Notharctus

Notharctus Temporal range: Early-Mid Eocene (Wasatchian-Uintan) ~ Pal. Eocene Oligo. Miocene P P
Notharctus tenebrosus
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Primates
Suborder:	Strepsirrhini
Family:	†Notharctidae
Subfamily:	†Notharctinae
Genus:	†Notharctus Leidy, 1870
Species
†N. pugnax Granger and Gregory 1917 †N. robustiorLeidy 1872 †N. tenebrosusLeidy 1870 †N. venticolus Osborn 1902
Synonyms
Hipposyus Leidy 1872 Limnotherium Marsh 1871 Telmalestes Marsh 1872 Thinolestes Marsh 1872 Tomitherium Cope 1872 Synonyms of N. tenebrosus Limnotherium tyrannus Marsh, 1871 Hipposyus formosus Leidy, 1872 Tomitherium rostratum Cope, 1872 Thinolestes anceps Marsh, 1872 Limnotherium affine Marsh, 1872 Notharctus osborni Granger and Gregory, 1917 Synonyms of N. robustior Telmatolestes crassus Marsh, 1872

Notharctus is a genus of adapiform primate that lived in North America and Europe during the early to middle Eocene. The fingers of the genus were elongated for clamping onto branches, including the development of a thumb. Its spine is flexible and the animal was about 40 centimetres (16 in) in length, excluding the long tail.¹ The genus contains at least 4 species. Notharctus belonged to an extinct primate group known as Adapiformes and fossils have been found in 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. In addition to this, small orbits in the genera indicate that they were diurnal. The diet of Notharctus was likely leaves, and the genus moved similarly to modern day lemurs in life.

Notharctus is currently grouped within the Notharctidae, though it has been historically considered an adapid.² Within the Notharctidae, Notharctus is considered within the subfamily Notharctinae.³ The name of the genus is derived from the Latin "noth", meaning false, and the Greek "arctos", meaning bear.⁴

Notharctus was one of the first discovered fossil primates from North America. The type species, Notharctus tenebrosus, was discovered by Joseph Leidy, who had difficulties assigning it to any specific group of mammals, noting similarities in its jaw anatomy to carnivorans and to "pachyderms".⁵ Leidy opted to refer Notharctus to the pachyderms, but remarked in his description of the genus that the jawbone was incredibly anatomically similar to modern platyrrhine monkeys.⁶ This similarity was also noted by Othniel Charles Marsh, who compared the jaw to Hyopsodus. Marsh assigned these remains to the genus Limnotherium, then later he found more remains which he described as Thinolestes.⁶ Marsh found limb bones in October of 1872, which shared similarities with lemurs, causing him to assign the genus to the Quadrumana (an obsolete classification including all non-human primates). In 1876, Edward Drinker Cope proposed Notharctus and Pelycodus belonged to a new order, the Mesodonta, due to the fact that Pelycodus had been associated historically with creodont claws. This order acted as an "intermediate" between creodonts and prosimians, according to Cope.⁷ Cope later in 1885 considered Notharctus a lemuroid, yet he still considered the lemuroids close to the obsolete Insectivora and Condylarthra .⁸⁶ In 1895, Karl Alfred von Zittel continued this error, referring Notharctus alongside Pelycodus, Microsyops and Hyopsodus within the Pachylemuridae. Notharctus was then later grouped within Adapis by Jacob Wortman in 1904 and Schlosser in 1887, who presumed the mesodonts were a polyphyletic assemblage. Wortman referred this group to the Neopithecini, a group also including simians but not lemurs. Henry Fairfield Osborn agreed with the classification of Notharctus as a primate, grouping Notharctus closely to Pelycodus in 1902, yet he still retained usage of the suborder Mesodonta, which united the Hyopsidontidae, the Notharctidae and the Anaptomorphidae.⁶ The family Notharctus was assigned to was given 2 names, with Marsh proposing Limnotheridae in 1875 while Osborn proposed Notharctidae in 1902.⁵ With Walter W. Granger's discovery of a nearly complete skeleton, also in Wyoming, it was firmly established as a primate. William K Gregory considered Notharctus a lemuriform, rather than an adapiform (a grouping which did not exist at the time).⁵ In 1917, Granger and Gregory published a revision of the genus, and in 1920 Gregory published a monograph which outlined the classification history and anatomy of Notharctus.⁶

Notharctus is one of the best represented adapiform primates, with the postcranial and cranial morphology being well known. Notharctus is distinguished from other fossil primates by its stout jaws, typically fused mandible, and quadrate molars with a pronounced hypocone and mesostyle.⁹ The different species of Notharctus have been estimated at different sizes, with N. robustior estimated to weigh 6.9 kg (15 lb) while N. tenebrosus has been estimated to weigh 4.2 kg (9.3 lb).¹⁰ The postcranial anatomy of Notharctus has been described as similar to modern day platyrrhines.¹¹

