Tetrapod limb

A tetrapod limb is a paired appendage of a tetrapod, such as a forelimb or hindlimb, organized around a shared inherited skeletal pattern. In a familiar forelimb, one proximal bone, the humerus, is followed by the radius bone and ulna, then more distal elements and limb digits. The corresponding structures can differ greatly in proportions and use. Comparing their relationships, rather than just their overall shape, is central to recognizing homology.

The transition from a fin to a limb was not an instantaneous appearance of an entire new skeleton. A 2006 study of Tiktaalik described a pectoral fin with an expanded distal internal skeleton and articulations permitting a substrate-supported posture. The reconstructed joint configurations included flexed shoulder and elbow regions and an extended distal region. This indicates precursors of limb-like support in a finned animal; it does not make every distal fin element a limb digit.^[ref]

A useful comparison traces the humerus, radius bone and ulna in two differently shaped forelimbs, then asks which elements contact one another. A wing and a walking appendage can preserve corresponding elements while redistributing their lengths and mechanical roles. Shared ancestry and present function are related questions, but one cannot be read directly from the other.