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

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动物，其作用较小，食草性海洋哺乳动物（如海牛）的多样性明显低于食肉性海洋哺乳动物（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中海洋哺乳动物和食草动物数量的差异？草食动物的生态和生理要求与海洋食肉动物有何不同？这些差异是否导致了这两个群体之间多样性的差异？为了回答这些问题，我们建议研究两类动物在进化过程中发生的生态和生理转变——海牛类（即海牛和儒艮）和一种与河马大小的哺乳动物相近的类群——近毛类动物。我们将分三个时间间隔对动物进行检查。首先，我们将研究始新世（50 Ma）最早的sirenians和desmostylians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特征（如体型和肢体形态）的变化联系起来。接下来，我们计划研究中新世中期北太平洋的石柱动物和海龙动物之间的相互作用，以评估海洋资源如何在这些群体之间分配，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食草动物多样性。最后，我们在太平洋的工作将继续研究产生斯特勒海牛的血统，探索哪些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代表，这将使我们能够区分陆地和海洋系统的摄食。其次，氧同位素值的变异性将被用作每个群体早期成员使用水生栖息地的程度的代表。平均氧同位素值将作为对海洋生境和淡水生境依赖程度的初步指标。最后，钙同位素将使我们能够确定营养水平，并评估早期海陆动物和海陆动物是否在克服从陆地向海洋过渡的潜在能量障碍时，以更高能量的食物资源（如肉类）补充草食性饮食。总之，这三个系统将使我们能够评估这些群体的生态偏好，并更好地了解与海洋食草动物进化有关的因素。

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.