Wl. Jungers et al., PHALANGEAL CURVATURE AND POSITIONAL BEHAVIOR IN EXTINCT SLOTH LEMURS (PRIMATES, PALAEOPROPITHECIDAE), Proceedings of the National Academy of Sciences of the United Statesof America, 94(22), 1997, pp. 11998-12001
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.