Physiological constraints of aquatic invasions in tetrapods
- William Benjamin Gearty.
- [Stanford, California] : [Stanford University], 2019.
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- Gearty, William Benjamin, author.
- Payne, Jonathan L., degree supervisor.
- Boyce, C. Kevin, degree committee member.
- Sperling, Erik, degree committee member.
- Stanford University. Department of Geological Sciences.
- Tetrapods invaded land more than 350 million years ago and have diversified enormously over time. Re-invasion of the aquatic realm has occurred across the tetrapod tree over the past 300 million years, but secondarily aquatic tetrapod lineages have not diversified to the same extent upon reinvasion of the water nor have they lasted for nearly as long. Such invasions are relatively uncommon most likely due to the differences in the physical and chemical conditions on land versus in water. Such differences require or favor different solutions to physiological challenges such as dehydration, thermoregulation, and salt balance as well as to other functions such as reproduction and locomotion. Many morphological changes associated with the transition to an aquatic lifestyle have been identified in extinct and extant tetrapods, such as vertical flattening of the tail and paddle-like limbs for more efficient swimming and dorsoventral flattening of the head for capturing prey in water. However, one remaining question in the evolution of these invaders is how these differing boundary conditions impacted the evolution of body size within these invading lineages. Chapters 1 and 2 address this question in two disparate tetrapod groups, Mammalia and Crocodyliformes. Comparative phylogenetic methods reveal that modern and fossil marine representatives of both groups have evolved to significantly larger sizes than their terrestrial counterparts, while they also maintain smaller degrees of variation and significantly faster rates of evolution. Physiological models that combine constraints from thermoregulation, metabolism, lung capacity, and feeding indicate that the regulation of body heat plays a major role in determining minimum viable sizes for mammals and crocodyliforms, while metabolism and feeding capacity may limit maximum sizes. Chapter 3 investigates similar aquatic invasions in snakes. While there is a significant increase in size as these marine snakes invade the ocean, it doesn't compare to the multiple orders of magnitude increases seen in mammals and crocodyliforms. Instead, it appears that the prior evolution of viviparity, coupled with the circumstantial occupancy of warm and low-elevation environments, facilitated these marine invasions by many ecologically diverse lineages. Combined, these studies reveal the strict demands of the aquatic medium that have caused a variety of adaptions by tetrapods before and during their aquatic invasions. In most cases, one or many key innovations are required in order to make such invasions possible. Further, these findings suggest that broad-scale patterns of morphological evolution within higher taxa are often determined more by physiological constraints than by ecological interactions or environmental fluctuations.
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- Submitted to the Department of Geological Sciences.
- Thesis Ph.D. Stanford University 2019.