Anhanguera taxonomy revisited: is our understanding of Santana Group pterosaur diversity biased by poor biological and stratigraphic control?

Anhanguera taxonomy revisited: is our understanding of Santana Group pterosaur diversity biased by poor biological and stratigraphic control? Felipe L. Pinheiro1 and Taissa Rodrigues2 1 2 Laboratório de Paleobiologia, Universidade Federal do Pampa, São Gabriel, RS, Brazil Departamento de Ciências Biológicas, Universidade Federal do Espírito Santo, Vitória, ES, Brazil ABSTRACT Submitted 4 January 2017 Accepted 8 April 2017 Published 4 May 2017 Corresponding author Felipe L. Pinheiro, Academic editor Graciela Piñeiro Additional Information and Declarations can be found on page 24 DOI 10.7717/peerj.3285 Copyright 2017 Pinheiro and Rodrigues Distributed under Creative Commons CC-BY 4.0 Background. Anhanguerids comprise an important clade of pterosaurs, mostly known from dozens of three-dimensionally preserved specimens recovered from the Lower Cretaceous Romualdo Formation (northeastern Brazil). They are remarkably diverse in this sedimentary unit, with eight named species, six of them belonging to the genus Anhanguera. However, such diversity is likely overestimated, as these species have been historically diagnosed based on subtle differences, mainly based on the shape and position of the cranial crest. In spite of that, recently discovered pterosaur taxa represented by large numbers of individuals, including juveniles and adults, as well as presumed males and females, have crests of sizes and shapes that are either ontogenetically variable or sexually dimorphic. Methods. We describe in detail the skull of one of the most complete specimens referred to Anhanguera, AMNH 22555, and use it as a case study to review the diversity of anhanguerids from the Romualdo Formation. In order to accomplish that, a geometric morphometric analysis was performed to assess size-dependent characters with respect to the premaxillary crest in the 12 most complete skulls bearing crests that are referred in, or related to, this clade, almost all of them analyzed first hand. Results. Geometric morphometric regression of shape on centroid size was highly statistically significant (p = 0.0091) and showed that allometry accounts for 25.7% of total shape variation between skulls of different centroid sizes. Premaxillary crests are both taller and anteroposteriorly longer in larger skulls, a feature consistent with ontogenetic growth. A new diagnosis is proposed for Anhanguera, including traits that are nowadays known to be widespread within the genus, as well as ontogenetic changes. AMNH 22555 cannot be referred to ‘‘Anhanguera santanae’’ and, in fact, ‘‘Anhanguera santanae’’, ‘‘Anhanguera araripensis’’, and ‘‘Anhanguera robustus’’ are here considered nomina dubia. Discussion. Historically, minor differences in crest morphology have been used in the definition of new anhanguerid species. Nowadays, this practice resulted in a considerable difficulty in referring well-preserved skulls into known taxa. When several specimens are analyzed, morphologies previously believed to be disparate are, in fact, separated by a continuum, and are thus better explained as individual or temporal variations. Stratigraphically controlled excavations on the Romualdo Formation have showed evidence for faunal turnover regarding fish communities. It is thus possible that some of the pterosaurs from this unit were not coeval, and might even represent OPEN ACCESS How to cite this article Pinheiro and Rodrigues (2017), Anhanguera taxonomy revisited: is our understanding of Santana Group pterosaur diversity biased by poor biological and stratigraphic control? PeerJ 5:e3285; DOI 10.7717/peerj.3285 anagenetic morphotypes. Unfortunately, amateur collecting of Romualdo Formation fossils, aimed especially at commerce, resulted in the lack of stratigraphic data of virtually all its pterosaurs and precludes testing of these further hypotheses. Subjects Paleontology, Zoology Keywords Pterosauria, Anhangueridae, Romualdo formation, Cretaceous, Allometry, Geometric morphometrics INTRODUCTION Anhangueridae is a clade of pterosaurs currently known from multiple localities worldwide, including named species from Brazil, the United States, Morocco, China and England (Rodrigues & Kellner, 2013). The majority of identifiable material comes from the Romualdo Formation (Araripe Basin, northeastern Brazil), a well-known fossil Lagerstätte where they are the most abundant and speciose clade of tetrapods, with eight named species (Tropeognathus mesembrinus, Maaradactylus kellneri and six species of Anhanguera) (Table 1), as well as several closely-related pterosaur taxa and dozens of referred specimens. Even though this anhanguerid taxonomy has already been disputed by several authors (Kellner & Tomida, 2000; Fastnacht, 2001; Unwin, 2001; Veldmeijer, 2003), the apparent species diversity seems nonetheless remarkable. Although the first descriptions of pterosaurs from the Romualdo Formation date from as early as the 1970s (Price, 1971), well-preserved skull material only began to be described in the 1980s and 1990s. In February 1985, Wellnhofer described a number of specimens from the Romualdo Formation, naming two new species based on fossils comprising skull material: ‘‘Santanadactylus’’ araripensis and ‘‘Araripesaurus’’ santanae; both genera were previously described based only on postcranial material. Later that same year, Campos and Kellner described the new genus and species Anhanguera blittersdorffi, based on a complete skull. In 1987, Wellnhofer described two further species, Tropeognathus mesembrinus and ‘‘Tropeognathus robustus’’. With increasing knowledge of these Romualdo Formation anhanguerids, some new taxonomic proposals arose, including placing all of these species in the genus Anhanguera (Kellner, 1990). Additional anhanguerid specimens, but no newly named species, were subsequently described by Wellnhofer (1991); among them AMNH 22555 is an incomplete skeleton including a skull and a fragmentary mandible. It was the most complete skeleton then known from the Romualdo Formation, and served as the basis for the first anhanguerid skeleton reconstruction ever made (Wellnhofer, 1991). This specimen was regarded by Wellnhofer (1991) as conspecific with the holotype of ‘‘Anhanguera santanae’’ (previously in the genus ‘‘Araripesaurus’’). Remarkably, two other almost complete skeletons, including skulls, were later described and referred to the species Anhanguera piscator (Kellner & Tomida, 2000) and ‘‘Coloborhynchus’’ spielbergi (Veldmeijer, 2003). Today, several skulls (both described and undescribed) are hosted in a myriad of publically accessible collections and thus enabling the examination of a larger sample of Romualdo anhanguerids (Tables 1 and 2). Recent proposals (Kellner & Tomida, Pinheiro and Rodrigues (2017), PeerJ, DOI 10.7717/peerj.3285 2/29 Table 1 Anhanguera taxonomy. Synopsis of the named species of Anhanguera from the Romualdo Formation. Species Taxonomic status in the present work Known specimens Dia (...truncated)