Calede Jonathan J M
Department of Evolution, Ecology, and Organismal Biology, The Ohio State University at Marion, 1459 Mount Vernon Avenue, Marion, OH 43302, USA.
R Soc Open Sci. 2022 Aug 24;9(8):220926. doi: 10.1098/rsos.220926. eCollection 2022 Aug.
The North American rodent fossil record includes hundreds of species representing both an incredible taxonomic diversity and great ecological disparity. Although it is during the Oligocene that taxonomic diversity first peaks, it is not until the Miocene, almost 10 Myr later, that many ecologies, particularly locomotory ecologies, are recorded. Here, I present a new Oligocene-aged species of beaver from Montana, sp. nov., which represents the oldest semi-aquatic rodent in North America and the oldest amphibious beaver in the world, pushing the advent of semi-aquatic ecology in beavers by 7 Myr. I also provide morphological data supporting a terrestrial ecology for the sister taxon to Castoridae. Together with existing data, these findings lead to a new hypothesis for the evolutionary ecology of castorids whereby swimming was exapted from burrowing during the Oligocene. This evolution of semi-aquatic locomotion may have taken place in North America instead of Eurasia. It started in small beavers with gigantism achieved only much later. Indeed, body size evolution in castoroids follows a directional drift. Beavers obey Cope's rule, a selection for larger size over time that appears associated with semi-aquatic ecology and may well explain their low modern diversity.
北美啮齿动物化石记录包含数百个物种，展现出令人难以置信的分类学多样性和巨大的生态差异。虽然分类学多样性在渐新世首次达到峰值，但直到近1000万年后的中新世，许多生态类型，尤其是运动生态类型才被记录下来。在此，我描述了一种来自蒙大拿州的渐新世新物种河狸，新物种，它是北美最古老的半水生啮齿动物，也是世界上最古老的两栖河狸，将河狸半水生生态的出现时间提前了700万年。我还提供了形态学数据，支持河狸科姐妹分类群的陆地生态。结合现有数据，这些发现为河狸科动物的进化生态学提出了一个新假说，即游泳是在渐新世从挖掘行为转变而来的。这种半水生运动的进化可能发生在北美而非欧亚大陆。它始于小型河狸，体型增大则是很久之后的事。实际上，河狸类动物的体型进化呈现出一种定向趋势。河狸遵循柯普法则，即随着时间推移选择更大的体型，这似乎与半水生生态相关，也很可能解释了它们如今较低的多样性。
