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A Virtual Phytosaur Endocast and Its Implications for Sensory System Evolution in Archosaurs Waymon Holloway [email protected]

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A Virtual Phytosaur Endocast and Its Implications for Sensory System Evolution in Archosaurs Waymon Holloway Holloway9@Marshall.Edu Marshall University Marshall Digital Scholar Theses, Dissertations and Capstones 1-1-2011 A Virtual Phytosaur Endocast and its Implications for Sensory System Evolution in Archosaurs Waymon Holloway [email protected] Follow this and additional works at: http://mds.marshall.edu/etd Part of the Terrestrial and Aquatic Ecology Commons Recommended Citation Holloway, Waymon, "A Virtual Phytosaur Endocast and its Implications for Sensory System Evolution in Archosaurs" (2011). Theses, Dissertations and Capstones. Paper 83. This Thesis is brought to you for free and open access by Marshall Digital Scholar. It has been accepted for inclusion in Theses, Dissertations and Capstones by an authorized administrator of Marshall Digital Scholar. For more information, please contact [email protected]. A VIRTUAL PHYTOSAUR ENDOCAST AND ITS IMPLICATIONS FOR SENSORY SYSTEM EVOLUTION IN ARCHOSAURS A Thesis submitted to The Graduate College of Marshall University In partial fulfillment of the requirements for the degree of Master of Science in Biological Sciences by Waymon Holloway Approved by Dr. F. Robin O’Keefe, Committee Chairperson Dr. Suzanne G. Strait Dr. Paul J. Constantino Marshall University June 2011 Acknowledgements This thesis was greatly improved by the comments of my advisor Dr. F. R. O’Keefe and committee members Dr. S. Strait and Dr. P. Constantino. Description of the internal cranial anatomy of the study specimen would not have been possible without the patient consideration of Dr. M. Carrano, who arranged a loan of the specimen from the NMNH, and S. Jabo, who facilitated the transport of the material. This research was supported by a MURC student travel grant to W. Holloway. Additional thanks are due to N. Gardner, Dr. L. Witmer, and R. Ridgely for their assistance with various computer hardware and software issues, C. Richards for her support, K. Macys for her support and invaluable role as a test audience for my research ideas, and my mother Patricia for her varied and meaningful support throughout my academic career. i Table of Contents Acknowledgements ……………………………………………………….....…….……...i List of Figures ………………………………………..………………………...………..iii Abstract ………………………………………………………………………….......…...iv Chapter 1 - Background information and rationale ………….……………….…...1-21 Introduction………………..…………………………………………………....1-5 Background of phytosaurs………………….………..…………………..….…5-11 Background of CT study techniques……...……….....……...……..………...11-18 Rationale……...………...………………………………..……………..….…18-21 Chapter 2 - Materials, methods, and results ………………………..…………..…21-38 Materials………………..………………………………………………….…21-23 CT scanning and 3D visualization…..…………………………………….….23-29 Cranial endocast……………………………………………………….……..29-36 Endosseous labyrinth………………………………..…………….………….36-38 Chapter 3 - Discussion and Conclusions …………….………………………..…...38-52 Endocast Comparison…………………………………………………...……38-41 Comparison of archosaurian endocasts………………………....……...…….41-49 Future Work……………………......………………………….………..….…49-50 Conclusions………………………...………………………….…………..…50-52 Appendix ……………………………..……………………………….……….……….. 53 References ………………………..……………………………………...…………...54-62 ii Table of Figures Figure 1.1: Crurotarsi phylogeny………………………………………………………….6 Figure 1.2: Phytosaur morphology………………………………………………….…...11 Figure 1.3: CT scanner components………………………………………………..……13 Figure 1.4: Tyrannosaurus rex cranial endocast and pneumaticity……………...………15 Figure 1.5: CT slice density data………….………………………………….………….17 Figure 2.1: Skull of USNM 17098…………………………………....…….……………22 Figure 2.2: CT visualization process…………………………………………….…...25-29 Figure 2.3: Cranial cavity of USNM 17098 with break…………………………….…....32 Figure 2.4: Cranial endocast of USNM 17098…………………………………………..33 Figure 2.5: Head posture of USNM 17098……………………………………..………..38 Figure 3.1: USNM 17098 and Crocodile endocasts……………………....……………..40 Figure 3.2: Crurotarsi endocasts and phylogeny………...………………….…………....43 Figure 3.3: Theropoda endocasts and phylogeny ……………………………………….46 Figure 3.4: Sauropodomorpha endocasts and phylogeny …………………….