Notharctus tenebrosus was an early primate species that lived during the Early Eocene, 54-38 million years ago.
It was first discovered by Ferdinand V. Hayden in southwestern Wyoming in the year 1870, but the fragments were found in a bed of elephant fossils and remained unidentified.
Later, an almost complete skeleton was discovered in Wyoming by Walter W. Granger, and it was finally established as a species of primate. Notharctus tenebrosus is most reminiscent of modern-day lemurs, although they are not closely related.
Notharctus tenebrosus belongs to the taxonomic group of adapiform primates. The adapiform primates were some of the very first primates to possess specialized arboreal (tree-dwelling) adaptations such as grasping hands, flexible backs and binocular vision.
The genus name, Notharctus, literally means "false bear", while the specific epithet, tenebrosus, means "gloomy" or "dark".
It was a diurnal creature, with an average body mass of 4.2 kilograms. It would have moved in a lemur-like arboreal fashion of vertical clinging and leaping from branch to branch and tree-to-tree (Beard & Godinot, 1988; Martin, 1990). Lemurs are one of the few primates that propel themselves with quadrupedal leaping (using all 4 limbs to leap).
Analyses of N. tenebrosus'postcranial skeleton not including the skull, have confidently identified N. tenebrosus to be a primitive arboreal species (Beard & Godinot, 1988; Martin, 1990). Studies of the organism’s teeth, indicate that it ate leaves, nuts and seeds (Martin, 1990). Some of those features have been used to infer the diet include, mandibular symphseal fusion (the connection between the two sides of the lower jaw) and the morphology of the molars (Beecher, 1983; Martin, 1990).
Notharctus tenebrosus weighed 4.2 kilograms and its body measured ~40 cm long, excluding its tail. It had well-developed molar cusps to aid in shearing leaves, when feeding. It also had a fused mandibular symphysis (joint between two sides of mandible). There was sexual dimorphism (morphological differences between the two sexes of the same species) present in N. tenebrosus, which is evident in the size of the canine teeth (males had much larger canines). The snout is fairly elongated, which would have given the species a lemur-like appearance (Fleagle, 1988).
Notharctus tenebrosus is a long, slender animal. It has long, gracile, hindlimbs, tail and trunk region. It has opposable thumbs to aid in grasping and the fingers and toes are long and possess nails (Fleagle, 1988).
The species is thought to have had a modified grooming claw, thought to be an intermediate stage between a grooming claw and a nail (Maiolano et al., 2012).
Notharctus tenebrosus ate leaves, nuts or seeds, as indicated by molar morphology (size and shape of the teeth). (Beecher, 1983; Ravosa & Hylander, 1994).
The morphological evidence confidently suggests N. tenebrosus is a diurnal (active during the day) creature. It had average body mass of 4.2 kilograms. Its body measured ~40 cm long, excluding its tail. It moved in a lemur-like arboreal fashion of vertical clinging and leaping that is powered by all four limbs (Beard & Godinot, 1988; Martin, 1990).
Unfortunately, little is known of their social living. It is assumed that they were highly social and lived in groups, much like modern primates. Besides that inferred information, little else is known.
