Notharctus, extinct genus of small primates (family Adapidae) that shares many similarities with modern lemurs, although its exact relationship to lemurs is controversial. The genus is well known from complete fossil remains found in Europe and North America in early Eocene deposits dated to about 54 million years ago.
The skull of Notharctus was about 5 cm (2 inches) long, with a prominent muzzle. It had long canine teeth and four premolars, which differentiate it from modern lemurs. The legs and tail were long and slim. The first digit of the feet and hands was set off from the rest and may have been opposable. Notharctus was probably an agile climber, and during the Eocene, it inhabited the dense subtropical forests that flourished over much of North America and Eurasia. Notharctus and its close relatives became extinct by the end of the Eocene Epoch. Other more modern lemurs survived in tropical forests elsewhere and became particularly successful on Madagascar, where they remained relatively free from competition.