The lower incisor enamel microstructure of the fossil rodent family Eomyidae was believed to be three‐layered and highly derived but rather uniform throughout the clade. Here, we describe a new four‐layered schmelzmuster in Eomyidae consisting of a three‐fold portio interna with longitudinal oriented, uniserial Hunter‐Schreger bands and a one‐fold portio externa, accounting for a unique enamel microstructure character combination in Rodentia. This new schmelzmuster type has developed early in eomyid evolution and is detectable already in the late Eocene (Chadronian) of North America. In European eomyids, it first occurs in the early Miocene (MN 3), implying that this four‐layered schmelzmuster was not present in all members of the family but restricted to species included in Eomyini and some genera currently considered Eomyidae incertae sedis within Eomyidae. Additionally, our analysis recognizes three taxa with schmelzmuster divergent from all other eomyids. Incisor enamel microstructure does not advocate a close phylogenetic relationship of Eomyidae to either fossil or extant Heteromyidae and Geomyidae, nor to fossil Heliscomyidae and Florentiamyidae. Our results rather support the view that Eomyidae are placed outside Geomorpha.

Recently discovered fossil material from the late Arikarcean of western Nebraska is referable to a new genus and species of castorid, Priusaulax browni. This species is the earliest record of a castoroidine beaver in North America. The skull possesses a combination of derived castoroidine features (divergent tooth rows, loss of stapedial foramen, premolars larger than molars, grooved palate) and primitive agnotocastorine features (elongated rostrum, procumbent incisors, contact between lachrymal and jugal bones). Morphologically and temporally, Priusaulax is transitional between the earlier agnotocastorines with more primitive characters and the later castoroidines with more derived characters.

The fauna of geomyoid rodents from the Runningwater Formation of western Nebraska is quite diverse, containing three heteromyids, two geomyids, two llorentiamyids and an indeterminate geomyoid.Two new genera, Stratimus (a heteromyid) and F animus (a florentiamyid) and five new species are described (Stratimus strobeli, Schizodontomys amnicolus, Pleurolicus hemingfordensis, Ziamys hugeni, and Fanimus ultimus).The Hemingfordian geomyoid fauna from Nebraska is unique and has little similarity to other rodent faunas of the same age.A marked increase in the overall diversity of geomyoids in the Hemingfordian is provided by the recognition of the Nebraska species.These species display a diversity intermediate between that of Arikareean faunas that are dominated by geomyids and Barstovian faunas that are dominated by heteromyids.