Miocene modernization of the North American lizard fauna

Scarpetta, Simon George
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Terrestrial ecosystems changed substantially during the Cenozoic era. The early Cenozoic was characterized by a warm and wet climate and ecosystems reminiscent of the modern tropics. Plant and animal communities similar to the modern biota were hypothesized to have appeared during the Miocene epoch. The phylogenetic, ecologic, and biogeographic assembly of the modern biota in North America and during the Miocene is well studied among fossil mammals and plants, but is not well documented in ectothermic vertebrates, such as lizards. I employed paleontological, biological, and geological data to address the formation of the modern lizard biota in North America, focusing on pleurodontan lizards (horned lizards, anoles, and relatives) and alligator lizards, which are prominent groups in modern ecosystems. My dissertation comprises three research foci: 1) Descriptions of fossil lizards, primarily from the Miocene but also from the Oligocene and Eocene, to evaluate paleodiversity, paleoecology, and paleobiogeography of focal lizard groups, 2) divergence time estimation using published genetic data and fossil calibrations to temporally constrain the evolution of focal lizard groups, and 3) exploration of fossil identification methodologies, especially fossil diagnoses made in the context of alternative phylogenetic hypotheses. I found one relatively modern lizard fauna from the middle Miocene of Wyoming that corroborates the records of terrestrial plants and mammals. However, I also described fossil occurrences that defy biogeographical (e.g., Abronia from the middle Miocene of California) and ecological (e.g., Uma from the late Miocene of California) expectations based on the biology of living lizards. The fields of historical biogeography and paleoecology draw heavily from the modern biota to make inferences into the past, but I suggest caution in that practice, because the distributions and ecological tolerances of organisms change through time. In the process of diagnosing and identifying a new fossil lizard (Kopidosaurus), I found that attention to phylogenetic hypotheses derived from molecular datasets altered the inferred phylogenetic position of the new taxon. Phylogenetic analyses are a popular avenue among paleontologists for fossil diagnosis, and neglecting alternative phylogenetic hypotheses can hinder fossil diagnosis and all downstream hypotheses that are contingent on accurate fossil diagnosis.