Forelimb long bones of Nacholapithecus (KNM-BG 35250) from the middle Miocene in Nachola, northern Kenya

This paper provides a thorough description of humeral, ulnar, and radial specimens of the Nacholapithecus holotype (KNM-BG 35250). A spool-shaped humeral trochlea (and keeled sigmoid notch of the ulna) is a hallmark of elbow joint evolution in hominoids. In lacking this feature, the elbow of Nacholapithecus is comparatively primitive, resembling that of proconsulids. However, the humeroulnar joint in Nacholapithecus is specialized for higher stability than that in proconsulids. The humeroradial joint (humeral capitulum) resembles that of extant apes and Sivapithecus. This condition may represent an intermediate stage leading to the fully modern elbow in extant apes. If this is the case, specialization of the humeroradial joint preceded that of the humeroulnar joint. Nacholapithecus elbow joint morphology suggests more enhanced forearm rotation compared to proconsulids. This observation accords with the forelimb-dominated positional behavior of Nacholapithecus relative to proconsulids, which has been proposed on the grounds of limb proportions and the morphology of the phalanges, shoulder girdle, and vertebrae.