Major evolutionary innovations in lifestyle can lead to adaptive radiations, change the nature of selective forces, and rearrange ecological interactions. One of the many determinants of lifestyle is body mass, and so change in body mass is often accompanied by change in lifestyle. In the case of mammals, their earliest members from the Late Triassic were small. They remained relatively small throughout the Mesozoic, most likely due to interactions with the diverse large bodied diapsids that dominated Mesozoic large-body ecomorphs, especially dinosaurs in the terrestrial realm. It was not until the early Cenozoic, after the extinction of the non-avian dinosaurs and large bodied marine diapsids, that the body size of mammals expanded, which lead to new mammalian ecomorphs. The initial stages of the Cenozoic mammalian exploration of large-body ecomorphs included at least two novel semiaquatic forms – large amphibious herbivores and large coastal marine carnivores. These ecomorphs were to evolve multiple times throughout the Cenozoic and led to the freshwater Hippopotamidae, the marine Pinnipedia (seals and sea lions), as well as to the fully aquatic marine mammals, the sirenians (manatees and dugongs) and cetaceans (whales and dolphins).
This dissertation employes bone histological methods and the analysis of new fossil specimens to add to our paleontological knowledge of large bodied semiaquatic mammals. The earliest occurrences of these large-bodied semiaquatic forms are poorly understood, and gaps exist in our knowledge of their evolution and distribution. To help address these deficiencies I used bone histological methods to establish the lifestyle of the earliest large-bodied mammal, the Paleocene-Eocene Coryphodon. I follow this with the description and analysis of new fossil material that expand our knowledge of the diversity and biogeography of fossil pinnipeds and their earliest stem-relatives, the geologically longest-lived lineage of semiaquatic marine carnivores.
The Late Paleocene to Early Eocene Coryphodon was a member of the Pantodonta, the first mammalian group to achieve large body size following the end-Cretaceous mass extinction. Using histological and microanatomical techniques to examine the structure and growth of its bones, I show that Coryphodon was semiaquatic and further demonstrate that it had significant adaptations to this way of life rather than merely occupying waterways in a transient manner. A framework for behavioral inference for extinct taxa is presented, which incorporates evidence from multiple timescales. I argue that this approach facilitates the framing of the causal relationships between, and interpretation of, the different kinds of evidence that can be brought to bear on our understanding of the origins of major changes in lifestyle. In the case of Coryphodon, the concurrent achievement of large body size and semiaquatic lifestyle suggests a reciprocal rather than unidirectional causal relationship between body size and specialization at the beginning of the Cenozoic.
Turning to the origins and biogeography of the pinnipeds, the Pleistocene sea lion Proterozetes ulysses belongs to a monophyletic group of North Pacific otariids that includes the living genera Eumetopias (Stellar’s sea lion) and Zalophus (California sea lion). Fossil mandibles of Proterozetes from the Port Orford Formation of Oregon are described and found to be intermediate in morphology and size to the other North Pacific otariids, most resembling Eumetopias. The presence of this distinct but morphologically intermediate taxon in the eastern North Pacific during the Pleistocene establishes the Pleistocene as a time of increased otariid diversity, in contrast to diversity decline seen in other marine mammal clades. The timing of sea lion diversification along the west coast of North America appears to follow diversification in Japan, reinforcing the possibility of an eastward dispersal trend in the North Pacific after the end-Pliocene. Pleistocene sea lions in the Eastern North Pacific similarly show multitaxon assemblages with overlapping body size and feeding styles, indicating the high diversity possible once the coastal carnivore ecomorph was achieved.
Finally, paleontological investigation of the early history of pinnipeds similarly reveals assemblages of increased diversity and overlapping body size. The pinnipedimorph Enaliarctos was a marine-adapted carnivore with dental and locomotor features intermediate between terrestrial arctoid carnivores and living pinnipeds. Newly discovered fossils, described herein, fill a major biogeographic gap in the record of the genus, previously known nowhere between southern California and central Oregon, and show that stem pinnipeds existed for longer and survived later than previously thought, from the latest Rupelian to the late Burdigalian (28.1-16.6 mya). This discovery of an increased biogeographic and temporal range of Enaliarctos indicates that the genus may have interacted with a much wider range of other taxa than previously thought, including plotopterid birds, odontocete whales, and crown pinnipeds such as early odobenids, early otariids, and desmatophocids. The expansion of the known ranges of Enaliarctos species and the description of previously undescribed morphological attributes, particularly of the mandible and lower dentition, provides insight into the origins of pinniped diversity and their possible interactions with other early Neogene coastal marine organisms. Though early pinniped relatives were smaller than living members, increasing body size does not appear to have been necessarily superior once the coastal marine ecospace was colonized.
