Plesiadapis

Plesiadapis is one of the oldest known primate-like mammal genera which existed about 55-58 million years ago in North America and Europe.[2] Plesiadapis literally means "near-Adapis", which is a reference to the Eocene lemuriform, Adapis. Plesiadapis tricuspidens, the type specimen, is so named because of the three cusps present on its upper incisors.

Origins and discovery
The first discovery of Plesiadapis was made by François Louis Paul Gervaise in 1877, who first discovered Plesiadapis tricuspidens in France. The type specimen is MNHN Crl-16, and is a left mandibular fragment dated to the early Eocene period.

This genus probably arose in North America and colonized Europe on a landbridge via Greenland. Thanks to the abundance of the genus and to its rapid evolution, species of Plesiadapis play an important role in the zonation of Late Paleocene continental sediments and in the correlation of faunas on both sides of the Atlantic. Two remarkable skeletons of Plesiadapis, one of them nearly complete, have been found in lake deposits at Menat, France.[2] Although the preservation of the hard parts is poor, these skeletons still show remains of skin and hair as a carbonaceous film—something unique among Paleocene mammals. Details of the bones are better preserved in fossils from Cernay, also in France, where Plesiadapis is one of the most common mammals.

Anatomy and remains
Nearly all of what is known about the anatomy of plesiadapiforms comes from fragmentary jaws and teeth, so most definitions of plesiadapiform genera and species are based on dentition. Plesiadapis's dentition shows a functional shift toward grinding and crushing in the cheek teeth as an adaptation towards increasing omnivority and herbivority. The dental formula for Plesiadapis is 2.1.3.3

2.1.3.3 [4] The skull of Plesiadapis is relatively broad and flat, with a long snout with rodentlike jaws and teeth and long, gnawing incisors separated by a gap from its molars. Orbits are still directed to the side, unlike the forward-facing eyeballs of modern primates that enable three-dimensional vision.[5] Although its braincase was small by today's standards, it was larger than in the contemporary hoofed mammals, for instance. Plesiadapis had mobile limbs that terminated in strongly curved claws, and it sported a long bushy tail which is beautifully preserved in the Menat skeletons. The way of life of Plesiadapis has been much debated in the past. Climbing habits could be expected in a relative of the primates, but tree-dwelling animals are rarely found in such high numbers. Based on this and other evidence, some paleontologists have concluded that these animals were mainly living on the ground, like today's marmots and ground squirrels.[2] However, more recent investigations have confirmed that the skeleton of Plesiadapis is that of an adept climber, which can be best compared to tree squirrels or to tree-dwelling marsupials such as possums.[5] The short, robust limbs, the long, laterally compressed claws, and the long, bushy tail indicate that it was an arboreal quadruped. Remains found showed that it had a body mass of around 2.1 kilograms.