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

by David Starr Jordan

"This theory of the vertebrate paired limb seems to me, I confess, to be a more plausible one on the face of it than either of the two which at present hold the field. If untrue, it is so dangerously plausible as to surely deserve more consideration than it appears to have had. One of the main differences between it and the other two hypotheses is that, instead of deriving the swimming-fin from the walking and supporting limb, it goes the other way about. That this is the safer line to take seems to me to be shown by the consideration that a very small and rudimentary limb could only be of use if provided with a fixed point d'appui. Also on this view, the pentadactyle limb and the swimming-fin would probably be evolved independently from a simple form of limb. This would evade the great difficulties which have beset those who have endeavored to establish the homologies of the elements of the pentadactyle limb with those of any type of fully formed fin."

Uncertain Conclusions.—In conclusion we may say that the evidence of embryology in this matter is inadequate, though possibly favoring on the whole the fin-fold theory; that of morphology is inconclusive, and probably the final answer may be given by paleontology. If the records of the rocks were complete, they would be decisive. At present we have to decide which is the more primitive of two forms of pectoral fin actually known among fossils. That of Cladoselache is a low, horizontal fold of skin, with feeble rays, called by Cope ptychopterygium. That of Pleuracanthus is a jointed paddle-shaped appendage with a fringe of rays on either side. In the theory of Gegenbaur and Kerr Pleuracanthus must be, so far as the limbs are concerned, the form nearest the primitive limb-bearing vertebrate. In Balfour's theory Cladoselache is nearest the primitive type from which the other and with it the archipterygium of later forms may be derived.

Boulenger and others question even this, believing that the archipterygium in Pleuracanthus and other primitive sharks and that in Neoceratodus and its Dipnoan and Crossopterygian allies and ancestors have been derived independently, not the latter from the former. In this view there is no real homology between the archipterygium in the sharks possessing it and that in the Dipnoans and Crossopterygians. In the one theory the type of Pleuracanthus would be ancestral to the other sharks on the one hand, and to Crossopterygians and all higher vertebrates on the other. With the theory of the origin of the pectoral from a lateral fold, Pleuracanthus would be merely a curious specialized offshoot from the primitive sharks, without descendants and without special significance in phylogeny.

As elements bearing on this decision we may note that the tapering unspecialized diphycercal tail of Pleuracanthus seems very primitive in comparison with the short heterocercal tail of Cladoselache. This evidence, perhaps deceptive, is balanced by the presence on the head of Pleuracanthus of a highly specialized serrated spine, evidence of a far from primitive structure. Certainly neither the one genus nor the other actually represents the primitive shark. But as Cladoselache appears in geological time, long before Pleuracanthus, Cladodus, or any other shark with a jointed, archipterygial fin, the burden of proof, according to Dean, rests with the followers of Gegenbaur. If the remains found in the Ordovician at Cañon City referred to Crossopterygians are correctly interpreted, we must regard the shark ancestry as lost in pre-Silurian darkness, for in sharks of some sort the Crossopterygians apparently must find their remote ancestry.

Fig. 63.—Heterocercal tail of Sturgeon, Acipenser sturio (Linnæus). (After Zittel.)

Forms of the Tail in Fishes.—In the process of development the median or vertical fins are, as above stated, older than the paired fins or limbs, whatever be the origin of the latter. They arise in a dermal keel, its membranes fitting and accentuating the undulatory motion of the body.

In this elementary fin-fold slender supports (actinotrichia), the rudiments of fin-rays, appear at intervals. In those fins of most service in the movement of the fish, the fin-rays are strengthened, and their basal supports specialized.

Dean calls attention to the fact that in fishes which swim, when adult, by an undulatory motion, the paired fins tend to disappear, as in the eel and in all eel-like fishes, as blennies and eel-pouts.