Notharctus tenebrosus

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Notharctus tenebrosus was an early primate that lived during the early Eocene, roughly 54 to 38 million years ago. Its fossils were found in southwestern Wyoming. The first fossil, discovered by Ferdinand V. Hayden in 1870, led scientists to think it was a small pachyderm because of the many fossil remains in the area. A nearly complete skeleton found later by Walter W. Granger confirmed it was a primate. Notharctus tenebrosus looks similar to modern lemurs in appearance, but it is not directly related. It belonged to an extinct group called Adapiformes, which includes some of the earliest primates known for adapting to life in the trees. Fossils of adapiform primates have been found in North America, and they are thought to have been active during the day.

The name means "false bear" (Notharctus) and "dark" or "gloomy" (tenebrosus). The animal weighed about 4.2 kilograms and was about 40 centimeters long without its tail. It had a fused jaw joint and teeth suited for chewing leaves, including strong molar crests and peg-like incisors, with some features in the canines that varied by sex. The upper molars had specialized features, and it had a relatively long snout. Its limbs were long, and it had large, opposable thumbs and big toes with long fingers and toes and nails, helping it grasp branches. A grooming claw is suggested. Notharctus tenebrosus was likely an arboreal quadruped that could also leap between branches, and based on its teeth, it probably fed mainly on foliage.