Plesiadapis tricuspidens (Gervais 1877) Paleocene ~55 mya. The Plesiadapiformes were widely thought to be the earliest representatives of the primate order, but here they nested closer toTupaia and rabbits. Derived from a sister to Tupaia, Plesiadapis phylogenetic preceded Gomphos, a primitive rabbit. Leaping from branch to branch, like a lemur, translated to hopping while on the ground.
Distinct from Tupaia, the skull of Plesiadapis had a deeper shorter rostrum and a higher orbit. The premaxilla was longer, but the maxilla was greatly reduced, restricted to chiefly below the orbit. The jugal was more robust. No postorbital bar backed up the eyeball. The ear hole was raised and much larger. The canine was gone. The first incisor was larger, the second much smaller. The three premolars and three molars formed a grinding surface largely beneath the cheek. The mandible was more robust with deeper surfaces for muscle attachement and a more robust angular process and a longer coronoid process. The lower incisor (only one remained) was the largest tooth and worked against the complex back surface of the anterior upper incisor.
The cervicals were shorter. The dorsals and lumbars were more robust along with the caudals. Chevrons developed at a likely sitting point.
The limbs and girdles were all more robust. The radius was anteriorly boewed and the ulna developed a large olecranon process (elbow). The unguals were large and deep. The feet were much larger than the hands. The joints were nearly all transversely aligned indicating a simple extension/flexion motion for the fingers and toes.