Ancestry of the Tetrapods

Abstract

DURING the last few decades it has become increasingly clear that theTetrapods were derived from a group of fishes having their closest affinities with the Devonian Crossopterygii and Dipnoi. In 1932, Save-Soderbergh published a valuable description1 of some fossils, from the “Upper Devonian” of East Greenland, which afford a valuable bridge between the Crossopterygii and the Carboniferous Stegocephalians. These forms, the Ichthyostegalia, are much closer to the Stegocephalians than to the Crossopterygii in structure. The homology of most of their bones, in terms of the normal Tetrapod skull, is clear ; but Säve-Söderbergh, when comparing the Ichthyostegid skull with that of the Crossopterygii, found it necessary to adopt a very cumbersome modification of the terms used in the description of the latter group. The nomenclature adopted had a praiseworthy intention, to indicate the homologies of the comparatively specialized Tetrapod skull on the basis of the structure of generalized fossil fishes. Unfortunately, the nomenclature of the fishes concerned is based on Tetrapod names, which have largely been misapplied in fishes. As a result of this, the Tetrapod parietal and post-parietal were regarded by Säve-Söderbergh as “fronto-parietal” and “parieto-extrascapular”, and a number of other changes was suggested. Säve-Süderbergh recognized two bones in the place of the Tetrapod pre-frontal, and his attempts to find these elements in higher Stegocephalia². have led to further complications. I have suggested³ that the Tetrapod parietal contains no part of the Cross-opterygian “parietal“, but is represented in the ”frontal“ of such a fish as Osteolepis ; and more recently I have directed attention⁴ to theprobable errors in homology made by Säve-Süderbergh in the supra- and pre-orbital regions.