which
sculpture does or does not cover the postorbital wing. The third (ch.34) relates to whether the
occipital shelf is single or bifurcate.
We ran a parsimony analysis, using the character matrix of Sweetman and Gardner 
with our additional characters (S1 File). The first analysis was run with the Branch and Bound
option of PAUP (for direct comparison with the analysis of Sweetman and Gardner ) (see
S1±S3 Figs). A second analysis used TNT (version 1.5, ), New Technology search option
with Ratchet (20 iterations) and 1000 Random addition sequences, followed by a scrutiny of
the resulting trees using Traditional search. In each case, we ran the analysis (a) with and
without the all-0 hypothetical ancestor of Sweetman and Gardner , rooting either with a basal
node (PAUP) or with Anoualerpeton priscum (TNT), and also (b) with and without character
29 as the derived state is an uncertain autapomorphy of Celtedens. Despite the differences
between analyses, and the programme used (PAUP, TNT), the resulting tree topologies were
closely similar. Fig 38 shows the strict consensus of 19 trees from a TNT analysis of the full
matrix (34 characters) and with the hypothetical ancestor as the outgroup (L = 51), but the tree
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Fig 36. Lateral profiles of maxillae in albanerpetontids (not to scale). A, Anoualerpeton priscum (Middle Jurassic, UK); B,
An. unicum (Early Cretaceous, Morocco); C, Shirerpeton isajii (Early Cretaceous, Japan); D, Wesserpeton evansae (Early
Cretaceous, UK); E, Albanerpeton arthridion (Early Cretaceous, North America); F, Albanerpeton gracile (Late Cretaceous,
North America); G, Albanerpeton nexuosum (Late Cretaceous, North America); H, Albanerpeton galaktion (Late Cretaceous,
North America); I, Albanerpeton inexpectatum (Miocene, France); J, Albanerpeton pannonicum (Pliocene, Hungary). Outlines
redrawn or reconstructed from: A-B ; C, original, SBEI 2459; D ; E ; F-H ; I ; J . Images C-H have been
"mirror-imaged" for ease of comparison.
topology was the same with character 29 deleted, and with the hypothetical ancestor replaced
with Anoualerpeton. Shirerpeton is placed in an unresolved polytomy with Albanerpeton
species. The same analyses run with PAUP (Branch and Bound), yielded 53 trees (L = 57) of
which the Strict Consensus was identical to that in Fig 38. Examination of the individual trees
39 / 58
Fig 37. Comparison of frontal shape in albanerpetontids, scaled so that posterior widths are roughly equal. A,
Anoualerpeton unicum (Early Cretaceous, Morocco); B, Celtedens ibericus (Early Cretaceous, Spain), based on ; C, cf.
Celtedens ibericus (Early Cretaceous, Spain); D, Wesserpeton evansae (Early Cretaceous, UK); E, Shirerpeton isajii (Early
Cretaceous, Japan); F, Albanerpeton arthridion (Early Cretaceous, North America); G, Albanerpeton gracile (Late Cretaceous,
North America); H, Albanerpeton pannonicum (Pliocene, Hungary); I, Albanerpeton inexpectatum (Miocene, France). Figures
redrawn from: A ; B ; C, original, MCCM-LH-15710; D ; E, original, SBEI 2459; F ; G ; H ; I .
40 / 58
Fig 38. Strict consensus of 19 MPT's from an analysis of the full matrix (NT search in TNT with
Ratchet followed by a Traditional search of the resulting trees in RAM). Of the 19 MPT's, 11% placed
Shirerpeton as the sister taxon to a monophyletic Albanerpeton; 42% placed it as the sister taxon to A.
arthridion; and 47% placed it crownward of A. arthridion (see Fig 39). An analysis with the limited matrix and
without Hypothetical Ancestor yields trees with the same topology for the in-group taxa.
from both analyses identified three alternative positions for Shirerpeton (Fig 39): i) as sister
group to Albanerpeton as a whole; ii) as sister taxon to A. arthridion; or iii) in variable positions
crownward of A. arthridion. The first of these arrangements was the rarest (15%), with the
remaining trees showing the second or third topology in roughly equal numbers. However, a
Bootstrap analysis (2000 replicates) using TNT (NT + Ratchet [20 iterations]) yielded a more
resolved tree (Fig 40) resembling that of Sweetman and Gardner  in recovering a
`robustsnouted clade' comprising A. nexuosus, A. pannonicum, A. inexpectatum, and the Paskapoo
taxon, and a `gracile-snouted clade' comprising A. gracile, A. galaktion, and A. cifellii, with A.
arthridion as the sister taxon to both clades, and Wesserpeton, the Uña taxon, Celtedens, and
Anoualerpeton as successive outgroups. Shirerpeton is nested within Albanerpeton as the sister
taxon of the `robust-snouted clade', but Bootstrap support values for all nodes are very low and
only the `robust-snouted clade' survived a decay analysis beyond one step. Moreover, when
synapomorphies are followed on the tree, none unequivocally supports the placement of
Shirerpeton as the sister taxon of the `robust-snouted clade' and only two frontal characters (ch.22,
28) support the placement of Shirerpeton crownward of Wesserpeton.
