Derived morphology of the wrist articulations and theories of hominoid evolution. Part I. The lorisine joints.

The claim that lorisines possess wrist joint specialisations that parallel those in hominoids is refuted. The cautious, quadrupedal climbing lorisines therefore do not provide an adequate model for the locomotion of emergent hominoids, as has been suggested. The notion that the Hominoidea possess unique wrist joint synapomorphies facilitating an increased range of forearm supination, and correlated thereby with a capacity for suspensory forelimb locomotion, seems to be sustained. The lorisines do possess their own derived characters at the wrist, largely centred on the midcarpal joint, which accommodate an exaggerated habitual movement comprising ulnar deviation with extension and rotation of the sense of pronation; this produces the powerful pincer-like split hand. Rather similar but more extreme specialisations are described in schizodactylous marsupials. In all these specialised hands the habitual movement at the midcarpal joint appears to be merely an exaggeration of that occurring in the clasping type of hand found in the primitive therian mammals. This movement, about an axis running from dorsally on the radial side of the carpus with an angulation proximally towards the volar aspect on the ulnar side, apparently provided for adjustments in angulation between forearm and manus in the semi-sprawling gait of primitive non-cursorial therian mammals.