The Evolution of Herbivorous Marine Mammals: Ecological and Evolutionary Transitions in the Sirenia and Desmostylia

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The Evolution of Herbivorous Marine Mammals: Ecological and Evolutionary Transitions in the Sirenia and DesmostyliabyPaul L. KochIn modern marine ecosystems, mammals play a minor role as primary consumers of marine plants, and the diversity of herbivorous marine mammals (e.g., sea cows) is significantly lower than that of carnivorous marine mammals (e.g, seals or whales). The diversity of marine herbivores in the past was much higher than today. What explains this disparity in the number of marine mammal herbivores between modern and ancient ecosystems? How do the ecological and physiological requirements imposed by a herbivorous diet differ from those faced by marine carnivores? Do these differences contribute to the differences in diversity between these two groups?To answer these questions, we propose to look at the ecological and physiological transitions that occurred during the evolution of two groups - sirenians (i.e. manatees and dugongs) and a related group of hippo-sized mammals called desmostylians. We will examine animals from three time intervals. First, we will examine the ecology of the earliest sirenians and desmostylians from the Eocene (50 Ma), focusing on animals involved in the transition from terrestrial to marine systems, and correlating this change in habitat with changes in physical features, such as body size and limb morphology. Next, we plan to study the interaction of desmostylians and sirenians from the north Pacific during the Middle Miocene to assess how marine resources were partitioned between these groups, and how Pacific ecosystems were able to sustain a higher level of diversity of marine mammalian herbivores than today. Finally, our work in the Pacific will continue by looking at the lineage that gave rise to the Steller's sea cow, exploring what environmental and physiological factors facilitated the evolution of the largest marine herbivore ever.Our study will chiefly rely on three stable isotope systems. First, carbon isotopes will be used as a proxy for diet that will allow us to discriminate between feeding in terrestrial versus marine systems. Second, variability in oxygen isotope values will be used as a proxy for the extent to which early members of each group used aquatic habitats. Mean oxygen isotope values will serve as a preliminary indicator of reliance on marine versus freshwater habitats. Finally, calcium isotopes will allow us to identify trophic level and assess whether early sirenians and desmostylians supplemented their herbivorous diet with a higher energy food resource (e.g, meat) as they overcame potential energetic barriers associated with the transition from land to sea. Together, these three systems will allow us to assess the ecological preferences of these groups, and better understand the factors involved in the evolution of marine herbivores.