Notharctus tenebrosus was an early primate from the early Eocene, some 54-38 million years ago. Its fossil was found by Ferdinand
V. Hayden in 1870 in southwestern Wyoming. When first found, Notharctus tenebrosus was thought to be a small pachyderm due
to the concentration of pachyderm fossils in the area. However, after Walter W. Granger's discovery of a nearly complete skeleton,
also in Wyoming, it was firmly established as a primate.