The cranium of the Hemingfordian beaver Anchitheriomys nanus Korth differs from the previously described skull of the Barstovian A. fluminis (Matthew) in being smaller and having less procumbent incisors, grooves on the palate, and the ethmoid and optic foramina more posterior in position on the skull. These features are viewed as being primitive in A. nanus relative to A. fluminis. The basicranium of A. nanus is preserved (unknown for A. fluminis) and demonstrates the derived castorid character of lacking the stapedial foramen in the auditory bulla, but has the apomorphic features of an elongated anterior process and a concave external meatus of the bulla (evidently a functioning part of the glenoid fossa). The paroccipital process is greatly broadened, another unique feature among castorids.
The cranial morphology of the earlier A. nanus suggests that the anchitheriomyines are more closely related to the advanced castorids (Castorinae and Castoroidinae) than the primitive agnotocastorines, with which they were previously allied. Anchitheriomys is part of a Miocene dispersal episode from Eurasia, in concert with numerous other rodents and insectivores.