…………47 iii Abstract A virtual phytosaur endocast and its implications for sensory system evolution in archosaurs By Waymon Holloway Due to the overall morphological similarities between the Triassic archosaurs of the order Phytosauria and extant crocodilians, most studies have assumed that the two shared similar lifestyles. Many studies involving phytosaurs have focused on the external cranial morphology of various taxa. Internal cranial anatomy has received relatively little attention. As a result, comparative morphology studies of the braincases interior, or endocast, of phytosaurs are an area of potential exploration. Just as modern medical X- ray computed tomography (CT) can be used to create three-dimensional images of internal structures of living subjects, such technology offers a non-invasive means of studying the internal anatomy of fossil specimens. CT scan data, used to virtually model and analyze the endocast of a phytosaur specimen, reveals an endocast morphology conserved among phytosaurs, crocodilians, and other Crurotarsi. This conservatism persists in taxa with numerous synapomorphies and distinctive ecologies, unlike the endocast specialization seen in other archosaurs like Dinosauria. iv Chapter 1 - Background information and rationale Introduction Paleontology is sometimes believed to be a restricted field of study because of a common misconception that very little determination about the lifestyle of an animal can be made from studying only its skeletal system. Although it is true that the skeletal system alone can be a poor indicator of life habits beyond those dependent upon a certain body shape or presence of certain osteological features, a close examination of the bones of an animal, living or dead, can reveal a great deal of information about other systems of the body (e.g. Witmer, 2005). For example, certain bones often exhibit muscle scars that would indicate the point of attachment, size, and orientation of various muscles and muscle groups. These data can then be used to create a model for the specifics of the method of locomotion, possible feeding habits, and any number of other muscle- dependent activities utilized by the animal. Similarly, the absence of bone in certain places or the size and shape of cavities within a skeletal structure can be an excellent source of information about the soft tissues of an animal. One region of the body that exhibits a number of important cavities is the skull. In particular, within the skull is housed the brain. Through advancements in technology, it has become possible to study the structure of the space occupied by the brain in great detail. Similar to more traditional paleontological methods of inferring information about the soft tissues based on features seen in the osteology of a specimen, an X-ray computed tomography (CT) aided virtual reconstruction of the endocasts of fossil specimens can be created and analyzed in order to describe previously overlooked aspects of the cranial anatomy (e.g. 1 Conroy & Vannier, 1984; Carlson et al., 2003), adding to the behavioral models associated with these specimens (e.g. Witmer & Ridgely, 2009), and enabling a better understanding of the taxonomic relationships of these specimens (Franzosa & Rowe, 2005). Understanding the functional anatomy and behavior of an extinct species can be an important source of information about why extant species live the way that they do. Although predicting precise aspects of organismal function based on the form or structure of individual morphological features is unlikely to be successful, the overall morphology of an animal and the behavior or ecology exhibited by that animal are linked closely enough to warrant general models (Lauder, 1995). In the case of morphologically convergent taxa (i.e. those that share a similar morphology that is not a result of a close phylogenetic relationship), similar behaviors are often assumed for both taxa based on an inference that similar morphologies are closely associated with similar behaviors. Such assumed behavioral similarities can be further supported by similarities in other aspects like habitat. Evolutionary convergence can also be seen in the morphology of more specific features of taxa that share overall similar lifestyles. The brain is one area of possible convergence between lifestyle-convergent taxa, even when those taxa are not considered to be convergent in terms of their overall morphologies. An example of this can be seen between pterosaurs and the earliest known bird, Archaeopteryx . Both pterosaurs, as a group, and Archaeopteryx are generally considered to have been aerial carnivores (e.g. Witmer, 2004), yet they each evolved from an entirely different evolutionary lineage (Laurin & Gauthier, 1996) and are not convergent in terms of their overall morphologies, 2 most notably their wing forms (e.g. Gauthier & Padian, 1985). In the brain and inner-ear canals of Archaeopteryx , an expansion and reorganization of areas that are neurologically important for flight can be seen, and although these features are more fully developed in modern birds, the development and reorganization present in Archaeopteryx would have been sufficient for a lifestyle involving full flight ability (Domínguez Alonso et al., 2004). The brain and inner-ear of pterosaurs exhibit an expansion and reorganization very much like that seen in Archaeopteryx (Witmer et al., 2003). This independently evolved similarity is an excellent example of convergence
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