Anthropoid phalanges from the Oligocene of Egypt

Proximal phalanges recovered from Oligocene sediments of the Jebel Qatrani Formation, Fayum Province, Egypt, are attributed to the anthropoids Aegyptopithecus and Apidium. A multivariate discriminant function analysis using phalangeal shape indices proved reliable for distinguishing between manual and pedal proximal phalanges of extant primates, and these data can be applied to classify phalanges of fossil primates. Manual and pedal phalanges of Aegyptopithecus show features clearly related to a quadrupedal arboreal lifestyle involving frequent and powerful digital flexion: plantar tubercles are prominent, the flexor sheath ridges are well developed, shaft height increases proximally, shaft cortices are thickest in their distal halves, and the phalanges display strong dorsoventral curvature. Proximal phalanges of Aegyptopithecus resemble those of extant Alouatta, and to a lesser extent those of colobine primates, in these respects. These fossil phalangeal specimens resemble quadrupedal cercopithecids in having large palmar and plantar tubercles, flaring basal apophyses, robust shafts, and dorsally extended and proximodorsally tilted metacarpal and metatarsal facets. Manual and pedal phalanges of Apidium have straight, robust shafts with cortices of equal thickness throughout, relatively dorsoventrally compressed condyles, weak flexor ridges, and a flared base for insertion of the collateral ligaments and interosseous musculature. These features suggest a locomotor repertoire for Apidium that is well suited for stable digital extension during quadrupedal running on horizontal substrates with little emphasis on grasping capabilities. These features compare most favourably with those of the phalanges of extant Saimiri.