However, after Walter W. Granger's discovery of a nearly complete skeleton, also in Wyoming, it was firmly established as a primate.

Notharctus tenebrosus belonged to an extinct primate group known as Adapiformes and fossils have been found in North America.

Notharctus tenebrosus had a fused mandibular symphysis and molar teeth with well-developed shearing crests, while the incisors are peg-like in form.

On the hands and feet, the pollex and hallux are large and opposable, and the fingers and toes are long and possess nails,[1] while on the foot the calcaneus is relatively short.

Based upon limb bone morphology Notharctus tenebrosus most likely moved by leaping and was an arboreal quadruped.