THE DIVERSIFICATION OF THE ENTOPTYCHINAE DURING THE OLIGOCENE: INSIGHTS FROM THE GREAT NORTHWEST

The fossil record of the northwestern United States captures numerous diversification events of the Cenozoic. One of them, the rise in species richness of rodents during the Oligocene, is recorded in several deposits of Montana and Oregon, including the John Day Formation. The dominant driver of this increased rodent biodiversity is the radiation of three families: the Aplodontiidae, Castoridae, and Geomyidae. We here present an updated phylogenetic analysis for Oligocene-aged geomyids (subfamily Entoptychinae) including 43 operational taxonomic units, four of which represent previously undescribed species from the great northwest, and 108 morphological characters. Our results show that the species Entoptychus cavifrons and E. minor should be synonymized, and that Gregorymys kayi is in fact a member of the genus Entoptychus. Additionally, our analysis suggests the existence of two clades within the otherwise paraphyletic genus Pleurolicus.

Our time-calibration of the tree enables us to investigate the speciation and extinction rates within Entoptychinae as well as the biogeographic history of the clade. We use a Bayesian analysis of macroevolutionary mixtures (BAMM) to determine speciation, extinction, and net diversification rates across the tree of Entoptychinae. We recover very high speciation rates at the base of the tree, leading to the establishment of all genera; speciation rates are much lower within genera. Extinction rates are very high within the genus Entoptychus as well as the clade including all Pleurolicus species from Montana, Wyoming, and Oregon; they are very low within the genera Gregorymys and Ziamys as well as the species of Pleurolicus from the Great Plains. The biogeographic analysis undertaken using BioGeoBEARS shows that the high speciation rates are associated with cladogenetic events in the northern Great Plains and the high extinction rates are associated with taxa ranging across the great northwest, specifically the northern Rocky Mountains and the Columbia Plateau. Together, these analyses paint an unexpected picture of the evolutionary history of entoptychines wherein tectonically active topographically complex regions were associated with high extinction rates and tectonically passive areas with high speciation rates.