Species: Notharctus tenebrosus, N. robustior, N. gracilis, N. robinsoni, N. venticolus, N. pugnax
When and where Notharctus lived: Found in the early to Middle Eocene (~53- 45 mya)in North America. Notharctus tenebrosus was named by Leidy (1870).
Time and Habitat Range: Early to Middle Eocene, approximately 53 to 45 mya (Gunnell 2008). It is a likely a descendant of Cantius of the late Wasatchian (Szalay and Delson 1979). Primates related to Notharctus (the adapids) are also common in Europe. Notharctus lived in a tropical forest habitat.
Notharctus weighed between 0.5 to 7kg. The larger ones were about the size of present-day lemurs.
The skull tends to be narrower, with a longer snout and smaller orbits than the tarsiiform primates of the Eocene. The smaller orbits suggest diurnal behavior, being active during the day and at dusk and dawn. Sexual dimorphism is suggested because some specimens found in the same localities have significantly different sizes of sagittal crests and and/or molars and canines (Robinson 1957; Rose 2006).
P4 is often molariform. Notharctus molars display sharper cusps and are more crested than most other early Eocene primates, indicating a folivorous (leaf-eating) diet. The typical molar is identifiable by a smaller trigonid compared to the talonid (Rose 2006).
Mandible: Most Notharctus have a symphysis at the anterior junction of the mandibles. This mandibular fusion is thought to be typical, though not universal in Notharctus (Robinson 1957; Rose 2006).
The hind limbs tended to be longer than the forelimbs; the femur is longer than the tibia. Flexible joints, grasping hands, an opposable thumb, and nails on the digits all signify an arboreal/terrestrial pattern of locomoition. In particular, the joint configuration is homologous to lemurs, though the ankle joints appear to be slightly less efficient in Notharctus. Nevertheless, they definitely possessed the skeletal characteristics that allowed them to deftly move about in the trees by leaping, grasping, and running (Szalay and Delson 1979).
Through the Eocene, the trend in size is toward larger features in Notharctus. Still smaller Notharctus species sustained their populations throughout the Eocene (e.g. N. gracilis). In fact, the smaller organisms seem to have lasted longer and spread through a wider range than the larger, later species (Robinson1957; Gazin 1959).
The Notharctus extinction began right after the end of the middle Eocene. The Western Interior Uplift contributed to the increasing aridity of the area in which Notharctus had proliferated. Some relatives such as Mahgarita survived until approximately 41mya in present-day Texas (Rose 2006). Eventually, the primates in North America disappeared. The North American primates are not the ancestors of the South American primates. It is suggested that these monkeys arrived from the Old World Adapidae, migrating from southern North America to South America (Gingerich 1979).
Gazin, C.L. 1958. A review of the middle and upper Eocene primates of North America. Smithsonian Miscellaneous Collections 136:1. Smithsonian Institution, Washington, D.C.
Gingerich, P.D. 1979. Phylogeny of middle Eocene Adapidae (Mammalia, Primates) in North America: Smilodectes and Notharctus. Journal of Paleontology 53:153-163.
Gunnell, H/F. K.D. Rose, and D.T. Rasmussen, 2008. Euprimates in C.M. Janis, G.F. Gunnell, and M.D. Uhen, Evolution of Tertiary Mammals of North America, volume 2: Small mammals, Xenarthrans and Marine Mammals. Pp. 239-262.
Robinson, P. 1957. The species of Notharctus from the middle Eocene. Postilla No. 28. Yale Peabody Museum of Natural History, New Haven, Conn.
Rose, K.D. 2006. The Beginning of the age of mammals. Johns Hopkins University Press. Baltimore, Md.
Szalay, F. S. and E. Delson. 1979. Evolutionary history of the primates. Academic Press, Inc. San Diego.
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