Arctocyonidae

Arctocyonidae (from Greek arktos and kyôn, "bear/dog-like") is as an extinct family of unspecialized, primitive mammals with more than 20 genera. Animals assigned to this family were most abundant during the Paleocene, but extant from the late Cretaceous to the early Eocene (66 to 50 million years ago). Like most early mammals, their actual relationships are very difficult to resolve. No Paleocene fossil has been unambiguously assigned to any living order of placental mammals, and many genera resemble each other: generalized robust, not very agile animals with long tails and all-purpose chewing teeth, living in warm closed-canopy forests with many niches left vacant by the K-T extinction.

Arctocyonids were early defined as a family of creodonts (early predators), then reassigned to the condylarths (primitive plant-eaters, now understood as a wastebasket taxon). More recently, these animals have been thought to be the ancestors of the orders Mesonychia and Cetartiodactyla, although some morphological studies have suggested that Arctocyonidae is also a wastebasket taxon for basal ungulates, and is in fact polyphyletic. As of 2015, the largest cladistic study of Paleocene mammals to date supports the idea that the animals in this group are not related, with Arctocyon and Loxolophus sister to pantodonts+periptychids, Goniacodon and Eoconodon sister to carnivores+mesonychids, most other genera allied with creodonts and palaeoryctidans, and Protungulatum not a placental mammal at all. If this analysis holds true, then any "Arctocyonid" characteristics are the result of coincidence (selection by the observer of characteristics shared by many early Tertiary mammals) or convergence (similar habits in life).