Notharctus tenebrosus (Leidy 1870, Gregory 1920) ~54-38mya, 40 cm in length from snout to rump was an adapid, one of the earliest known primates in the lineage of humans. Related plesiadapid fossils are older, but they are in the lineage of rabbits. It used to be thought that tree shrews, such as Ptilocercus, phylogenetically preceded primates. Here instead Notharctus was derived from a sister to the miacid carnivore, Vulpavus and phylogenetically preceded Aegyptopithecus.

Distinct from Vulpavus, the skull of Notharctus had a complete postorbital ring, like Ptilocercus, and a cranial crest. Both eyes were directed forwards (narrower interoccular distance) for binocular vision and were higher on the skull. The jawbones fused at the chin, creating a single solid mandible. A bony cover enclosed the tiny earbones. The incisors and canines were smaller. Premolars 1 and 2 were larger. Premolars 3 and 4 were smaller. Lower molar 1 was lower, without the carnasial shape.

The cervicals were shorter and there were fewer of them, down to seven. The lumbars increased in number. The sacrals were relatively shorer. The caudals were more gracile, and probably more expressive.

The scapula was smaller. The forelimbs were longer and more slender. With these it was able to spring from branch to branch across great gaps. The radius bowed away from the ulna enabling greater pronation and supination. The metacarpals were shorter. The fingers were much longer. Digit I was more robust and rotated medially, opposing the others. Notharctus held on to branches by grasping them with its hands, not by embedding its claws.

The hind limbs were longer and more gracile. The medial toe likewise became thumb-like, rotating medially to oppose the others. The foot had a better "thumb" than the hand did. The metatarsals were shorter. The toes were longer and more gracile. Metatarsal III extended further than II, IV or V. Pedal digit III also extended further than the others. More primitive than living lemurs, Notharctus still had claws, not nails.

If like a living lemur, Notharctus practicsed vertical clinging and leaping, a new method of locomotion. Sitting erect is comfortable due to the angle of the neck as it inserts into the skull.

Diet includes plants, insects, small vertebrates and eggs. Lemurs usually have only one baby after a 126-day pregnancy. Newborns are not born premature. They ride on their mother's back as she forage and they mature in 18 months. A typical lifespan is 18 years. Whereas tree shrews are solitary, lemurs are social, living in tribes of fewer than 15 individuals. Reciprocal grooming is practiced. Lemurs with larger canines tend to be more agressive.