Although multituberculates are among the best-represented mammals of the Late Cretaceous and early Paleogene in North America, their evolution during the first several tens to hundreds of thousands of years following the Cretaceous–Paleogene (K–Pg) impact event is largely obscure. A better understanding of the early Paleogene record of multituberculates is crucial, for their dominance in early Paleocene mammalian faunas is unquestionably a result of rapid evolution during the immediate post-impact interval, and they accordingly played an important role in the evolution of mammalian communities more generally. I report on a new multituberculate from the early Paleocene of southwestern Alberta, in rocks of the Willow Creek Formation, the first such occurrence in this otherwise poorly known unit. The new multituberculate, Aenigmamys aries gen. et sp. nov., most closely resembles the ptilodontid Kimbetohia campi in comparable parts of the dentition and sheds light on the early evolution of Ptilodontidae, one of the major cimolodontan families that diversified during the Paleocene. The presence of Aenigmamys in mammalian faunas that lived soon after the K–Pg boundary implies a still-deeper evolutionary history for Ptilodontidae that may have extended into the Late Cretaceous. Aenigmamys is part of a new mammalian fauna from southwestern Alberta, the taxonomic composition of which includes a diversity of multituberculates, cimolestans, primates, and condylarths. The fauna correlates with those of middle Puercan age from other parts of the Western Interior of North America, and its high taxonomic diversity further corroborates previous hypotheses that multituberculate recovery — and mammalian recovery more generally — occurred relatively quickly after the K–Pg extinction event.

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