New wrist bones of the Malagasy giant subfossil lemurs

Recently discovered wrist bones of the Malagasy subfossil lemurs Babakotia radofilai, Palaeopropithecus ingens, Mesopropithecus dolichobrachion, and M egaladapis madagascariensis shed new light on the postcranial morphologies and positional behaviors that characterized these extinct primates. Wrist b ones of P. ingens resemble those of certain modern hominoids in having a re latively enlarged ulnar head and dorsally extended articular surface on the hamate, features related to a large range of rotation at the inferior radi oulnar and midcarpal joints. The scaphoid of P. ingens is also similar to t hat of the extant tree sloth Choloepus in having an elongate, palmarly dire cted tubercle forming a deep radial margin of the carpal tunnel for the pas sage of large digital flexors. In (contrast, wrist remains of Megaladapis e dwardsi and M. madagascariensis exhibit traits observed in the hands of ext ant pronograde, arboreal primates; these include a dorsopalmarly expanded p isiform and well-developed "spiral" facet on the hamate. Moreover Megaladap is spp. and Mesopropithecus dolichobrachion possess bony tubercles (e.g., s caphoid tubercle and hamate hamulus) forming the carpal tunnel that are rel atively similar in length to those, of modern pronograde lemurs. Babakotia and Mesopropithecus differ from Megaladapis in exhibiting features of the m idcarpal joint related to frequent supination and radioulnar deviation of t he hand characteristic of animals that use vertical and quadrumanous climbi ng in their foraging behaviors. Comparative analysis of subfossil lemur wri st morphology complements and expands upon prior inferences based on other regions of the postcranial skeleton, and suggests a considerable degree of locomotor and postural heterogeneity among these recently extinct primates. (C) 2000 Academic Press.