A New High-Latitude Tylosaurus (Squamata, Mosasauridae) from Canada with Unique
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A new high-latitude Tylosaurus (Squamata, Mosasauridae) from Canada with unique dentition A thesis submitted to the Graduate School of the University of Cincinnati in partial fulfillment of the requirements for the degree of Master of Science in the Department of Biological Sciences of the College of Arts and Sciences by Samuel T. Garvey B.S. University of Cincinnati B.S. Indiana University March 2020 Committee Chair: B. C. Jayne, Ph.D. ABSTRACT Mosasaurs were large aquatic lizards, typically 5 m or more in length, that lived during the Late Cretaceous (ca. 100–66 Ma). Of the six subfamilies and more than 70 species recognized today, most were hydropedal (flipper-bearing). Mosasaurs were cosmopolitan apex predators, and their remains occur on every continent, including Antarctica. In North America, mosasaurs flourished in the Western Interior Seaway, an inland sea that covered a large swath of the continent between the Gulf of Mexico and the Arctic Ocean during much of the Late Cretaceous. The challenges of paleontological fieldwork in high latitudes in the Northern Hemisphere have biased mosasaur collections such that most mosasaur fossils are found within 0°–60°N paleolatitude, and in North America plioplatecarpine mosasaurs are the only mosasaurs yet confirmed to have existed in paleolatitudes higher than 60°N. However, this does not mean mosasaur fossils are necessarily lacking at such latitudes. Herein, I report on the northernmost occurrence of a tylosaurine mosasaur from near Grande Prairie in Alberta, Canada (ca. 86.6–79.6 Ma). Recovered from about 62°N paleolatitude, this material (TMP 2014.011.0001) is assignable to the subfamily Tylosaurinae by exhibiting a cylindrical rostrum, broadly parallel-sided premaxillo-maxillary sutures, and overall homodonty. I further refer this material to Tylosaurus based on the lack of a dorsal midsagittal ridge on the premaxilla. Unexpectedly, TMP 2014.011.0001 exhibits widely spaced, high-aspect-ratio marginal tooth crowns and low-profile maxillae, similar to the typical juvenile condition of Tylosaurus, despite its likely adult age based on an estimated body length of at least 6.5 m. The specimen also exhibits anterior maxillary tooth roots covered by downward extensions of the maxillary cortical bone, a feature previously unknown in Tylosaurinae. TMP 2014.011.0001 hints at an undiscovered, temporally more stable ii Tylosaurus diversity in the northern latitudes of the seaway throughout the Late Cretaceous, possibly even into the latest Cretaceous and inclusive of the Arctic Circle. Analogous dental morphologies in other non-mosasaurid taxa, as well as a standard model of tooth function based on tooth morphology, indicate TMP 2014.011.0001 may have been specially adapted for piscivory. This study suggests the possible presence of a Cretaceous boreal marine community that was distinct from those across the more southern stretches of the Western Interior Seaway, in the western and southern United States. iii iv ACKNOWLEDGEMENTS I must first thank Dr. T. Konishi. Although not technically my committee chair, he was my supervisor for this undertaking, and has proved an excellent guide, in both my research and my teaching. I also wish to thank Drs. B. C. Jayne and E. J. Tepe for their thoughtful and incisive advice, feedback, and support on my committee, with particular gratitude to Dr. B. C. Jayne for serving as my committee chair. Thanks as well to J. Jakielaszek and B. Neuman, who discovered TMP 2014.011.0001, A. McGee at the Royal Tyrrell Museum of Palaeontology, who prepared the specimen, and B. Strilisky and Dr. D. Brinkman, of the same institution, who loaned us the specimen. The following individuals and institutions facilitated my data collection: M. Currie at the Canadian Museum of Nature, Dr. G. Storrs at the Cincinnati Museum Center, W. Simpson at the Field Museum of Natural History, C. Byrd and Dr. L. Wilson at the Sternberg Museum of Natural History, and Dr. D. Burnham at the University of Kansas Natural History Museum. This entire endeavor was possible thanks to the scholarship and teaching assistantship offered to me by the University of Cincinnati Department of Biological Sciences. I also received further financial support from the Department, including a Wieman-Benedict Research Award, and from several Conference Travel Awards from the University of Cincinnati Graduate Student Governance Association and a Jackson School of Geosciences Student Member Travel Grant from the Society of Vertebrate Paleontology. The photos used in Figure 11C and Figure 14A and C were taken by Dr. T. Konishi while funded by a Natural Sciences and Engineering Research Council of Canada Discovery Grant (no. 238458-01) to Dr. M. Caldwell. I would additionally like to thank R. Russell, R. Sanchez, and B. Strilisky, as well as Drs. J. Carter, D. Coleman, M. Day, Z. Johanson, H. Street, and D. Surge for providing information and/or photographs v pertaining to particular specimens; R. Carr and Drs. D. D’Amore, J. Massare, and N. Morehouse for helpful discussions; D. Foffa for kindly sharing his data; and Drs. M. Friedman and S. Smith for the use of their micro-CT equipment. Finally, I would like to thank my wife, Hannah, and our family and friends for their continued love and support over the course of my graduate studies and throughout my life. vi TABLE OF CONTENTS Abstract ........................................................................................................................................... ii Acknowledgements ..........................................................................................................................v List of Figures ................................................................................................................................ ix Introduction ......................................................................................................................................1 Institutional Abbreviations...................................................................................................3 Anatomical Abbreviations ...................................................................................................3 Geological Setting ............................................................................................................................3 Materials and Methods .....................................................................................................................5 Systematic Paleontology ..................................................................................................................6 Description .......................................................................................................................................9 Premaxilla ............................................................................................................................9 Maxilla ...............................................................................................................................11 Vomer ................................................................................................................................14 Septomaxilla ......................................................................................................................16 Mandible ............................................................................................................................19 Marginal Dentition .............................................................................................................20 Intrageneric Comparison ................................................................................................................23 Marginal Dentition .............................................................................................................23 Maxillary Depth .................................................................................................................26 Discussion ......................................................................................................................................27 Diagnosis............................................................................................................................27 vii Osteology ...........................................................................................................................31 Septomaxilla ..........................................................................................................31 Downward Maxillary Extensions ..........................................................................33 Ontogenetic Changes in Dentition .........................................................................33 Paedomorphosis .....................................................................................................35 Paleobiogeography .............................................................................................................36 Northernmost Tylosaurus ......................................................................................36 Tylosaurus Distribution in Western Interior Seaway .............................................41 Food Habits and Paleoecology...........................................................................................44 Conclusions ....................................................................................................................................47 Literature Cited ..............................................................................................................................49