A sample of upper and lower dentitions of a large paramyid is described from the upper part of the Willwood Formation of the Bighorn Basin (late Wasatchian biozone Wa-6, = “Lysitean”). The specimens are referred to Notoparamys costilloi, originally described from the Lysitean of the Huerfano Formation of southern Colorado. The sample shows considerable variation, interpreted here to represent one highly variable species. This leads to the conclusion that many characters that have been used to distinguish species and even genera among paramyids might in some cases reflect intraspecific variability.
“Pseudotomus” coloradensis and Leptotomus loomisi are probably junior synonyms of Notoparamys costilloi. The incisor enamel of N. costilloi is pauciserial, as in other paramyids. The enamel of P4 (and presumably the molars) has three zones: outer and inner zones of radial enamel and a middle zone of multiserial Hunter-Schreger bands. This constitutes the S-type schmelzmuster, a condition more derived than in some other paramyids but characteristic of sciuromorphs, sciuravids, hystricognaths, and various other rodents. Occlusal study reveals a relatively continuous mesiolingual movement of the lower molars across the uppers during phases I and II, as in Paramys.
