RECONSTRUCTING BURROWING BEHAVIOR IN THE MIO-PLIOCENE SQUIRRELS OF THE COLUMBIA BASIN

The detailed fossil, climatic, and paleoenvironmental records of the Columbia Basin have long made it the focus of research on the causes and effects of the Cenozoic spread of grasslands. Among large mammals, these effects include an increase in cursoriality both in many ungulate lineages and in pursuit predators, but the impacts on small mammal locomotion were likely just as profound. Due to their wide range of lifestyles – from burrowing prairie dogs to gliding “flying” squirrels – sciurids offer an especially valuable opportunity to study the locomotor impacts of the transition to grasslands. Burrowing is recognized as an adaptation to open environments and identifying fossorial taxa in the fossil record could reveal a great deal about the tempo and mode of ground squirrel evolution. A data set consisting of modern taxa suggests that humeral morphology – specifically the robusticity of the humerus at mid-shaft and the presence of a small humeral head and enlarged deltopectoral crest – can be a reliable indicator of fossoriality. Here I apply these findings to the rich sciurid fossil record of the mid-Columbia Basin, and in particular to Otospermophilus mckayensis from the Hemphillian McKay Formation (Oregon) and Ammospermophilus hanfordi from the Blancan Ringold Formation (Washington). A principal components analysis suggests that these taxa were not only fossorial, but that their mode of burrowing was likely identical to that of extant ground squirrels. This result is unsurprising, given the young age of these squirrels and their close relationship to living species of Otospermophilus and Ammospermophilus. However, the record of squirrels in the Columbia Basin dates back to the Oligocene of the John Day Basin, and future analyses of sciurid postcrania in the region could shed light on the timing and drivers of the evolution of burrowing.