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Fig 39. The three configurations recovered within the individual trees of the different analyses
described in the text. In (C) the position of Shirerpeton is unstable with respect to individual Albanerpeton
sp. The `A. inexpectatum clade' consistently comprises A. inexpectatum, A. pannonicum and the Paskapoo
taxon, with the variable addition of A. nexuosum.
In summary, the results of the phylogenetic analyses consistently support the placement of
Shirerpeton crownward of Anoualerpeton and Celtedens, and provide more limited support for
a position crownward of the Uña taxon and Wesserpeton, in a closer relationship with species
currently referred to the genus Albanerpeton. This is discussed further below.
SBEI 2459 preserves the disarticulated, but associated, bones of a single small individual.
Although a few surface elements have suffered damage, individual bones are largely uncrushed
42 / 58
Fig 40. Bootstrap analysis (1000 replicates) using TNT (NT search + Ratchet [20 iterations]) of the full
matrix, with the hypothetical ancestor as outgroup. The same analysis run using the limited matrix
yielded the same topology, with minor differences in the Bootstrap values.
and the μCT scans have revealed significant new information on albanerpetontid cranial
morphology. The specimen preserves several elements that were previously either unknown or
incompletely known from other albanerpetontid taxa±notably the nasal, septomaxilla,
prefrontal, lacrimal, jugal, quadrate, squamosal, parietal, supraoccipital, sphenoid, and other braincase
components. Moreover, some of the unidentified elements suggest that the albanerpetontid
skull may have retained other elements subsequently lost in crown Lissamphibia (e.g. the
The albanerpetontid rostrum was first reconstructed by Estes & Hoffstetter  on the basis
of disarticulated, but well-preserved, bones of the Miocene species, A. inexpectatum. The nasals
were not preserved but, based on surrounding bones, the authors reconstructed them as small
oval elements lying between the frontal, premaxillae, and prefrontals, but excluded from the
narial margins. The prefrontal was inferred to be fused to the lacrimal, with some uncertainty
as to the limits of each bone. Gardner [72±73] broadly agreed with this interpretation.
However, on the basis of articulated Early Cretaceous material of Celtedens ibericus from Las
Hoyas, Spain, McGowan  concluded that small ovoid nasals `did not appear to exist', and
43 / 58
re-interpreted Estes and Hoffstetter's  fused prefrontals as elongate, slender nasals. In
McGowan's interpretation, the posterolateral facet on the frontal accommodated the lacrimal,
not a prefrontal, with the nasal articulating further anteriorly and entering the narial margin.
However, the Las Hoyas specimens are flattened and are usually split between part and
counterpart, complicating interpretation (SEE pers. obs.). The rostral morphology of Celtedens
ibericus is therefore uncertain. More recently, three-dimensionally preserved and partly
articulated material of A. pannonicum  clarified the relationships of the nasals, prefrontals,
lacrimals, and frontal to one another, at least in Neogene Albanerpeton. SBEI 2459 supports the
interpretations of Estes & Hoffstetter  and of Venczel & Gardner , rather than
McGowan . It demonstrates the presence of a discrete prefrontal that articulated with the frontal
medially and lacrimal laterally, and of a separate nasal. As in A. pannonicum, the nasals of
Shirerpeton were relatively large. Our reconstruction (Fig 3) suggests that the nasals were separated
for most of their length by the internasal process of the frontal, but entered the narial margins
and contacted the lacrimals to exclude the prefrontals from those margins. In shape (relative
angles of the dorsal and lateral processes, position and depth of the lateral groove), the lacrimal
of Shirerpeton more closely resembles that of A. inexpectatum than A. pannonicum, allowing
for the non-fusion of the prefrontal (Fig 41).
A small septomaxilla has also been revealed in Shirerpeton. This bone is variably present in
members of all extant lissamphibian clades  but has not previously been described in an
The quadrate of Albanerpeton inexpectatum was mentioned briefly by previous authors [
], and figured at small scale. In Shirerpeton, the facets suggest that the quadrate had a firm
articulation with the squamosal and pterygoid. SBEI 2459 has also confirmed the presence of
discrete jugals in albanerpetontids, as in Eocaecilia  but not extant lissamphibians.
