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A New Close Mammal Relative and the Origin and Evolution of the Mammalian Central Nervous System

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A New Close Mammal Relative and the Origin and Evolution of the Mammalian Central Nervous System Copyright by Rachel Veronica Simon Wallace 2018 The Dissertation Committee for Rachel Veronica Simon Wallace Certifies that this is the approved version of the following Dissertation: A new close mammal relative and the origin and evolution of the mammalian central nervous system Committee: Timothy Rowe, Supervisor Christopher J. Bell Richard A. Ketcham Chris Kirk Matthew W. Colbert A new close mammal relative and the origin and evolution of the mammalian central nervous system by Rachel Veronica Simon Wallace Dissertation Presented to the Faculty of the Graduate School of The University of Texas at Austin in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy The University of Texas at Austin May 2018 Dedication This dissertation is dedicated to my mom and dad, for encouraging me through my entire educational journey; to my husband, for motivating me to never give up and to push for success; and to my daughter, Beatrix, for giving me a new perspective of my life. Acknowledgements I would like to thank my advisor, Dr. Timothy Rowe, for sharing all of his wisdom with me for the past eight years. I am also thankful to Dr. Rowe for all of his patients. The hours of conversation we have had together have opened my eyes to evolutionary and developmental anatomy. I would like to thank my committee, Drs. Chris Bell, Rich Ketcham, Chris Kirk, and Matt Colbert, for setting time aside to read my dissertation and provide valuable feedback. I feel fortunate to have worked with such a harmonious and insightful committee. I thank my committee for the valuable discussions I have had with each of them on an individual basis throughout the years. The Jackson School has provided numerous, invaluable resources to me. Thank you for supporting me with TA opportunities that have shown me how much I value teaching. I also want to thank the Lundelius Grant for Student Research in Vertebrate Paleontology as a helpful source of financial support during those long summers when TA-ships are dry. And of course, I am thankful to Drs. Jessie Maisano and Matt Colbert (also a committee member) for all of their assistance with CT data acquisition in The University of Texas High-Resolution X-ray Computed Tomography Facility. The struggles through graduate school would not be possible without social support. For that, I thank UT Paleo graduate students for insightful conversations and emotional support. It is always nice to know you are not alone! And above all, no greater support has come from anyone than from my husband, Gerard Wallace (soon to be Dr. Wallace), and our beautiful, funny little girl, Beatrix. Thank you, everyone, for making this all possible. v Abstract A new close mammal relative and the origin and evolution of the mammalian central nervous system Rachel Veronica Simon Wallace, Ph.D. The University of Texas at Austin, 2018 Supervisor: Timothy B. Rowe Mammals are distinguished by the presences of the cerebral neocortex, and dentary- squamosal jaw joint, among other traits. These traits likely evolved outside of Mammalia, in Mammaliaformes. Recent fossil discoveries from South America elucidate the evolution of the brain in taxa outside of Mammaliaformes. A new skull of a close mammal relative, Pseudotherium argentinus, from the Late Triassic Ischigualasto Formation in Argentina was scanned at The University of Texas High-Resolution X-ray Computed Tomography (CT) Facility. CT data reveal a unique combination of ancestral and derived characters. Phylogenetic analysis supports a sister- taxon relationship between Pseudotherium and the derived Tritylodontidae. An endocast reconstruction further supports a derived phylogenetic position among cynodonts. A comparison of the Pseudotherium endocast with other cynodont endocasts suggests that the cerebral hemispheres enlarged in probainognathian cynodonts, while the total endocranial volume remained relatively constant until the origin of Mammaliaformes. This expansion is also present in the foramen magnum. Encephalization volume and skull length were compared with foramen magnum size in extinct and extant cynodonts, birds, and lizards. Endocranial volume and foramen magnum size, and foramen magnum size and skull length, are correlated following a 3/2 power vi law. Therefore, foramen magnum size can be used to predict endocranial volume for fossils. Regression analyses show foramen magnum size is significantly increased in Mammaliaformes. This change reflects an increase in medulla oblongata size and supports a concerted model of brain evolution outside of Mammalia. Because the increase in medullary size coincides with the origin of the neocortex, I hypothesize that the mammalian pyramidal tract originated in Mammaliaformes. The hypotheses from this dissertation are testable given recent fossil discoveries. However, morphological data of those specimens are not readily available. Morphological descriptions in this dissertation are accompanied by detailed figures, and the data will be digitally archived and made publically available on DigiMorph.org. The intent is to model a communication and data-sharing standard to be implemented in all future fossil discoveries. vii Table of Contents Chapter 1: First record of a basal mammaliamorph from the Carnian-Norian Ischigualasto Formation fo Argentina..................................................................................................1 Introduction .................................................................................................................3 Geological and Paleontological Settings ....................................................................6 Materials and Methods................................................................................................7 Computed tomography ...................................................................................7 Institutional Abbreviations .............................................................................9 Nomenclature ..................................................................................................9 Supplemental Data ..........................................................................................9 Results .........................................................................................................................9 Systematic Paleontology .................................................................................9 Description of the skull ................................................................................12 The Dentition ................................................................................................32 Postcanine Orientation and Root Constriction .....................................34 Discussion .................................................................................................................35 Phylogenetic Analysis...................................................................................35 Additional Phylogenetic Context ..................................................................38 Mammaliamorph Characters that are Lacking in Pseudotherium ...............39 Additional Characters Pseudotherium Shares with Mammaliamorpha ........39 Comparison to 'brasilodontids' ......................................................................41 Conclusions ...............................................................................................................41 Chapter 2: Endocranial volume of Pseudotherium argentinus (Cynodontia, Mammaliamorpha) and the evolution of the mammalian brain...................................82 Introduction ...............................................................................................................82 viii Institutional Abbreviations .......................................................................................84 Materials ...................................................................................................................85 Methods ....................................................................................................................86 Challenges of Comparing Encephalization Volume Reconstructions in Non- Mammalian Cynodonts ...........................................................................86 Computed Tomography ................................................................................88 Encephalization Volume and Encephalization Quotient calculations ..........89 Description of the Pseudotherium Braincase and Endocast .....................................89 Braincase .......................................................................................................89 Endocast ........................................................................................................92 Forebrain (Telencephalon + Diencephalon) ........................................92 Midbrain (Mesencephalon) ..................................................................94 Hindbrain (Metencephalon + Myelencephalon) ..................................94 Vessels .................................................................................................95 Encephalization Quotient of Pseudotherium .......................................96 Discussion .................................................................................................................96 Caveats and limitations in endocasts ............................................................96 Encephalization in Early Pan-Mammalian Evolution ..................................97
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