Anatomy of the hominoid wrist joint: Its evolutionary and functional implications |
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Authors: | Esteban E Sarmiento |
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Institution: | (1) Department of Mammalogy, American Museum of Natural History, New York, New York |
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Abstract: | Observations on the behavior of living hominoids show generic differences in the use and posture of the wrist joint. Both
orang-utans and hylobatids usually use the wrist in suspensory behaviors. However, orang-utans emphasize markedly adducted
and flexed wrist postures, while hylobatids emphasize violent forearm and wrist rotation. African apes, especially the gorilla,
use the wrist more frequently than other hominoids for terrestrial quadrupedal weight-bearing. Humans use the wrist less frequently
for supportive purposes than do other hominoids. These behavioral differences correspond to structural specializations in
the proximal carpal joint of each of the hominoid genera. Although each of the hominoid genera has apparently modified its
proximal carpal joint best to serve its characteristic behaviors, all hominoids share a unique proximal carpal joint that
permits approximately 160ℴ of forearm rotation. The hylobatid proximal carpal joint is specialized in exhibiting a marked
development of those structures limiting forearm rotation, but it is in most respects the least derived— that is, closest
to the nonhominoid anthropoids. Chimpanzees show a proximal carpal joint that is more generalized than those of the other
great apes but more derived than that of hylobatids. The human and gorilla proximal wrist joints, on the other hand, show
marked modifications for weight-bearing in terrestrial behaviors. Orang-utans have the most derived proximal carpal joint,
which in many respects parallels that of the slow-climbing nonhominoid primates. The comparative anatomy and structural specializations
of the wrist joint support (a) an early divergence of hylobatids from the common hominoid stock, (b) a common ancestry for
gorillas and humans separate from the other hominoids, and (c) a long independent evolutionary period for orang-utans since
their divergence from the common hominoid stock, or one that was marked by strong selection pressures for wrist specializations.
Unfortunately, the generalized condition of the chimpanzee’s wrist joint and the very derived condition of the orang-utan
wrist provide uncertain evidence as to which of the two was first to diverge from the common hominoid stock. Identification
of hominoid wrist specializations as reflecting real phylogenetic relationships or parallelisms depends on how well the phytogeny
inferred from wrist morphology accords with those arrived at from the study of other systems. |
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Keywords: | os daubentonii ulnocarpal joint forearm rotation triangular articular disc hominoids |
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