A comparative study of carpal joint structure and function in six Malagasy lemuriforms was undertaken to test predicted morphoclines in carpal joint morphology between pronograde and orthograde arboreal primates. Patterns of movement at the wrist during locomotion were observed and described for the lemuriform species Lemur fuluus and Propithecus uerreauxi. Lemur fuluus, which assumes a pronograde posture during locomotion, extends and pronates the wrist during the support phase of quadrupedal walking and running stride cycles. Furthermore, the forearm of this species exhibits some transverse movement across the proximal wrist joint during the support phase. In contrast, the indriid Propithecus maintains the hand and wrist in a flexed and partially supinated position during vertical clinging and suspensory postures.
Habitual quadrupedal and vertical postures in Malagasy primates are in turn related to very different patterns of carpal joint morphology and articular mechanics. Those lemurs which are predominantly pronograde share a series of structural features related to stabilizing the antebrachiocarpal joint during extension and mediolateral deviation and the midcarpal joint during pronation: a n intraarticular labrum is present on the inner portion of the radiocarpal ligament, the radiocarpal articular surface is quite flat dorsoventrally, the capitatetrapezoid embrasure is expanded dorsally, and development of the radial and ulnar styloids is more pronounced. The wrists ofPropithecus, Avahi, and Lepilemur (vertical clingers) differ from those of quadrupedal lemuriforms in pos- sessing a suite of morphological features related to stabilizing the wrist during antebrachiocarpal flexion and midcarpal supination: the radiocarpal articular surface is deeply curved and tilted anteriorly, the dorsal radiocarpal ligament is very broad, thick, and fibrous, the hamate's triquetral facet is directed proximodistally, and the capitate-trapezoid embrasure is dorsally constricted and expanded palmarly. These observed contrasts in carpal form and function are used to define further the morphological features related to orthograde posture in several lineages of arboreal primates. o 1996 Wiley-Liss, Inc.
The wrist joints of arboreal mammals are extremely complex in both their morphology and function. Positional behaviors on irregular, discontinuous, arboreal supports require compromises between wrist joint mobility and stability which place a wide range of perhaps any other order of placental mammals , and not surprisingly the anatomy of the wrist is exceptionally variable within the Order Primechanical demands on carpal joint marphology 1972)' Primates are more