What is "arctocyonidae"

Arctocyonidae (from Greek arktos kyôn, "bear/dog-like") is an extinct family of unspecialized, primitive mammals with more than 20 genera. The family was most abundant during the Paleocene, but extant from the late Cretaceous to the early Eocene . These animals are thought to be the ancestors of the orders Mesonychia and Cetartiodactyla, although some morphological studies have suggested that Arctocyonidae is a 'wastebasket' group for basal ungulates, and is in fact, polyphyletic