Plesiadapis

in TrentoThe 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 epoch. This genus probably arose in North America and colonized Europe on a land bridge 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. 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. Classification '' (left), an early true primate. Both come from Eocene Wyoming, though the latter is slightly geologically younger. The following are possible shared derived features of Plesiadapiformes: maxillary-frontal contact in orbit, the presence of a suboptic foramen, an ossified external auditory meatus, the absence of a promontory artery, the absence of a stapedial artery, and a strong mastoid tubercle. Although the placement of the Plesiadapis lineage is still up for debate, the consensus in the 1970s was that they were closest to early tarsier-like primates. Plesiadapiformes have also been proposed as a nonprimate sister group to Eocene-Recent primates. A study done in 1987 linked Plesiadapiformes with adapids and omomyids through nine shared-derived features, six of which are cranial or dental: (1) auditory bulla inflated and formed by the petrosal bone, (2) ectotympanic expanded laterally and fused medially to the wall of the bulla, (3) promontorium centrally positioned in the bulla, and large hypotympanic sinus widely separating promontorium from the basisphenoid, (4) internal carotid entering the bulla posteriolaterally and enclosed in a bony tube, (5) nannopithex fold on the upper molars, and (6) loss of one pair of incisors. Species Fifteen species of Plesiadapis recognised by Fredrick S. Szalay and Eric Delson in 1979. The following table is based on that work: == Description ==

Plesiadapis is one of the most completely known early primatomorphs, with a significant amount of the skeleton known, unlike most other plesiadapids which are quite fragmentary. Skull The skull of Plesiadapis is relatively broad and flat, with a long snout with rodent-like 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. Dentition Plesiadapis' dentition shows a functional shift toward grinding and crushing in the cheek teeth as an adaptation towards increasing omnivory and herbivory. The dental formula is usually , with two incisors, one canine, three premolars, and three molars in either half of the upper jaw; and one incisor, one canine, three premolars, and one molar in either half of the lower jaw. The incisors are quite long. Already, Plesiadapis had lost the first premolar from the mammalian common ancestor, but later primatomorphs would lose the second premolar as well. P. dubius consistently lacks the lower second premolar, and about half of P. rex specimens lack it too. P. gidleyi and European Plesiadapis lack the lower canines. == Palaeoecology ==

Remains belonging to Plesiadapis have been discovered in the Rocky Mountains region of North America and parts of western Europe. Large Plesiadapis species in Europe evolved to become increasingly adapted for folivory across the Palaeocene–Eocene boundary, although they were not as adapted for a folivorous diet as Platychoerops daubrei. ==References==