However, unlike previous reconstructions  that present the jugal as a straight bar between the
maxilla and suspensorium, in Shirerpeton the jugal has a posterodorsal curvature and may
have had a ligamentous attachment with the skull roof. There are no obvious posteroventral
facets or processes to indicate how (or whether) the jugal contacted the suspensorium.
Although the albanerpetontid parietal is probably as taxonomically distinctive in its shape
and ornamentation as the frontal, it has only rarely been preserved and/or described (Fig 42).
Estes & Hoffstetter  provided images of the almost complete parietals of A. inexpectatum,
showing the bones to be paired, with an acuminate lateral postorbital wing, a sculptured
triangular central area, and unsculptured occipital shelves (Fig 42E). The parietals of Celtedens
ibericus [6,8] have a similar overall shape, although the details are less clearly preserved (Fig 42C).
For the remaining taxa, however, the parietal is either unknown or fragmentary. McGowan
 figured the anterior portion of a right parietal from the Middle Jurassic of Britain,
referable to Anoualerpeton priscum (Fig 42A), and Wiechmann  figured fragments of two left
bones of `Celtedens' guimarotae (not reproduced here as the figures are difficult to interpret).
Sweetman & Gardner  did not describe the parietal of Wesserpeton, but a partial (anterior)
left bone was recovered from the type deposit (Natural History Museum, London, NHMUK
PV R 36956, Fig 42B). The Tetori holotype specimen (Figs 3 and 42D) is therefore exceptional
in preserving both parietals almost in their entirety. They differ from the known parietals of
other taxa in three major respects: the postorbital wing is relatively longer; there is a more
limited lateral extension of the sculpture so that it does not encroach on the postorbital wing; and
the occipital shelf is shorter and bifurcated. This bifurcation is clearest on the right bone where
part of a vertebra extends between the posteromedial and posterolateral rami. The detailed
three-dimensional preservation of the parietals in Shirerpeton also demonstrates, for the first
time, how they articulated with the supraoccipital and epipterygoids.
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Fig 41. Skull roofing bones in albanerpetontids (not to scale). A, Shirerpeton isajii; B, Celtedens ibericus;
C, Albanerpeton inexpectatum; D, Albanerpeton pannonicum. B-D, redrawn from , but with B based on
Perhaps the greatest differences in morphology between Shirerpeton and the European
Neogene taxa are found in the braincase. Estes and Hoffstetter  figured stereopairs of an isolated
three-dimensional braincase of A. inexpectatum, but these images are too small to yield any
detail. Moreover, this specimen, formerly in the collections of the Museum National d'Histoire
Naturelle, Paris, is now apparently lost . Nonetheless, it is clear from the images and the
description that the component elements of the braincase were fully conjoined. A second
three-dimensional braincase, attributed to the Pliocene A. pannonicum, has also been
described . Again, the components are co-ossified, leaving the homologies of some regions
uncertain. Although Maddin et al.  reported the presence of an ossified tectum synoticum
in A. pannonicum, with an anterior projection, the lack of sutures left the authors unable to
determine whether this ossification represented a supraoccipital. In Shirerpeton, however, it is
clear that the supraoccipital is an independent ossification. Neither of the Neogene braincases
45 / 58
Fig 42. Parietal morphology in albanerpetontids (not to scale). A, Anoualerpeton priscum (Middle Jurassic, UK); B, c.f.
Wesserpeton evansae (Early Cretaceous, UK); C, Celtedens ibericus (Early Cretaceous, Spain); D, Shirerpeton isajii, Early
Cretaceous, Japan); E, Albanerpeton inexpectatum, Miocene, France). In each case, the small asterisk marks the lateral limit of the
frontal facet, lateral to which is the postorbital wing. Abbreviations: Occ.s, occipital shelf; PoW, postorbital wing. Figures redrawn
from: A  (mirror-imaged for comparison); B, original, NHMUK PV R 36956; C ; D, original, SBEI 2459; E .
has an anteromedian process as long as that of Shirerpeton, but A. pannonicum preserves a
short process that may be incomplete. No living lissamphibian has a discrete supraoccipital
ossification, although De Beer  suggested that it might be included within the os basale of
caecilians. A supraoccipital ossification is also absent in putative stem-lissamphibians like the
temnospondyl Doleserpeton , but it is present in the lepospondyl Brachydectes  where
the element is remarkably similar to that of Shirerpeton.
