A new stem geomyoid helps elucidate the palaeoecology and evolutionary history of geomorph rodents

Geomyoid rodents are one of the most species-rich and ecologically diverse rodent groups in North America since the Oligocene, but their evolution remains poorly understood. The taxonomic affinities of many species are enigmatic, and several taxa have been assigned to various families, alternatively considered clades or paraphyletic, or moved in and out of the crown group. Additionally, morphological phylogenetic analyses have showed conflicts with analyses based on molecular data. We present here an updated phylogenetic framework integrating morphological data for extant and fossil taxa and a backbone tree constraint from molecular data. Included in the tree is a fossil from Colorado, which represents a new, very large, stem geomyoid species. This new material, which includes a complete astragalus, enables the exploration of the ancestral locomotion of geomyoids. Using a training set of living and extinct rodents, we infer the locomotion for the new species and 11 other fossil taxa. Our results support a semi-fossorial ecology for the new species. This is consistent with predictions from ancestral character state reconstructions based on extant taxa that living pocket mice represent a reasonable model of the ancestral bauplan of geomyoids. http://zoobank.org/urn:lsid:zoobank.org:pub:63145542-2FCD-4AEE-9A6E-52104269FE12