PHALANGEAL CURVATURE AND POSITIONAL BEHAVIOR IN EXTINCT SLOTH LEMURS (PRIMATES, PALAEOPROPITHECIDAE)

Recent paleontological discoveries in Madagascar document the existenc e of a diverse clade of palaeopropithecids or ''sloth lemurs'': Mesopr opithecus (three species), Babakotia (one species), Palaeopropithecus (three species), and Archaeoindris (one species), This mini-radiation of now extinct (''subfossil'') lemurs is most closely related to the l iving indrids (Indri, Propithecus, and Avahi). Whereas the extant indr ids are known for their leaping acrobatics, the palaeopropithecids (ex cept perhaps for the poorly known giant Archaeoindris) exhibit numerou s skeletal design features for antipronograde or suspensory positional behaviors (e.g., high intermembral indices and mobile joints). Here w e analyze the curvature of the proximal phalanges of the hands and fee t, Computed as the included angle (theta), phalangeal curvature develo ps in response to mechanical use and is known to be correlated in prim ates with hand and foot function in different habitats; terrestrial sp ecies have straighter phalanges than their arboreal counterparts, and highly suspensory forms such as the orangutan possess the most curved phalanges, Sloth lemurs as a group are characterized by very curved pr oximal phalanges, exceeding those seen in spider monkeys and siamangs, and approaching: that of orangutans. Indrids have curvatures roughly half that of sloth lemurs, and the more terrestrial, subfossil Archaeo lemur possesses the least curved phalanges of all the indroids. Taken together with many other derived aspects of their postcranial anatomy, phalangeal curvature indicates that the sloth lemurs are one of the m ost suspensory clades of mammals ever to evolve.