Ventrally, Shirerpeton lacks a median basioccipital but the paired exoccipital plates
presumably had the same embryonic origin. Whether the notochord was enclosed by the plates, or
overlay them in the midline is unclear. Maddin et al.  postulated the presence of a
transverse suture between anterior and posterior parts of the braincase floor in both A.
inexpectatum and Celtedens ibericus. This is also the case in Shirerpeton, although our examination of
the key Las Hoyas specimen (Museo de las Ciencias de Castilla-La-Mancha, Cuenca, Spain,
MCCM-LH-15710) showed that labelled suture in this specimen  is actually a break within
the exoccipital plate, with the other part of this element preserved on the counterpart (blue
outlines in Fig 43). Moreover, the more anterior element interpreted by  as the dorsum
of the braincase, is actually a sphenoid like that of Shirerpeton (red outlines in Fig 43). In
46 / 58
Fig 43. Celtedens ibericus, Las Hoyas specimen, Museo de las Ciencias de Castilla-La-Mancha,
MCCM-LH-15710, digital photograph of skull of part and counterpart. A,B MCCM-LH-15710b, with A, whole
skull and B, enlargement of braincase region; C,D, MCCM-LH-15710a, with C, whole skull and D, enlargement of
braincase region. In B and D, blue outlined element is the exoccipital plate, and red outlined element is the
sphenoid. Scale bar = 1 mm.
A. pannonicum, Maddin et al.  concluded that the ventral floor of the neurocranium was
`composed largely of the parasphenoid' with `minor contributions at the posterolateral corners
from the otic capsules'. This is very different from the condition in Shirerpeton where the floor
is clearly formed by the exoccipital plates. The configuration of the parasphenoid remains
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unknown. In living frogs and salamanders, the exoccipital plates do not generally meet in the
midline, but this may not apply to the solidly fused os basale of caecilians .
Both previous accounts of the albanerpetontid braincase [3, 13] recorded the presence of
one or more hypoglossal foramina piercing the exoccipital. Foramina are also present in the
Tetori specimen but we were unable to follow a clear path through the bone and therefore the
identification of these foramina remains uncertain.
The albanerpetontid palate remains an enigma. One small element in the Tetori skull has
been tentatively identified as a possible ectopterygoid or palatine but the other ventral
components of the skull (parasphenoid, pterygoids, vomers, elongated median hyoid element) are
not preserved. It seems likely that these ventral skull components were displaced either
laterally or anteriorly and were on the part of the block that was lost before the dorsal elements
Systematic position of Shirerpeton within Albanerpetontidae
As most albanerpetontid material is disarticulated, the elements most frequently used in
diagnosis are the premaxilla and the distinctive median frontal [6, 30, 34, 74]. The oldest, and
apparently more plesiomorphic, taxa (Anoualerpeton, Celtedens) have a frontal that has been
described as `hour-glass' or `bell-shaped' [6, 8, 73±74], in that the bone is significantly longer
than it is wide, and the lateral orbital borders are concave (Fig 37A±37C). The anterior limit of
the frontal orbital margin (posterior edge of the prefrontal facet) lies anterior to the mid-point
of the anteroposterior long axis. The second frontal morphotype, which is considered to
characterise the derived genus Albanerpeton [28, 30, 34, 72], has lateral borders that are straight (or
nearly so) and a posterior margin that is significantly wider than the anterior one (Fig 37F±
37I). The anterior margin of the orbit lies at or posterior to the mid-point of the long axis of
the frontal. The frontal length and width are roughly equal, so that the bone is strongly
triangular, a shape enhanced by an acuminate internasal process that may be flanked by smaller
anterolateral processes. However, length/width proportions differ considerably between early
species like A. arthridion and derived Neogene species such as A. inexpectatum and A.
pannonicum. The Barremian genus Wesserpeton  has a frontal that is intermediate in morphology
between the two main types, in that it is triangular and acuminate like that of Neogene
Albanerpeton, but is somewhat longer in relation to its posterior width, and the anterior margin of
the orbit is roughly level with the mid-point of the frontal long axis (Fig 37D).
As reconstructed, the frontal of Shirerpeton has a length/posterior width proportion of 1.23,
as compared to 0.87±1 for A. inexpectatum ; 0.88±0.95 for A. pannonicum ; ~1.2 in A.
gracile, A. nexuosum, A. galaktion [29, 30], and A. arthridion ; 1.0±1.3 for Wesserpeton ;
1.3±1.4 for Celtedens ibericus (depending on the reconstruction used); and 1.57 for
Anoualerpeton unicum and An. priscum . Of all species currently known, the reconstructed frontal
shape and proportions of Shirerpeton are most like those of the North American A. arthridion
(Fig 37F), the oldest (uppermost Aptian or lower Albian) species referred to Albanerpeton,
although the internasal process is longer in the Japanese taxon.
