Evolutionary change in the primate wrist and inferior radio‐ulnar joints

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Abstract
The joints at the junction of the forearm and hand are described in a number of Primates, including Man. It is shown that the original lower articular extremity of the ulna recedes from its primitive articulation with the triquetral and pisiform to become the ulnar styloid process, while a neomorphic ulnar head is elaborated entering into a new synovial inferior radio‐ulnar joint. The lower capsule of this joint becomes the triangular articular disk. In the anthropoid apes a meniscus (with a laterally‐directed concave free border bounding the ulnar styloid) develops in the interval between the receding ulna and the carpus. In gibbons this meniscus presents a lunula (os Daubentonii) which is also present in the human fetus as a transient cartilaginous nodule often erroneously homologized with the os intermedium. The upper articular surface of the human wrist joint is formed by the radius, the triangular articular disk and the homologue of the ape meniscus. Between the two latter components is an aperture, similar to that of apes, leading into a pea‐sized diverticulum of the wrist joint — the pre‐styloid recess. The mechanism of the evolution of new synovial joints is discussed.

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