Primary chondrification foci in the wing basipodium of Struthio camelus with comments on interpretation of autopodial elements in Crocodilia and Aves.

UNLABELLED: The present analysis consists of (1) description of the primary chondrification patterns and their transformation into ossified elements in the basipodium of Struthio camelus; (2) comparison of these with the conditions found in Alligator and Gallus; and (3) interpretation of the autopodial elements of Archaeopteryx.

CONCLUSIONS: (1) The existence of five discrete metacarpal condensations in the 16-day embryo of Struthio argues for unique linear patterning process for each, and these are interpreted as digits 2,3,4 originating from metacarpal condensations 2,3,4. Nine chondrogenic foci appear in the Struthio carpus: radiale, centrale, intermedium, ulnare, pseudoulnare, pisiform, distal carpal 2+3, distal carpal 4, and distal carpal 5. It is evident that: (a) the avian "radiale" represents fused chondrogenic foci of the intermedium plus the radiale; (b) a neomorph carpal element, the pseudoulnare (probably avian autapomorphy), replaces the ulnare cartilage in Struthio; (c) the pseudoulnare in Struthio and Hinchliffe's element "X" are not identical to each other. (2) Spatio-temporal conditions of the autopodium are less constrained in the development of Struthio than they are in Gallus. This favors the ostrich model as the more appropriate for interpretation of the autopodial skeleton in the oldest birds and their ancestors. (3) An interpretation of the elements of the hand skeleton of Archaeopteryx is as follows: (a) digits 2,3,4; (b) distal carpal 2+3 (the semilunate); (c) distal carpal 4 (a missing element filling the gap between the semilunate and metacarpal IV); (d) the radiale+intermedium complex (the proximal carpal bone); (e) the pseudoulnare (the proximal carpal bone).