A NEW PLESIADAPIFORM (MAMMALIA, PLESIADAPIFORMES) FROM THE EARLY EOCENE OF THE BIGHORN BASIN, WYOMING

Dentaries of a very small, specialized plesiadapiform from the lower E ocene Willwood Formation are the basis for a new genus and species, Pi cromomys petersonorum. It was the smallest known plesiadapiform, weigh ing only about 10 g. Like some other plesiadapiforms, the new taxon ha s a strongly procumbent, hypertrophied medial incisor, followed by ver y reduced dentition between the incisor and P-4. Unlike all other ples iadapiforms, it has a unique semimolariform P-4 reminiscent of M(1) in picrodontids, as well as lower molars bearing an accessory trigonid c usp anterobuccal to the protoconid. The closest known relative of Picr omomys appears to be Bridgerian Alveojunctus, and we group these two g enera here in the new family Picromomyidae. Picromomyids share derived similarities with several plesiadapiform families but are probably mo st closely related to either Micromomyidae or uintasoricine Microsyopi dae.