Cimolestes Marsh is a North American eutherian mammal primarily known from latest Cretaceous deposits in Alberta, Wyoming, Saskatchewan, and Montana. At present, five species of Cimolestes are considered valid, all Lancian in age; they include one of the largest North American Late Cretaceous therian mammals as well as one of the smallest, a size range far exceeding that within other genera of tribosphenic therians contemporary with Cimolestes, such as the leptictoid eutherian Gypsonictops Simpson or genera of alphadontid or pediomyid marsupials. Moreover, the species of Cimolestes display a disparity of dental morphology in addition to size well in excess of interspecific differences within these genera. Given these considerations, Cimolestes is clearly a grade-taxon, uniting species sharing an adaptively subzalambdodont dentition, but showing divergent specializations within this pattern that are inconsistent with monophyly of its presently included species. To correct these imbalances, this paper limits Cimolestes to Cimolestes incisus Marsh and Cimolestes stirtoni Clemens; Cimolestes magnus Clemens and Russell, Cimolestes cerberoides Lillegraven, and Cimolestes propalaeoryctes Lillegraven are reclassified in the new genera Altacreodus, Ambilestes, and Scollardius, respectively. Altacreodus magnus, having a massive shearing dentition, is reconfirmed as showing a relationship to some Tertiary ‘creodonts’ not shared by other species of Lancian cimolestids; Ambilestes cerberoides exhibits a distinctive molar wear pattern that emphasized horizontal grinding, not orthal shear; Scollardius propalaeoryctes, the smallest species in this revision and having hyper-faunivorous molars, was not ancestral to Paleogene Palaeoryctidae, as indicated in part by contradictions in premolar number and morphology.

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