Evolution of the Tetrapod Forelimb and Functional Morphology of the Humerus across Water-Land Transitions

Abstract

In transitioning from water to land, tetrapods faced a plethora of changing environmental challenges, such as preventing desiccation, breathing in air, and locomoting on land. Throughout evolutionary history, the shift from an aquatic to terrestrial environment has driven the evolution of robust and muscularized appendages that allow vertebrates to overcome the forces of gravity and navigate land. Understanding the functional evolution of limbs and limb-like structures has been a major locus for scientific investigation for decades, yet the development of new technologies allows for entirely novel sources of insight. In this dissertation I implement cutting-edge methods in quantitative functional morphology to investigate the functional underpinnings and adaptations in the limbs and appendages of vertebrates spanning the water-land transition.

In \textit{Chapter 1} I implement contrast-enhanced µCT scanning and digital myology to investigate the skeletal and muscular adaptations of the fins of benthic anglerfish to understand how limb-like fins can evolve for use underwater. In \textit{Chapters 2} and \textit{3}, I develop and implement cutting-edge methods in constructing ecologically informed functional adaptive landscapes to understand the functional evolution of the humerus in response to transitions in ecology. In \textit{Chapter 2} I apply these analyses to understand the functional adaptations of the humerus of marine, semi-aquatic and terrestrial turtles, one of the few taxonomic groups that actively spans the water-land boundary. In \textit{Chapter 3}, using functional adaptive landscapes, I reconstruct the functional evolution of the humerus in stem and early tetrapods across the water-land transition and infer when tetrapods became functionally adapted to terrestrial locomotion. Finally, in \textit{Chapter 4} I investigate the changes in functional morphology of the humerus through ontogeny in early amphibians to provide deeper insight into the locomotor ecologies of the dominant tetrapods of early terrestrial ecosystems.

The results I present in this thesis demonstrate that the utilization of new approaches in quantitative biology can produce novel insights into the evolutionary drivers and ecological adaptations surrounding the shift from water to land, and can provide new answers to old questions in evolutionary biology.