A Guide to the Study of Fishes, Volume 1 (of 2)

by David Starr Jordan

Fig. 299.—Section of vertebræ of sharks, showing calcification. (After Hasse.) 1. Cyclospondyli (Squalus); 2. Tectospondyli (Squatina); 3. Asterospondyli (Carcharias).

Nothing is known of the Polyospondyli, and they may never have existed at all. The Diplospondyli do not differ very widely from the earlier Asterospondyli (Cestraciontes) which, as a matter of fact, have preceded the Diplospondyli in point of time, if we can trust our present knowledge of the geological record.

Other Classifications of Elasmobranchs.—Characters more fundamental may be drawn from the structure of the pectoral fin. In this regard four distinct types appear. In Acanthoessus this fin consists of a stout, stiff spine, with a rayless membrane attached behind it. In Cladoselache the fin is low, with a very long base, like a fold of skin (ptychopterygium), and composed of feeble rays. In Pleuracanthus it is a jointed axis of many segments, with a fringe of slender fin-rays, corresponding in structure to all appearance to the pectoral fin of Dipnoans and Crossopterygians, the type called by Gegenbaur archipterygium on the hypothesis that it represents the primitive vertebrate limb.

In most sharks the fin has a fan-shape, with three of the basal segments larger than the others. Of these the mesopterygium is the central one, with the propterygium before it and the metapterygium behind. In the living sharks of the family of Heterodontidæ, this form of fin occurs and the teeth of the same general type constitute the earliest remains distinctly referable to sharks in the Devonian rocks.

Primitive Sharks.—Admitting that these four types of pectoral fin should constitute separate orders, we have next to consider which form is the most primitive and what is the line of descent. In this matter we have, in the phrase of Hæckel, only the "three ancestral documents, Palæontology, Morphology, and Ontogeny."

Unfortunately the evidence of these documents is incomplete and conflicting. So far as Palæontology is concerned, the fin of Cladoselache, with that of Acanthoessus, which may be derived from it, appears earliest, but the modern type of pectoral fin with the three basal segments is assumed to have accompanied the teeth of Psammodonts and Cochliodonts, while the fin of the Chimæra must have been developed in the Devonian. The jointed fin of Cladodus and Pleuracanthus may be a modification or degradation of the ordinary type of shark-fin.

Assuming, however, that the geological record is not perfect and that the fin of Cladoselache is not clearly shown to be primitive, we have next to consider the evidence drawn from morphology.

Those who with Balfour and others (see page [69]) accept the theory that the paired fins are derived from a vertebral fold, will regard with Dean the fin of Cladoselache as coming nearest the theoretical primitive condition.

The pectoral fin in Acanthoessus Dean regards as a specialized derivative from a fin like that of Cladoselache, the fin-rays being gathered together at the front and joined together to form the thick spine characteristic of Acanthoessus. This view of the morphology of the fin of Acanthoessus is not accepted by Woodward, and several different suggestions have been recorded.