Date of Award

Fall 1-1-2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geology and Geophysics

First Advisor

Gauthier, Jacques

Abstract

Among the living terrestrial vertebrates, the greatest diversity of verifiable species is seenamong the squamates, the group which contains the lizards and snakes. With a global distribution limited only by their general intolerance to long periods of cold temperatures, they are a ubiquitous component of modern ecosystems. Furthermore, over their roughly 180-million-year evolutionary history they have achieved tremendous morphological disparity and a range of body sizes that spans three orders of magnitude. However, our understanding of the origins and early evolution of squamates remains obscured due to persistent conflict in hypotheses generated from morphological and molecular data. This discordance is one of the largest of its kind in the vertebrate tree of life, and has major implications for our understanding how squamates acquired their immense diversity and disparity. My dissertation uses cutting edge 3D imaging techniques to interrogate the morphological components of the taxon-character datasets that are used to generate these hypotheses. In doing so I address two fundamental questions that underpin the study of squamate evolution in the light of persistent phylogenetic conflict. In chapters 1 and 2, I address the question of whether fossil squamates can be reliably placed on the tree of life despite conflicting interpretations of squamate evolution, and demonstrate that indeed they can. My first chapter describes and names a new species of Jurassic pan-gekkotan with major implications for the acquisition of the characteristic anatomy of modern Gekkota. My second chapter provides amonographic description of an Oligocene stem-glass lizard (Parophisaurus pawneensis) which provides critical insight into the ancestral limbed morphology of this modern limbless group, and which brings data from morphology and the fossil record into congruence with estimations of divergence times and biogeography from molecular data. In chapters 3 and 4, I address the question of whether new morphological data from under sampled regions of squamate anatomy may find previously unknown anatomical support for molecular phylogenetic hypotheses. My third chapter uses 3D CT imagery to bypass the traditional obstacles of size and disarticulation to comparatively describe the individual elements of the squamate ankle and wrist. From these, I identify a suite of new phylogenetic characters. The distribution of these characters is broadly more compatible with the molecular squamate tree, and they provide new (and in some instances the only) morphological support for traditional molecular clades. In chapter four I use 3D geometric morphometric techniques on four elements of the ankle and wrist to assess their association with phylogeny and ecology. I find significant phylogenetic signal that validates their use in inferring evolutionary relationships in squamates, and in some elements also find significant association with ecological factors. Investigating the link between shape and function reveals that ancestrally acquired versatility in the squamate ankle may allow them to utilize specialized locomotory strategies without aggressively specializing their anatomy.

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