Cheiropterygium

All anatomists are agreed that the limbs of the higher Vertebrata are derived from those of Fishes, but the gulf between the two types of limbs is so great that there is room for a very great diversity of opinion as to the mode of evolution of the cheiropterygium.

The relation of the ichthyopterygium to the cheiropterygium is also at present obscure; the structure of the latter is fundamentally identical in all those animals in which it occurs.

If the limbs of Ceratodus may be taken as a pattern of an ichthyopterygium there has been a secular shortening of some, and suppression of other, axial parts or internodes: thus we get the shortened radiating fan-shaped limb—the propert cheiropterygium; that, in turn, had to be transformed into the avian fore limb or wing .

The chiridial appendage (cheiropterygium) is primarily a walking leg (Fig. 50, a), designed to aid in pushing the body along the ground or, when extremely developed, to support the body above the surface and assume the entire function of locomotion.

While all these differences may be simply primitive characters, separating members of two distinct classes, nevertheless they raise the question whether the tetrapod resembles in the pectoral fin of rhizodonts may not be fortuitous, and without phylogenetic significance; but in view of the differences in function of the Sauripterus paddle and a true cheiropterygium such underlying similarities as may exist can hardly be attributed to convergence, and when taken in connection with the resemblances in the skull above noted (pp. 332-337) they gain in importance.