Allomys, MARSH, 1877

Hopkins, Samantha S. B., 2008, Phylogeny and evolutionary history of the Aplodontoidea (Mammalia: Rodentia), Zoological Journal of the Linnean Society 153 (4), pp. 769-838 : 794

ALLOMYS MARSH, 1877

Definition: The node group arising from the last common ancestor of Alwoodia magna Rensberger, 1983 and A. nitens Marsh, 1877 .

Revised diagnosis: Brachydont allomyines with numerous prominent accessory crests connecting cusps of cheek teeth and crenulate enamel in the basins of the lower teeth. The interpremaxillary foramen is only a pit. The hypolophulid is present, although it is weak in some taxa. The hypoconulid has a distinct crest, which in many Allomys is connected to the hypolophulid. The valley dividing the labial crest of the hypoconulid from the entoconid is at least partially closed, and is completely closed in some Allomys . The entoconid is shifted anteriorly on M 1, and an anteroconid is present anterior to the protoconid on P 4. The labial end of the protoconule joins the anterostyle and crosses the anterior basin of P 4. The protoconule has two parallel anterior crests on M 1–2.

Discussion: The only monophyletic group recovered within the traditional ‘Allomyinae’ was a grouping of Allomys and Alwoodia species. Although the species previously placed in the genus Alwoodia do seem to form a distinct clade, several of the phylogenetic analyses nest it within the clade formed by previously described Allomys species. The major feature distinguishing Alwoodia from Allomys is the presence or absence of accessory crests; however, specimens of Alwoodia can be found with a small number of accessory crests, particularly Alwoodia harkseni Korth, 1992a . Given the high morphological similarity between the two genera, and the fact that genera cannot be nested under a system of binomial nomenclature, the two species previously placed in Alwoodia are now assigned to Allomys . As Alwoodia is a monophyletic group, it is retained as a subgenus of Allomys . Allomys cristabrevis also seems to fall within this group, although more complete material would be necessary to be certain of its position. As A. cristabrevis is only known from a single isolated m1 or m2, determining its systematic affinities with certainty is currently difficult.

Referred taxa: See Table 1.

Geographic and stratigraphic range: From the Late Oligocene to the Early Miocene of Oregon, Montana, Wyoming, and South Dakota.