PURGATORIUS (PLESIADAPIFORMES, PRIMATES?, MAMMALIA), A PALEOCENE IMMIGRANT INTO NORTHEASTERN MONTANA: STRATIGRAPHIC OCCURRENCES AND INCISOR PROPORTIONS

Abstract

The earliest unquestionable records of Purgatorius (Plesiadapiformes, Primates?, Mammalia) in northeastern Montana and other areas of the Western Interior of North America are of early Paleocene, specifically middle or late Puercan (Pu 2–3 Interval Zones, undifferentiated) North American Land Mammal Age (NALMA). The report of an occurrence of Purgatorius in the Late Cretaceous (Lancian NALMA) was based on an isolated, worn lower molar found in a channel filling that is now recognized as containing a time-averaged assemblage of Lancian and earliest Puercan (Pu 1 Interval Zone) fossils. Purgatorius has not been found in large samples of other local faunas of either Pu 1 or Lancian age. In contrast, Purgatorius is abundantly represented in Pu 2–3 local faunas in the northern Western Interior suggesting that it dispersed into the area during the interval between ca. 64.75 and 64.11 Ma. Fragmentary dentaries provisionally referred to Purgatorius janisae from the Garbani Channel fauna (Pu 2–3) document its lower dental formula as: 3.1.4.3. Morphology of the canine and incisor alveoli suggests a derived gradient in crown size of: I1> or = I2 ≫ I3 ≪ C. Isolated upper incisors possibly referable to P. janisae exhibit some typical plesiadapiform specializations. Morphology of the postcanine dentition of Purgatorius is what might be expected to characterize a primitive member of the Primates, but specializations of its incisors suggest P. janisae is best considered a basal member of the Plesiadapiformes sensu lato and neither a member of nor directly ancestral to the Euprimates.