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Comparative Morphology of the Vestibular Semicircular Canals in Therian Mammals

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Comparative Morphology of the Vestibular Semicircular Canals in Therian Mammals Copyright by Jeri Cameron Rodgers 2011 The Dissertation Committee for Jeri Cameron Rodgers Certifies that this is the approved version of the following dissertation: Comparative Morphology of the Vestibular Semicircular Canals in Therian Mammals Committee: Timothy B. Rowe, Supervisor Christopher J. Bell James T. Sprinkle Edward C. Kirk Lawrence M. Witmer Comparative Morphology of the Vestibular Semicircular Canals in Therian Mammals by Jeri Cameron Rodgers, B.S.; M.S. 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 December 2011 Dedication To Michael, Genevieve, and Alexandra Rodgers Acknowledgements My sincerest thanks go to the long-suffering core of my committee: Tim Rowe, Chris Bell, and Jim Sprinkle. Each of these professors has shown patience and provided direction in so many different academic and personal areas of this doctoral trek. Tim Rowe, my advisor, accepted a student he knew nothing about and fulfilled his role as a mentor throughout many discussions and advisements. Chris Bell saw to the teaching of comparative osteology, proper usage of language, and provided an example of a great teacher. Jim Sprinkle gave me the first opportunity to be a teaching assistant, allowed me to accompany him in many student field trips, and showed how to collaborate on a scientific paper. Larry Witmer unknowingly initiated this dissertation through his offhand remark that “ears are hot.” Chris Kirk willingly stepped on to this committee in a moment of great need. These people truly deserve their designation as professors. Numerous non-committee people made this dissertation possible. Ted Macrini volunteered the use of many marsupial CT scans for this work, and also the use of Monodelphis domestica μMRI and HRXCT scans which he and Tim Rowe originally commissioned. He provided editing of the last chapter in this dissertation. Eric Ekdale shared his knowledge of the petrosal and bony labyrinth on many occasions, as well as helping me to learn the imaging programs with which he worked. Michael Cameron Rodgers spent untold hours listening to the requirements of a semicircular canal sensitivity program that I wanted. The staff of The University of Texas High-Resolution v X-ray CT facility (UTCT), particularly Jessie Maisano and Matt Colbert, spent many hours assisting me with scans and directions for working imaging programs and machines in general. Rich Ketcham provided the technical magic necessary to translate the *.nifti format to a format importable into VGStudioMax. Tim Hullar at Washington University in St. Louis, MO, corresponded with me at length regarding his program for canal sensitivity, and about the function of the vestibular system in general. Thanks are also due to the group known collectively as the paleo grad students for support, advice, and social opportunities. In particular, Lyn Murray, Jon Wagner, Jen Olori, Kerin Claeson, Sebastian Egberts, Heather Ahrens, Chris Jass, Christian George, Nick Smith, Michelle Stocker, and Sterling Nesbitt escorted me through these years. The specimens I scanned were paid for in part through the Jackson School of Geosciences Analytic Funds grants. The completion of this dissertation is a multi-generational success story. My parents and siblings though far away, provided support in many ways during critical phases of the work and only rarely asked when I would be done. In an extended family context George Rodgers and Robert Berlin helped me initiate and complete graduate school with support and patient listening. Michael, Genevieve, and Alexandra Rodgers grew up with a graduate student for a mother and did a marvelous job of raising me. My greatest thanks are reserved for them. vi Comparative Morphology of the Vestibular Semicircular Canals in Therian Mammals Publication No._____________ Jeri Cameron Rodgers, PhD. The University of Texas at Austin, 2011 Supervisor: Timothy B. Rowe The peripheral vestibular membranous ducts that detect angular motion are contained within bony semicircular canals of mammalian petrosals. I investigated morphology and function in the three membranous semicircular ducts through measurements on the bony semicircular canals of 31 skeletonized skulls from different genera. While the prevailing theory of semicircular canal researchers is that the locomotor agility of extant and extinct mammals can be understood by measuring the size of the three bony semicircular canal arcs, I propose that there are important and quantifiable features other than the adult size of radius of curvature of the semicircular canal arc (R) that influence angular movement detection in mammals and perhaps in their ancestors. Initially, I sought to verify that there was no significant asymmetry of R across the study specimens. However, there was significant asymmetry in canal pair angles vii between contralateral sets: ipsilateral canal pair angles differed by up to 14◦, and contralateral synergistic angle pairs differed by up to 18◦. Canal pair contralateral differences were lower for specimens of more agile taxa. In addition, the angle between the left and right lateral canals varied by up to 27◦ from parallelism, so the use of the lateral bony canal in one petrosal to represent the horizontal animal resting position could result in significant skull orientation errors. I utilized a program to quantify the effects of canal plane non-orthogonality and to calculate a maximum rotational sensitivity axis for a given taxon. My results concur with earlier research indicating that canal orientation significantly affects the location of maximum rotational sensitivity axes in the head, and should be considered in future quantitative research. Finally, I determined the volumes of the subarcuate fossa and the petrosal lobule in three Monodelphis domestica animals (76 days postnatal) by utilizing both cranial and tissue volumes in fresh specimens. The petrosal lobule fills 93-97% of the Monodelphis domestica fossa, a greater volume than the 50% estimated by previous researchers. These results highlight the difficulties of using histologic or preserved specimens to make quantitative determinations of brain tissue volumes, and reopen the question of whether the subarcuate fossa volume provides a record of the agility for an extinct taxon. viii Table of Contents List of Tables ........................................................................................................ xii List of Figures ...................................................................................................... xiv CHAPTER 1: INTRODUCTION TO THE BONY SEMICIRCULAR CANAL SYSTEM OF MAMMALS ............................................................................1 General Structure of the Membranous Semicircular Ducts ............................2 Purpose of This Doctoral Research .................................................................3 Study Considerations ......................................................................................3 Dissertation Overview ....................................................................................5 CHAPTER 2: COMPARISON OF CONTRALATERAL SEMICIRCULAR CANALS IN THERIAN MAMMALS ...........................................................9 ABSTRACT ....................................................................................................9 INTRODUCTION ........................................................................................10 Background ..........................................................................................11 Questions..............................................................................................13 Symmetry of R for contralateral canals (Question 1) ..........................14 Symmetry of canal pair angles (Question 2)........................................15 METHODS ...................................................................................................16 Reference Planes ..................................................................................17 Semicircular Canal Imaging ................................................................18 Comparison of R Calculation Methods................................................19 Defining the Orientation of Semicircular Canals .................................21 RESULTS .....................................................................................................22 Comparison of R calculation methods .................................................22 Radius of curvature for contralateral canals (Question 1) ...................24 Symmetry of angles between canal pairs (Question 2) ........................25 DISCUSSION ...............................................................................................28 Comparison of R calculation methods .................................................28 Radius of curvature for contralateral canals (Question 1) ...................29 ix Symmetry of angles between canal pairs (Question 2) ........................30 Study limitations ..................................................................................34 CONCLUSIONS...........................................................................................35 TABLES .......................................................................................................38 FIGURES ......................................................................................................53
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