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

食草性海洋哺乳动物的进化：锡雷尼亚和沙漠的生态和进化转变Paul L.Kochin在现代海洋生态系统中，哺乳动物作为海洋植物的主要消费者扮演着次要的角色，食草性海洋哺乳动物(如海牛)的多样性显著低于肉食性海洋哺乳动物(如海豹或鲸鱼)。过去海洋食草动物的多样性比现在高得多。如何解释现代和古代生态系统之间海洋哺乳动物食草动物数量的差异？食草性饮食对生态和生理的要求与海洋食肉动物面临的要求有何不同？这些差异是否导致了这两个群体之间的多样性差异？为了回答这些问题，我们建议观察两个群体-西里安人(即海牛和儒艮)和相关的河马大小哺乳动物群体-结缔组织动物在进化过程中发生的生态和生理转变。我们将从三个时间间隔检查动物。首先，我们将研究始新世(50 Ma)以来最早的西里安人和剑尾龙的生态学，重点关注从陆地系统向海洋系统过渡的动物，并将这种栖息地的变化与身体大小和肢体形态等物理特征的变化联系起来。下一步，我们计划研究中中新世期间北太平洋的剑齿类和西里亚类之间的相互作用，以评估海洋资源是如何在这些类群之间分配的，以及太平洋生态系统如何能够维持比今天更高水平的海洋哺乳动物草食动物的多样性。最后，我们在太平洋的工作将继续，通过观察产生Steller海牛的谱系，探索是什么环境和生理因素促进了有史以来最大的海洋食草动物的进化。我们的研究将主要依靠三个稳定的同位素系统。首先，碳同位素将被用作饮食的替代品，这将使我们能够区分陆地和海洋系统的摄食。其次，氧同位素值的可变性将被用来代表每个群体的早期成员使用水生栖息地的程度。平均氧同位素值将作为依赖海洋生境与淡水生境的初步指标。最后，钙同位素将使我们能够确定营养水平，并评估早期的西里安人和黄毛虫是否在克服与从陆地向海洋过渡相关的潜在能量障碍时，用更高能量的食物资源(如肉类)补充他们的食草性饮食。这三个系统加在一起，将使我们能够评估这些群体的生态偏好，并更好地了解海洋食草动物进化所涉及的因素。

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.