Meeting Abstract
The family Castoridae is today represented by only two species, the Eurasian and the North American beavers. Both animals are large semi-aquatic rodents. The fossil record includes many more species; 70 are known from North America alone, some in deposits as old as 37 million years. The bulk of this diversity arose 30 million years ago with the evolution of many burrowing species. The oldest North American semi-aquatic rodent, the beaver Monosaulax, is 18.8 million years old. Despite the wealth of data on the locomotion of many burrowing beavers of the Oligocene and the semi-aquatic ones of the mid-Miocene to today, castorid evolutionary ecology remains enigmatic; the oldest species of beavers are known only from fragmentary craniodental remains. I here present the oldest postcranial remains of an anchitheriomyine beaver dated to 30 million years ago discovered in Montana. The dentition indicates that this animal is a new Microtheriomys species. I use a multivariate analysis of astragalus shape to infer its locomotory ecology. Based on a training set including 259 specimens representing 117 species of extant rodents and fossil beavers with known locomotion, I determine that Microtheriomys represents the oldest semi-aquatic rodent in North America. I also provide the first quantitative determination of the locomotion of three additional beaver species and the castoroid Eutypomys. I include the results of this ecomorphological analysis into an updated phylogenetic framework for 33 castoroid species with known locomotion. My ancestral character state reconstruction shows the evolution of the semi-aquatic ecology only once from terrestrial ancestors but hints at a more complicated evolution of burrowing.
