A nearly complete skeleton of a new eusphenodontian from the Upper Jurassic Morrison Formation, Wyoming, USA, provides insight into the evolution and diversity of Rhynchocephalia (Reptilia: Lepidosauria)

Abstract

We describe a new, small-bodied rhynchocephalian reptile, Opisthiamimus gregori gen. et sp. nov., from the Upper Jurassic Morrison Formation of Wyoming, USA. Whereas many fossil rhynchocephalians are based on isolated incomplete jaws, the holotype of O. gregori includes most of the skull and postcranium and therefore represents one of the most complete specimens of Rhynchocephalia known from North America. We used micro-computed tomography to examine its skeletal anatomy in detail and to develop a three-dimensional reconstruction of the skull. The skull of O. gregori is similar to that of several non-neosphenodontian rhynchocephalians such as Planocephalosaurus (e.g. large orbits) and Clevosaurus (e.g. parietal parasagittal crests) yet exhibits a suite of other features related to the proal shearing mechanism that becomes increasingly elaborated among more phylogenetically nested taxa such as Sphenodon (e.g. lateral palatine tooth row parallels maxillary tooth row along its entire length, pyramidal dentary teeth with mesial shearing crests). The postcranial skeleton of O. gregori exhibits characteristics typical of a terrestrial rhynchocephalian. Our phylogenetic analyses use a substantially updated data set of 118 characters and 46 taxa, and both maximum parsimony and Bayesian frameworks. Results place O. gregori inside Eusphenodontia but outside Neosphenodontia, and therefore in a key position for contributing to character polarity for more deeply nested clades such as Clevosauridae, Sphenodontidae and Pleurosauridae. We also erect Leptorhynchia taxon nov., composed primarily of aquatically adapted taxa (e.g. Pleurosaurus, Sapheosaurus), which is supported by both cranial and postcranial characters. Because O. gregori is not particularly closely related to the other named Morrison rhynchocephalians (e.g. Opisthias rarus), it increases both the alpha and beta taxonomic diversities within the formation. Similarly, major differences in body size and inferred diet of the Morrison taxa imply considerable concomitant palaeoecological diversity just prior to a major global decline in rhynchocephalian diversity around the close of the Jurassic.

http://zoobank.org/urn:lsid:zoobank.org:pub:888E055B-8AC1-4BD0-A37C-8CB192F79673

Keywords:

• Rhynchocephalia
• Eusphenodontia
• Late Jurassic
• Morrison Formation
• phylogeny
• diversity

Acknowledgements

We are grateful to the many NMNH colleagues whose work contributed to the discovery, preparation and curation of these materials: in particular, we thank A. Millhouse for collections access and curatorial assistance; M. Brett-Surman, P. Kroehler, S. Jabo, J. Guibord, M. Oreska and C. Peredo for fieldwork; and J. Gregor and P. Kroehler for their careful preparation work. J. Hill, H. Little and S. Whittaker (NMNH), D. White (GWU, previously NMNH), A. Pritchard (VMNH, previously NMNH), J. Gladman and M. Walters (DU), and S. Taft and M. Fagan (UOHUK) helped facilitate and assist with μCT scans and/or processing of the digital data using 3D hardware and software. S. Whittaker provided access to and assistance with digital microscopy in the Scientific Imaging Lab at the NMNH. We especially thank J. Morrison (NMNH) for his skillfully rendered interpretive drawings in Figures 2, 3, 6, 13-15 and 17-23. K. de Queiroz (NMNH) provided access to the Amphibians and Reptiles collections in the Department of Vertebrate Zoology at the NMNH. G. Bever (JHMI), R. Martínez (UNSJ), A. Pritchard and V. Reynoso (UNAM) kindly provided digital images of Theretairus, Sphenotitan, Planocephalosaurus and Zapatodon, respectively, for character reassessment and scoring. J. Foster (UFHNH SPM), J. Kirkland (UGS), H.-D. Sues (NMNH) and K. Trujillo (LCCC) provided additional information on other North American rhynchocephalian specimens known to them. We thank M. Bies and the Bureau of Land Management for land access, assistance and special use permits (PA99-WY-054, PA03-WY-102) for collection of the fossils described herein. We are further grateful to H.-D. Sues and A. Pritchard for conversations on rhynchocephalian morphology and evolution. We are indebted to Editor-in-Chief P. Barrett for allowing us to submit our lengthy manuscript to the Journal of Systematic Palaeontology and to J. Olori (Associate Editor) and two anonymous reviewers for their constructive reviews and/or editorial assistance that improved our manuscript. Funding was provided by Smithsonian Small Grants to MTC. MEHJ was supported by the Australian Research Council (DE130101567) and was hosted by Elizabeth Freeland and David Land during visits to Washington.

Supplemental material

Supplemental material for this article can be accessed here: https://doi.org/10.1080/14772019.2022.2093139.

Associate Editor: Jennifer Olori