Plesiadapis is a Paleocene mammal that thrived in North America from about 62 to 56 million years ago.  It was arboreal and, though not a rodent, seemingly lived somewhat like a squirrel.  Plesiadapis evolved rapidly, and its distinctive jaws and teeth are important for biochronology of the North American middle and late Paleocene.  Plesiadapis has special interest because its molar teeth have long been compared to those of primates. 

A new relatively complete skeleton of Plesiadapis cookei (illustration) enables analysis and comparison of Plesiadapis across a range of primates and related mammals.  The skeleton is described by Douglas Boyer of Duke University and Philip Gingerich of the University of Michigan in a 2019 monograph titled “Skeleton of late Paleocene Plesiadapis cookei (Mammalia, Euarchonta):  life history, locomotion, and phylogenetic relationships.”  The monograph is volume 38 in the University of Michigan Papers on Paleontology series.

The skull is well preserved and shows that the brain was relatively small compared to those of later primates.  The skeleton shows that P. cookei weighed 1.8 to 2.1 kilograms (4.0 to 4.6 pounds).  Forty-three of an expected 53 vertebrae are preserved in the skeleton, as are ribs and most elements of each fore- and hind limb.  Fingers and toes end in long falciform phalanges that bore sharp claws.  The postcranial skeleton of P. cookei suggests that it was a forest-dwelling arboreal climber primarily adapted to large-diameter vertical and horizontal supports.  Headfirst descent of large supports was likely accomplished by claw-clinging, with a reversed (supinated) foot.  P. cookei was surely more cautious and less scansorial in its movements than squirrels or smaller-bodied plesiadapids.

Cladistic analysis of the skeleton in comparison to higher-level taxa supports a link between plesiadapids, saxonellids, and carpolestids, grouped as Plesiadapoidea, and does not contradict previous hypotheses suggesting a special relationship of primitive plesiadapoids to more advanced true primates (Euprimateformes).

The 269-page Plesiadapis monograph is available online open-access in the University of Michigan Deep Blue archive.  A 3D model of the reconstructed skeleton can be viewed on the University of Michigan UMORF web site, and 3D models of individual bones can be found on the Duke University MorphoSource web site.