Mesozoic Origins of Diversity and Global Biogeographic History in the Order Squamata
Open Access DepositedSquamata is a diverse clade of vertebrates comprising the ~11,000 species of lizards, snakes, and amphisbaenians. Their Mesozoic evolutionary origins date back to the Middle Triassic, with a patchy fossil record throughout the Jurassic and a well-represented fossil record in the Cretaceous (Simões and Pyron 2021). This includes groups that achieved high diversity prior to going extinct along with the dinosaurs at the KPg boundary but set the foundation for current squamate biodiversity. Modern squamates have achieved a near global distribution, but their biogeographic origins have not been examined in detail. In my dissertation, I explore the interplay of biogeography, fossils, and systematics for Squamata in Deep Time. In my first chapter, I performed a systematic analysis of the extinct lizard clade Polyglyphanodontia, a group of large-bodied Late Cretaceous lizards. This lineage showcases a range of unique cranial and dental morphology and body sizes corresponding to herbivory, something rare in reptiles. In my second chapter, I performed the first-ever ancestral-range estimation for Squamata, utilizing a molecular phylogeny with fossils as biogeographic constraints. In my third chapter, I performed a fossil-based ancestral-range estimation for Squamata with a biogeographic analysis that included fossil terminals in total-evidence trees and plate-tectonic models. These three chapters contribute greatly to our understanding of Late Cretaceous lizards and the biogeography of Mesozoic squamates that helped establish their modern diversity. In Chapter 1, I discovered and described new morphological characters from fossil specimens to place Polyglyphanodontia on the stem of Teiioidea (Teiidae + Gymnophthalmidae). I also provide updated definitions and morphological diagnoses for multiple higher-order clades. This clarifies the relationships within the Squamate Tree of Life, especially in the evolution of Laterata, a clade that includes Gymnophthalmidae, Teiidae, Lacertidae, and Amphisbaenia (Vidal and Hedges 2005). In both biogeographic analyses (Chapters 2 and 3), I estimate Eurasia as the origin of diversity for Squamata. In my tectonically informed analysis using total-evidence trees, I reconstruct Europe and northeastern Asia as ancestral range within Pangaea. Both analyses estimate multiple independent dispersals from Laurasia to Gondwana in the mid-Cretaceous, in several lineages. This provides foundational insight into the modern distribution and diversity of squamates. My thesis provides unique insights into the early origin and evolution of Squamata, one of the most diverse, widespread, and widely studied vertebrate groups. My phylogenetic reconstruction is foundational for future studies on the macroevolutionary trends in squamates, including development of herbivory. I have also generated the first two biogeographic ancestral range estimates to determine the geographic origin of major lineages in the Mesozoic. Overall, I promote an integrative perspective on the study of natural history, one that includes data from molecular, morphological, geographical, and paleontological sources. This best allows for robust conclusions on the evolutionary history of clades in Deep Time.
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