Procreodi is an order of Paleocene and Eocene mammals thought to lie at the base of the radiation of the paraphyletic ‘condylarths.’ Taxa within the order have been linked to the origins of other condylarth groups, and of some living orders. Within the order, there are specializations indicative of a range of behaviors, and a considerable size range including some of the largest Paleocene mammals. Arctocyon mumak is the largest known arctocyonid. Several craniodental specimens from the Tiffanian of western North America and one partial skeleton, preserving parts of the fore- and hind limbs, pelvic and pectoral girdles, and some vertebrae, with associated teeth and other bony elements, are described here for the first time. Skeletal elements of A. mumak are larger than those of other species of Arctocyon and Anacodon, but are otherwise similar in overall morphology. Certain features of the tarsus, such as the large plantar tubercle on the navicular and the well-developed groove below the sustentaculum tali, are shared between A. mumak and Anacodon to the exclusion of Artcocyon and are suggestive of plantigrady and a degree of fossoriality. Univariate and multivariate statistical analyses of six ecomorphological ratios successfully distinguishes a taxonomically diverse group of 47 extant taxa with differing locomotor specializations. When calculated for Arctocyon mumak, these ratios support the view that this taxon was a terrestrial, possibly semi-fossorial taxon. Other taxa within Procreodi are recovered as more arboreal or more terrestrial. Significant ecological and morphological variation exists within this understudied group.

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