Wikipedia
Purgatorius is the genus of the four extinct species generally believed to be the earliest example of a primate or a proto-primate, a primatomorph precursor to the Plesiadapiformes, dating to as old as 66 million years ago. The first remains (P. unio and P. ceratops) were reported in 1965, from what is now eastern Montana's Tullock Formation (early Paleocene, Puercan), specifically at Purgatory Hill (hence the animal's name) in deposits believed to be about 63 million years old, and at Harbicht Hill in the late Cretaceous and lower Paleocene Hell Creek Formation. Both locations are in McCone County.
They have also been widely discovered in the early Paleocene Bug Creek fauna, along with leptictids. These deposits were once thought to be latest Cretaceous, but it is now clear that they are Paleocene channels with time-averaged fossil assemblages. It is thought to have been rat-sized long and 1.3 ounces (about 37g) and a diurnal insectivore, who burrowed through small holes in the ground.