Members of the Mylagaulidae have been known for over a century to bear nasal horns; the only rodents, extinct or extant, ever to have done so. This striking feature is known from five of the over 30 species of mylagaulid rodents discovered across North America and Eurasia, all relatively large animals that were likely less fossorial than their relatives. We describe herein a sixth new species of horned mylagaulid. This new taxon from Sioux County, Nebraska, offers the opportunity to reassess the phylogenetic relationships of Mylagaulidae and test several evolutionary hypotheses. Our analyses demonstrate that horns evolved only once in Mylagaulidae, in the common ancestor of Ceratogaulus, first as short horns exapted from the thickened nasals of fossorial ancestors, and later as taller horns. The horns evolved following a positive allometric scaling with body mass that suggests a response to predation pressure in these nearly blind animals. The evolution of tall horns also corresponds to a jump in body mass. The largest mylagaulids are not horn-bearing species, however. Additional analyses of the complex pattern of body mass evolution we reveal will be necessary to explain the evolution of the largest head-lift digging rodents in Earth history. https://zoobank.org/urn:lsid:zoobank.org:pub:81FE999A-F79E-4BD4-9A81-2C7D3D5D81CD.

Cricetidae ranks as the second-most species-rich and abundant mammalian family, with limited studies on eastern North American records prior to the Pleistocene. While cricetids has been previously noted at the early Pliocene Gray Fossil Site (GFS), this study provides a detailed description of eight taxa: Postcopemys (two species), Symmetrodontomys, Oryzomyini, Peromyscus, Neotoma, Neotomodon, and Xenomys. Postcopemys is the most common cricetid taxon at GFS, followed by Peromyscus and Neotoma. These records expand the stratigraphic and geographic range of multiple genera. Distinctive morphological features of GFS taxa suggest presence of several new species. The GFS cricetid assemblage exhibits diverse body sizes and dietary preferences, setting GFS apart from other contemporaneous sites and emphasizing its spatial and temporal uniqueness. The Appalachian region represents a biodiversity hotspot today, and GFS was likely an important habitat for cricetid evolution during the Pliocene.

No description available.