CRANIAL DISPARITY IN EXTANT GEOMYOID RODENTS: IMPLICATIONS FOR SPECIES BOUNDARIES IN THE FOSSIL RECORD

Entoptychine gophers (family Geomyidae) are an incredibly species-rich group of rodents whose dramatic rise in diversity 26 million years ago is still poorly understood. The pattern of this radiation is itself not fully documented; there remain many specimens of gophers undescribed in collections and significant holes in the early fossil record of Entoptychinae. One of the challenges associated with the description of the fossil record of entoptychine gophers revolves around their cranial morphology; much work has been undertaken in describing and quantifying the dental morphology of entoptychine gophers but no quantitative analysis of their cranial morphology has been undertaken to date. Within extant geomyoids (gophers and their sister family Heteromyidae), such analyses have mostly been undertaken for individual species and, rarely, several species within a single genus. Only one study, focused on Heteromyidae, has rigorously investigated variation in cranial morphology across geomyoid genera. We are building an extensive dataset of cranial variables for extant geomyoids to provide a biological basis for the identification of fossil gopher skulls at the species level. Specifically, we seek to develop a quantitative framework based on modern taxonomic units to identify morphological clusters in multispecies fossil assemblages. Our current dataset includes 333 specimens from 23 species representing all known heteromyid genera and four geomyid genera. We used multivariate statistics, ordinations, and analyses of disparity to compare within-group variation to between-group variation in our modern dataset and develop guidelines for species boundaries. We applied a similar approach to investigate the taxonomic affinities of two specimens of entoptychine gophers: a large skull from the John Day Formation contemporaneous with several species of Entoptychus and a small skull from South Dakota found in the same deposits as two species of Gregorymys. Our results suggest that the two fossil specimens both represent outliers that do not fall in the expected range of variation of contemporaneous species based on observed within-group variance in modern geomyoids but constitute evidence for an even more dramatic radiation of gophers than currently documented around 26 million years ago.