Estes & Hoffstetter  figured the parietals of A. inexpectatum which, although slightly
damaged posteriorly, clearly differ in shape from those of the Japanese taxon in that the
unsculptured posterior occipital shelf is long and wide rather than bifurcate. There is also a
difference in proportion in relation to the width of the fronto-parietal contact and the length of
the postorbital wing. In A. inexpectatum, the postorbital wing is relatively short (Fig 42E) and
the sculpture extends almost to its tip. In Shirerpeton, the wing forms almost half the anterior
width of the bone, and is unsculptured (Fig 42D). The parietals are also preserved in Celtedens
ibericus [6, 8], but are crushed. The postorbital wings of Celtedens, as reconstructed (Fig 42C),
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are proportionally longer than those of Shirerpeton (more than 58% parietal width), but bear
sculpture as in A. inexpectatum. The same condition is seen in the few fragmentary parietals
preserved for the Middle Jurassic Anoualerpeton priscum , Fig 42A), and in the parietal
fragment attributed to Wesserpeton evansae (Fig 42B). Unfortunately, the tip of the wing is
missing in both UK taxa, but (from the angle of the converging margins) it does not appear to
have been long. A long, unsculptured postorbital wing may therefore be a diagnostic feature of
Shirerpeton, but without comparable elements from A. arthridion and members of the
`gracilesnouted' clade, this remains tentative.
Of the jaw elements, comparison between albanerpetontid species has focused mainly on
the premaxilla, an element that remains unknown in Shirerpeton. As listed , maxillary
characters have been limited to the presence or absence of labial sculpture; the position of the
anteriormost tooth relative to point of maximum indentation of the narial margin; and the
presence or absence of an expansion of the labial dental margin in association with enlarged
teeth. Shirerpeton shows the primitive state in the first two of these, and shares the heterodonty
and expanded dental margin with species of Anoualerpeton and Albanerpeton nexuosum.
However, as Fig 36 demonstrates, there are other maxillary features that vary between species.
These include the length and shape (rounded or pointed) of the anterior premaxillary process;
the degree of concavity of the anterior narial margin; and the shape of the dorsal facial process
in terms of angulation and profile. The difficulty is that few complete maxillae have been
described and most sample sizes are small. On the basis of existing material, the maxilla of
Shirerpeton (Fig 36C) appears to be distinct in combining heterodonty with a relatively short,
pointed premaxillary process, little curvature of the narial margin, and a low, rounded facial
process. Even fewer dentary characters have been identified, other than those relating to labial
sculpture and heterodonty, and the presence of a dorsal process posterior to the tooth row,
currently reported only in Albanerpeton inexpectatum . Again, Shirerpeton most closely
resembles Anoualerpeton and Albanerpeton nexuosum in having enlarged anterior teeth
supported by an expansion of the labial dental margin, although it differs in the greater degree of
sinuosity of the margin due to the concave-convex-concave profile. The dentary of Shirerpeton
also appears to differ in two other ways. The anterior limit of the prearticular facet seems to be
in line with the rear of the tooth row and posterior to the opening of the Meckelian fossa,
whereas it extends forward below the tooth row to a point roughly in line with the opening of
the Meckelian fossa in most other taxa where this region is known. Secondly, in many
albanerpetontids (including A. nexuosum and the two species of Anoualerpeton), the subdental shelf
rises steeply at the posterior end of the tooth row such that the posterior tooth loci are
significantly smaller (25% or less) than those close to the symphysis. In Shirerpeton, the subdental
shelf is shallower and, apart from the group of enlarged teeth, there is less of a difference in
size between anterior and posterior teeth. Thus the combined characters of the maxilla and
dentary in Shirerpeton allow it to be distinguished from other albanerpetontid taxa known
only from dental elements. This includes elements from the Upper Cretaceous of Central Asia
attributed to `Nukusurus' , which lack heterodonty and have an anteriorly extended
prearticular facet and very small posterior teeth.
Taken together with the morphological features, the results of the phylogenetic analyses
provide some support for the placement of the Japanese albanerpetontid crownward of the
European Wesserpeton and the Uña taxon (sensu ). Although the Strict Consensus (PAUP,
TNT) is partially unresolved (Fig 38), only a minority of individual trees placed the Japanese
taxon as the sister group to a monophyletic Albanerpeton. Of the remaining trees, roughly half
placed the Japanese albanerpetontid as the sister taxon to A. arthridion, which it resembles in
frontal morphology and small size, but not dentary morphology (proportionally deeper
anteriorly in A. arthridion, without the strongly sinuous alveolar margin ). The other half of the
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individual trees placed the Japanese taxon crownward of A. arthridion, but of unstable position
in relation to other included species. The Bootstrap analysis (Fig 40) placed Shirerpeton on the
stem of the `robust-snouted' clade. These topologies raise the question as to whether the
Japanese species should be attributed to Albanerpeton.
