Reconstructing the evolutionary histories of aquatic and macrocarnivorous lifestyles in reptiles

Date of Award

Spring 1-1-2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geology and Geophysics

First Advisor

Bhullar, Bhart-Anjan

Abstract

The Mesozoic Era (252–66 Myr ago) is often called the “Age of Reptiles,” as it saw reptiles become the dominant megafaunal constituents of both aquatic and terrestrial ecosystems globally for the first (and perhaps last) time in Earth’s history. During the Mesozoic, reptiles radiated into new ecological and functional spaces, evolving both macrocarnivorous habits (the tendency to eat massive prey) and highly or fully aquatic habits (the tendency to live primarily in the water). My thesis considers when and in what clades these ecological radiations took place, as well as what key morphological innovations may have potentiated them. To answer these questions, we rely on an imperfect fossil record, which often provides scant or conflicting lines of evidence about when and how these new ecologies first arose. To circumvent these issues, I use a combination of new and emerging computational methods that allow me to reconstruct the anatomical transformations associated with the evolution of aquatic and macrocarnivorous habits in Mesozoic reptiles. In my first dissertation chapter, I develop a supervised phylogenetic machine-learning approach to predict aquatic habits and associated soft-tissue phenotypes in tetrapods. I generate the largest morphometric dataset of tetrapod limbs to date, score the aquatic habits and soft-tissue phenotypes of all extant taxa in the dataset, iteratively train phylogenetic logistic regression models to predict these features, and compare the accuracy of competing predictors with receiver-operating characteristic analysis. I find that the relative lengths of different limb regions can determine the aquatic habits and flipper phenotypes of extinct taxa with >90% accuracy, and use this best predictor to reconstruct their evolutionary histories in all major lineages of Triassic marine reptiles. This work highlights when and in what reptilian clades aquatic habits first arose, and reveals key innovations in limb morphology that facilitated this recurring ecological transition. In my second dissertation chapter, I develop a three-pronged approach to distinguish two kinds of trace fossils (regurgitalites and coprolites) by evaluating the gastric etchings on specimen bones, the preservation of associated soft tissues, and the chemistry of the surrounding matrix. This method identifies a formerly ambiguous Triassic trace fossil as a regurgitalite and highlights the potential of regurgitalites to facilitate dietary reconstructions in extinct reptilian carnivores. In my third chapter, I reconstruct the key musculoskeletal transformations in the skull associated with the earliest advent of macrocarnivory in reptiles. A synthesis of existing morphological and trace-fossil evidence suggests that among reptiles, macrocarnivorous habits first evolved on the stem lineage of Archosauria (the clade that includes dinosaurs and crocodilians). Using digital segmentation and mesh-manipulation software, I reassemble the formerly flattened or disarticulated skulls of three key transitional fossils on the archosaur stem and reconstruct their jaw adductor muscles in silico. In doing so, I find that a fundamental reorganization of the jaw muscles in pan-archosaurs (a drastic expansion of the internal adductors at the expense of the typically dominant external adductors) enabled this clade to access a higher functional optimum for bite strength, gape, and skull stability, and thereby achieve more powerful bites than any other terrestrial carnivores in Earth’s history.

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