Paleogene Plesiadapiform Skeletons from Wyoming and the Origin of Primates

Regents Of The University Of Michigan - Ann Arbor, Ann Arbor MI

Investigators

Abstract

Primates include all living prosimians, monkeys, apes, and humans. Much is known of the evolutionary history of this group but its origins remain obscure. The earliest recognized fossil primates appear suddenly on northern continents at the beginning of the Eocene, about 55 million years ago, but their place of origin and their relationships with other fossil mammal groups remains an unresolved question. Tree shrews, dermopterans (flying lemurs), and chiropterans (bats) all have been suggested as the closest living relatives of primates. Among fossil mammals, a Paleocene group called Plesiadapiformes has most often been cited as possible primate ancestors, based on shared tooth structures. Alternatively, plesiadapiforms have been linked by skull and limited skeletal evidence to flying lemurs. Recently, exceptionally well-preserved plesiadapiform skeletons have been found in Paleocene and Eocene freshwater limestones in the Clarks Fork Basin of Wyoming. These skeletons are remarkable for their completeness and their preservation of even the most delicate of anatomical structures. These skeletons represent four different families of plesiadapiforms including Carpolestidae, Micromomyidae, Paromomyidae, and Plesiadapidae. Analysis of these skeletons will allow for: (1) new functional interpretations of the positional behavior and locomotor pattern of Plesiadapiformes, (2), a test of the hypothesized plesiadapiform-dermopteran link and (3) a re-analysis of plesiadapiform-primate relationships. Initial results indicate that a diversity of unique positional behaviors is represented in the four plesiadapiform families represented. All groups appear to have been arboreal and their skeletons show a variety of adaptations for above-branch walking, vertical clinging, climbing, grasping, leaping, and suspension. These diverse skeletal attributes parallel the wide variety of dental specializations developed in plesiadapiforms. New data from these specimens will be used to reconstruct a model of the skeletal anatomy of primitive Primates.