A New Genus of Late Oligocene Monkey (Cebidae, Platyrrhini) with Notes on Postorbital Closure and Platyrrhine Evolution

The platyrrhine Homunculus harringtoni Rusconi, 1933, of the Patagonian Late Oligocene, known from a skull fragment, is designated type of new genus Tremacebus, new subfamily Tremacebinae, family Cebidae. Comparisons are made with contemporaneous Dolichocebus Kraglievich, early Miocene Homunculus, and Recent platyrrhines, mainly AotusCallicebus, and Saimiri. Notable characters of Tremacebus include Callicebus-size skull with short face, braincase steeply vaulted, posterior occipital plate and foramen magnum directed more nearly backward than downward, orbits oriented fronto-laterally, interorbital region wide, muzzle broad, zygomatic arch relatively long, dental arch more nearly U- than V-shaped, occlusal surface of dental row moderately curved. Outstanding is the extremely large orbitotemporal opening (basis for generic name) which resolves one of two main cranial characters separating platyrrhines from tarsioids. The two halves of missing Tremacebus mandible are presumably synostosed at symphysis menti as in all platyrrhines, but in no tarsioid. Morphological stages and grades in evolution of primate postorbital closure are described and figured. Platyrrhine origin, history, and relationship are briefly discussed within the framework of continental drift. The Homunculidae are defined and a systematic arrangement is proposed for all New World primate families and genera known from Mexico southward.