A ‘giant’ purgatoriid ( P lesiadapiformes) from the P aleocene of M ontana, USA : mosaic evolution in the earliest primates

U rsolestes perpetior gen. et sp. nov. is a large purgatoriid plesiadapiform primate from the middle/late P uercan (earliest P aleocene) S impson Q uarry, M ontana, USA . It differs from P urgatorius V an V alen and S loan, 1965, the only other known purgatoriid and the oldest discovered primate, in its substantially larger body size as inferred from the dimensions of the known parts of its dentition, while resembling P urgatorius in possessing a large lower canine, unreduced p1, and in the upper molars, a protocone twist and weakly developed postprotocone fold; moreover, the m3 of U . perpetior possesses an expanded, plesiadapiform‐like posterior lobe of the talonid that supports the hypoconulid. The large lower canine and the greater development of trenchant molar crests suggest that U . perpetior was more insectivorous, differing appreciably from the omnivorous niche long hypothesized for P urgatorius and purportedly marking the insectivore–primate transition. Hence, U . perpetior adds significant new knowledge regarding the breadth of the adaptive zone occupied by purgatoriids in their position at the very beginnings of primate evolution.