The type species of Albanerpeton is the European Miocene A. inexpectatum . Although
the first albanerpetontids (represented by dentaries) recovered in North America were placed
in the genus Prosiren [
], the designated holotype of Prosiren was a vertebra later shown to
belong to a salamander . Based on their unique structure (tooth and symphysial
morphology) the dentaries originally attributed to Prosiren [
] were re-assigned to the distinctive
European genus Albanerpeton , as was all subsequent North American material. In the interim,
however, there have been both temporal range extensions (Middle Jurassic to Pliocene), and a
recognition of generic diversity among European Mesozoic albanerpetontids (Anoualerpeton,
Celtedens, Wesserpeton). Compared to these relatively short-lived genera, Albanerpeton as
currently defined extends from the? Aptian-Albian to the late Pliocene, an interval of more than
100 Myr. Given the difference in frontal shape between, for example, A. arthridion and A.
inexpectatum (Fig 37), it seems likely that there is greater taxonomic diversity within Albanerpeton
than is currently recognised by the nomenclature. This applies particularly to the earliest
North American species, and the unstable placement of the Japanese taxon in relation to these
species (especially A. arthridion) may partly reflect this. Resolution of this problem would be
advanced by the recovery and description of additional elements (especially parietals) for the
North American species, as current diagnoses and phylogenies rely too heavily on premaxillary
Based on the above discussion, we consider it preferable to give the new Japanese
albanerpetontid separate generic status. This reflects i) its distinctive character combination; ii) the
striking differences in braincase morphology between Shirerpeton and the Neogene
Albanerpeton species; iii) the unstable phylogenetic position of the Japanese taxon in relation to existing
Albanerpeton species; iv) a lack of overlap of preserved elements, resulting in more than 50%
missing data when coded into existing matrices (mainly due to a lack of premaxillae); v) the
very limited knowledge of albanerpetontid parietal and braincase morphology; and vi) the
fragmentary nature of most of the North American material. The separate taxonomic identity
also facilitates an objective discussion of albanerpetontid biogeography.
Systematic position of Albanerpetontidae among tetrapods
Discussion of the systematic position of albanerpetontids amongst tetrapods is complicated by
the lack of agreement on the evolutionary relationships and monophyly of lissamphibians [14,
79±80], and by a lack of information on the early history of each major group. Triadobatrachus
[77, 81±82] and Czatkobatrachus [83±85] provide some information on early frog evolution,
but it is very incomplete, as is that for early caudates [86±89]. Eocaecilia is more completely
known [5, 76] but, until the recent description of the Triassic Chinlestegophis  as a possible
stem-caecilian, it had been isolated in time. The recovery of new material pertaining to the
early history and divergence of lissamphibians is clearly crucial.
Albanerpetontids have the potential to shed light on early lissamphibian morphology, but
in the current state of phylogenetic flux, a detailed analysis of albanerpetontid relationships to
other tetrapods is beyond the scope of the current paper. Existing data matrices differ quite
strikingly in their coding of the same taxa (e.g. compare the codings in [5, 14] vs ) and
yield conflicting results, even when the same approaches are used. Nonetheless, the presence
of epipterygoids and a separate supraoccipital in albanerpetontids argues against a nested
position within Batrachia (as a caudate sister group) as some previous authors have suggested
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[1±5]. Moreover, although there is no question as to the monophyly of Albanerpetontidae, the
differences in braincase anatomy between Shirerpeton and the Neogene species of
Albanerpeton provide a clear argument against representing Albanerpetontidae as a single operational
unit in phylogenetic analyses as has often been the case [5, 10, 79, 90]. Shirerpeton is separated
from A. inexpectatum and A. pannonicum by around 100 million years, but is itself separated
by a similar time interval from the likely Permo-Triassic ancestral albanerpetontid of which
nothing is known.
Whatever the precise relationship between albanerpetontids and the three living lissamphibian
clades, their last common ancestor must have lived in the Paleozoic. This leaves a long hiatus
before the first recorded appearance of albanerpetontids in the Middle Jurassic, limiting
discussion as to their place of origin. The Jurassic record is currently restricted to Europe (France,
England, Portugal [21, 22, 71, 91]) and North Africa (Morocco ). Most Early Cretaceous
records are also European (Britain [34, 92], France [
], Italy [
], Spain , Sweden [
the group is represented in Europe for much of the Late Cretaceous (Spain, France, Hungary,
]), as well as intervals of the Palaeogene (Oligocene, Germany ) and Neogene
(France, Germany, Czech Republic, Austria, Hungary, Italy ). Albanerpetontids survived
in Europe until at least the late Pliocene (Italy ) where they are associated with the remains
of extant reptile and amphibian genera.
