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. Notharctus tenebrosus most resembles modern-day Madagascar lemurs.
Notharctus tenebrosus belonged to an extinct primate group known as Adapiformes and fossils have been found in Europe and North America. Adapiform primates were among the first primates to exhibit a set of adaptations for life in the trees, such as grasping hands, binocular vision, and flexible backs.
"Notharctus" means false bear, while "tenebrosus" means dark or gloomy. N. tenebrosus is one of 5-12 species in the Notharctus genus.
had a fused mandibular symphysis
and molar teeth
with well-developed shearing crests, while the incisors are peg-like in form. Notharctus tenebrosus
had canine teeth
that are sexually dimorphic. The upper molars of this species have a psuedohypocone
and the snout
is moderately long, with a long premaxillary bone
. Notharctus tenebrosus
had a lacrimal bone that was positioned at the end of the orbit but not anterior to it as found in extinct lemurs
. The vertebral formula of Notharctus tenebrosus
is 7 cervicals, 12 thoracics, 8 lumbars, 3 sacrals, and 19+ caudals. Observing the fossils, Notharctus tenebrosus
had long hindlimbs
, and tail
. On the hands and feet
, the pollex
are large and opposable, and the fingers
are long and possess nails, while on the foot the calcaneus
is relatively short. Notharctus tenebrosus
has an average body mass of 4.2 kilograms and was about 40 cm long without its tail.
Diet and locomotion
Based on dental morphology, Notharctus tenebrosus most likely had a folivorous diet.
Based upon limb bone morphology Notharctus tenebrosus most likely moved by leaping and was an arboreal quadruped.
- Mikko's Phylogeny archive