Notharctus has a small braincase compared to Lepilemur, which is considered the most basal living lemur. The snout is moderately long, with a long premaxillary bone, which is contrasted by a shortened jaw.⁹¹⁰ Notharctus had a reduced lacrimal bone that was positioned at the end of the orbit but not anterior to it.^{10 12} The sagittal and nuchal crests of the genus are large, similar to other adapids (though not as large as those in Adapis).¹³ Behind the orbits there is a notable constriction, which suggests a smaller brain size than the related Smilodectes.² Despite this, Notharctus had both a larger and narrower skull. The occipital bone is narrow and pointed. On the petromastoid there is a prominent tuberosity. In Notharctus, much like other adapids, the external pterygoid is joined with the bulla. The postglenoid foramen is prominent, another trait shared by other adapids.² The skull of N. robustior is larger than that of N. tenebrosus, and the sagittal and nuchal crests are larger as well.¹³ The mandibular symphysis of Notharctus is typically fused, though in some specimens they are not entirely coossified.⁹

The adult dental formula of Notharctus is 2.1.4.32.1.4.3 × 2 = 40.⁶ The molar teeth of Notharctus have well-developed shearing crests, while the incisors are peg-like in form. Notharctus had canine teeth that are sexually dimorphic, and the canines were enlarged compared to other adapids.⁸¹⁰ Unlike modern lemurids, the premolars are not caniniform.¹² The anatomy of the incisors and canines have been compared to indrids and and lemurids.¹⁴ The upper molars of this species have a pseudohypocone. The hypocones of the teeth are strong and cuspate, with strong cingulae surrounding the teeth.² Specifics of the teeth have been used historically to differentiate species of Notharctus, as the dentition of primates is distinctive.⁹¹⁵ The canines of Notharctus have been suggested to be sexually dimorphic.¹⁶

The vertebral formula of Notharctus tenebrosus is 7 cervicals, 12 thoracics, 8 lumbars, 3 sacrals, and 19+ caudals. The thorax is long and deep. ¹⁷ In life, Notharctus was likely pronograde in posture, similar to modern lemurs.¹⁷ The same can be said for the general postcranial anatomy, which has been compared to Propithecus and Lemur.¹⁰² The hind limbs Notharctus are long, as is the tail. The ilium is sickle shaped and the ischium is elongate.¹⁰

On the hands and feet, the pollex and hallux are large and opposable, and the fingers and toes are long and possess nails,¹⁰ while on the foot the calcaneus is relatively short. As in other adapiformes and lemuriformes, the shape of the posterior trochlear shelf alongside the presence of a projection on the talus allows for a more flexible positioning and support during leaping. The posterior trochlear shelf is larger than in modern lemuriformes.² In general, the anatomy of the tarsus is less rigid than in modern day lemurs. There is evidence that the genus had a type of grooming claw, thought to be an intermediate between a grooming claw and a nail.¹⁸ The anatomy of the grooming claw differs from the grooming claws in present day strepsirrhines, with the distinguishing feature being an enlarged cervical nutrient foramen.¹⁹

The humerus of Notharctus has been compared to those of both modern lemurs and modern platyrrhine monkeys, with the S shaped contour being similarly developed as in Alouatta. The entepicondylar foramen is prominent, and the head of the humerus is inclined posteriorly. The features of both the radius and ulna, as well as the pelvis, and scapula, and clavicle, are also similar to those of platyrrhines, though comparisons have also been made with those of indrids and lemurids.¹¹¹⁷

Based on dental morphology, Notharctus most likely had a folivorous diet. This is contradicted by other aspects of the dental anatomy, which suggest the genus may have been frugivorous.¹⁰² Despite this, analysis of the enamel of the molars indicates that the genus was primarily folivorous.²⁰

Based upon limb bone morphology Notharctus most likely was an arboreal quadruped. The robust limbs of Notharctus suggest it moved by leaping and may have been capable of vertical grasp clinging, similar to modern day sportive lemurs and indri.²¹⁷⁹

Notharctus is known from many middle Eocene North American and European localities. The paleoenvironment of the Bridger and Uinta formations, which the environments Notharctus inhabited may not have differed much from, was subtropical, with varying rainfall levels depending on the season. The winters were likely cool and moist, with the mean annual temperature being 65 °F (18 °C).¹³ Palms have been found in the Green River formation, which was likely comparable to the typical North American Eocene paleoenvironment.¹³

Notharctus was contemporary with an array of other fauna, including oxyaenids, omomyid primates, hyaenodonts, mesonychids and rhinocerotoids.²¹