There are no confirmed records of albanerpetontids in North America before the late Early
Cretaceous and this coincides with the first occurrence of specimens currently referred to
Albanerpeton [17, 26]. Gardner's phylogenetic work [16±17, 28, 34] recognises A. arthridion
from the Cloverley and Antlers formations of Oklahoma and Texas, USA, as the sister taxon to
all other (younger) Albanerpeton species and, Shirerpeton aside, our analyses support this
placement. The precise age of the oldest horizons bearing A. arthridion are uncertain, as
radiometric dates are lacking, but they are generally given as uppermost Aptian or lowermost
]. Albanerpeton then apparently radiated in North America through the Late
Cretaceous (at least seven attributed species), with records in every stage from the
Cenomanian to the Maastrichtian . Specimens from the middle to late Palaeocene of Canada 
show that the group survived the end-Cretaceous extinction in North America, but there are
currently no younger records. In Europe, the genus Albanerpeton is first reported from the
Late Cretaceous (Campanian) of France, but it is important to note that many Late Cretaceous
European albanerpetontid records are based on fragmentary specimens that are really
diagnostic only at family level. The two named European species, the Miocene A. inexpectatum and
Pliocene A. pannonicum, fall within a single well-supported clade  with material from the
Palaeocene Paskapoo Formation of Canada.
Up until recently, therefore, the distribution pattern was consistent with the hypothesis that
the genus Albanerpeton originated in North America, around the mid-Cretaceous, and
subsequently spread to Europe. However, as Gardner & Bőhme  observed, the expanding record
of albanerpetontids in Europe weakens that argument, as does the absence of any confirmed
record of albanerpetontids in North America prior to the latest Aptian/earliest Albian. This
apparent absence may be an artefact of the fossil record, there being no Middle Jurassic or
Berriasian-Barremian microvertebrate faunas known from North America. However,
albanerpetontids are also conspicuously absent from the Upper Jurassic Morrison Formation despite
the existence of several rich microvertebrate localities where they would be expected to occur
(based on other components of the faunal assemblage). It is therefore possible that
albanerpetontids first reached North America during the Early Cretaceous but, if so, from where ?
51 / 58
One possibility is that derived albanerpetontids spread to North America from Europe via a
North Atlantic route, as suggested for some mammals [98±99]. This hypothesis has received
support from the description of the European Wesserpeton and its placement as a sister taxon
to Albanerpeton. Sweetman & Gardner  suggested that the split between the ancestral
lineages of these two genera might have occurred in western Eurasia, with a Wesserpeton-like
ancestor subsequently migrating into North America via Europe and giving rise to
Albanerpeton. They further proposed (based on Wesserpeton) that `acquisition of a triangular (frontal)
shape preceded shortening of the bone and that this evolutionary trait was acquired in western
Eurasia some time prior to the Barremian' .
An alternative route into western North America, where records are concentrated, would
be from eastern Asia via the Beringian land bridge. This route has been proposed for several
groups of dinosaurs, lizards, mammals and choristoderes that appeared in North America
toward the end of the Early Cretaceous [100±103]. However, some authors [17, 31±32] have
argued against that scenario for albanerpetontids, on the basis that the first record in North
America (A. arthridion) both predated the opening of the Beringian land bridge and predated
any record of albanerpetontids in Asia. The first objection is moot as there is uncertainty both
on the precise dating of the relevant American deposits (e.g. [
]) and of the opening of the
land bridge (e.g. [101±104]). Nonetheless, up until now, the second objection stood, because
the earliest confirmed Asian records of albanerpetontids were from the early Cenomanian of
Uzbekistan [31±33,105±106]). To explain their apparently late appearance in Asia, Gardner &
Averianov  and Skutschas  favoured a mid-Cretaceous dispersal of albanerpetontids
from North America into Asia. Clearly, the recovery of a relatively derived albanerpetontid in
the Early Cretaceous of Japan is not consistent with that scenario. It is plausible that the
ancestral Wesserpeton-like stock proposed by Sweetman & Gardner  could have spread
eastwards rather than (or as well as) westwards from western Eurasia, although the potential for
interchange may have been limited by the opening of the Turgai Straits between Asia and
western Eurasia from the late Middle Jurassic onward [
]. Moreover, none of the Middle
Jurassic assemblages of Russia and Central Asia has yielded conclusive evidence of
]. This is surprising given the general resemblance of these assemblages to the
contemporaneous faunas of Britain where albanerpetontids do occur . An albanerpetontid
frontal was reported [
] from the Middle Jurassic Balabansai Formation of Kirghizia, but
this observation cannot be confirmed as the specimen was neither figured nor described and
cannot be located (.
It remains possible, of course, that the apparent absence of the group at some localities is an
artefact of local environmental or taphonomic factors [
]. Albanerpetontids have yet to be
recovered from the fossil rich Chinese Daohugou Beds (Jurassic) or the Jehol Biota (Early
Cretaceous), yet the discovery of Shirerpeton in Japan, then part of the eastern Asian mainland,
renders it highly likely that the group was present in China and other parts of Asia at this time.
