Monday, May 17, 2010

Bird and Pterosaur Carpus (1)

"The pterosaur carpus consists of two proximal [closest to the forearm] and four distal [furthest from the forearm] carpals, excluding the pteroid, which may itself be a modified distal carpal (Unwin et al. 1996). The proximal carpals are fused into a proximal syncarpal in osteologically mature specimens, while three of the distal carpals fuse to form a distal syncarpal (Bennett 1993). The remaining distal carpal,  referred to here as the medial carpal (Padian 1984), but which has also been termed the distal lateral (Wellnhofer 1985), or pre-axial carpal (Bennett 2001), articulates on a vertically elongate biconvex facet on the anterior surface of the distal syncarpal. The medial carpal bears a deep concave fovea that opens anteriorly, ventrally and somewhat medially, within which the pteroid articulates (figure 1b)."

Skeletal reconstruction of A. santanae. (a) Reconstructed skeleton of the right wing and membrane outlines in dorsal view, adapted from Wellnhofer (1991b), showing the pteroid in the antero-ventral orientation, supporting a broad propatagium (solid line) and in the medial orientation, supporting a narrow propatagium (broken line): scale bar=200 mm. (b) Right medial carpal in distal view and right pteroid in proximal view, showing the articular surfaces of the carpal–pteroid joint: scale bar=20 mm. (c) Right wrist in antero-medial view, showing articular motion of the pteroid. Two planes have been superimposed, intersecting at the carpal–pteroid joint—one parallel to the wing spar, one normal to it. During the initial phase of flexion the pteroid (solid line) occupies the normal plane, and angulation thus takes the form of pure depression. During the second phase the articular head of the pteroid rotates laterally with respect to the medial carpal, and angulation therefore gradually shifts from depression to adduction. The pteroid swings out of the normal plane until at the limit of flexion (broken line) it comes to occupy the parallel plane. Abbreviations: ch, cheiropatagium; cr, cruropatagium; ds, distal syncarpal; f, femur; fov, fovea of the medial carpal; h, humerus; lf, lateral facet of the pteroid; mc, medial carpal; mf, medial facet of the pteroid; pro, propatagium; ps, proximal syncarpal; pt, pteroid; r, radius; t, tibiotarsus; u, ulna; wf, wing-finger; wm, wing-metacarpal.

"During ontogeny two bones in the proximal row, probably the radiale and ulnare (Wellnhofer. 1970), fused to form a proximal syncarpal (Bennett 1993)."

No comments:

Post a Comment