Recent isotope analysis of reptile bone suggests that the Early Cretaceous Yixian Formation, at
least, was deposited under similar (cool) climatic conditions (mean air temperature of ~10ÊC,
]) to those of the Kuwajima Formation, but perhaps the lacustrine environments that have
been richly sampled in China were less suitable for small terrestrial albanerpetontids. Most
Mesozoic albanerpetontids are found in lowland estuarine or swampy environments . The
Japanese Kuwajima Formation conforms to this pattern (swampy inland wetland), but
albanerpetontid remains are extremely rare (to date, 3 out of 2459 catalogued specimens) and it
seems likely that they entered the deposit from further afield, possibly in the gut of a predator.
One further chapter in the history of Asian albanerpetontids is currently being written.
Daza et al.  recently described a collection of lizard fossils from an amber deposit in
Myanmar (Burma) dated to Albian-Cenomanian age. The image of one very small specimen,
52 / 58
tentatively identified  as an early chamaeleonid, was re-identified by one of us (SEE) as an
albanerpetontid. μCT has confirmed this identification and work on that specimen is ongoing.
It provides an important temporal link between the Japanese specimen and the younger
material from Uzbekistan, and offers support for the idea that albanerpetontids were well
established and relatively widespread across at least eastern and south-eastern Asia during the
The recovery of the new Japanese specimens sheds new light on albanerpetontid morphology
and biogeography, but raises as many questions as it resolves. There is clearly much more to
discover about these enigmatic little tetrapods, in terms of their morphology, relationships,
and evolutionary history. Recent discoveries have demonstrated that the group had a more
extensive temporal and geographical distribution in Asia than previously understood.
Awareness of this among researchers may lead to further discoveries.
S1 Fig. Strict consensus of 53 trees run in PAUP using the full matrix, with the
hypothetical ancestor as outgroup. This tree topology matches that recovered from the TNT analysis
shown in Fig 34. Of the 53 individual MPTs, 15% placed Shirerpeton as the sister taxon to a
monophyletic Albanerpeton; 45% placed it as the sister taxon to A. arthridion; and 40% placed
it crownward of A. arthridion.
S2 Fig. PAUP analysis of the full matrix run without the hypothetical ancestor. Left,
Bootstrap Analysis; right, 70% Majority Rule Tree of 53 individual trees.
S3 Fig. Bootstrap analysis using PAUP of the limited matrix. There is less resolution with
respect to Wesserpeton and the Uña taxon.
S1 File. Characters and data matrix used in the phylogenetic analysis.
Our thanks to Yuki Iwama (Nagoya Municipal Industrial Research Institute) and Akira
Monkawa (Tokyo Metropolitan Industrial Technology Research Institute) for μCT scanning; Reiko
Kohno, Archaeology Department National Science Museum, Tokyo, for access to a 3D printer;
Akemi Wakimoto, Kento Otsuka, and Tsuyoshi Hibino (Hakusan City Board of Education,
Ishikawa Prefecture) for access to the specimens described here and much of the initial
preparation; Steven Sweetman (University of Portsmouth) for access to the partial parietal of c.f.
Wesserpeton and for donating it to the Natural History Museum, London; Filip Weichmann
(Free University Berlin) and Pavel Skutschas (St Petersburg State University) for information
on Guimarota and Russian material respectively; Angela Delgado (Autonoma University
Madrid) for arranging the loan and transportation of MCCM-LH-15710 from the Museo de
las Ciencias de Castilla-La-Mancha (Cuenca); Juan Daza (Sam Houston State University) for
confirming our identification of the Myanmar amber specimen; Nick Crumpton (University
College London) and Masatoshi Okura (Aichi Prefecture) for taking the digital images; and
three reviewers (Annelise Folie, Marton Venczel, James Gardner) for helpful comments on an
53 / 58
earlier version of the manuscript. Yui Inagaki, a Junior High School student from Kanagawa
City, found the second albanerpetontid jaw while the original manuscript version was in
review: our special thanks to her. We are also indebted to the mayors and administrations of
Kuwajima District (Hakusan City, Ishikawa Prefecture, Japan) and Ishikawa Prefecture for
their generous hospitality and support over many years. We also acknowledge the Willi
Hennig Society for access to the phylogeny programme TNT.
Conceptualization: Susan E. Evans.
Investigation: Ryoko Matsumoto, Susan E. Evans.
Methodology: Ryoko Matsumoto.
Writing ± original draft: Susan E. Evans.
Writing ± review & editing: Ryoko Matsumoto, Susan E. Evans.
3. Estes R, Hoffstetter R. Les urodeÂles du Miocène de la Grive-St.Alban (Isère, France). Bull Mus Natl
Hist Nat, Paris 1976; 57:297±343.
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