If with Gegenbaur we regard the paired fins as derived from the septa between the gill-slits, or with Kerr regard them as modified external gills, the whole theoretical relation of the parts is changed. The archipterygium of Pleuracanthus would be the nearest approach to the primitive pectoral limb, and from this group and its allies all the other sharks are descended. This central jointed axis of Pleuracanthus is regarded by Traquair as the equivalent of the metapterygium in ordinary sharks. (See Figs. 44, 45, 46.)
According to Traquair: "The median stern [of the archipterygium], simplified, shortened up and losing all its radials on the postaxial side, except in sometimes a few near the tip, becomes the metapterygium, while the mesopterygium and propterygium are formed by the fusion into two pieces of the basal joints of a number of preaxial radials, which have reached and become attached to the shoulder-girdle in front of the metapterygium."
According to Dr. Traquair, the pectoral fin in Cladodus neilsoni, a shark from the Coal Measures of Scotland, is "apparently a veritable uniserial archipterygium midway between the truly biserial one of Pleuracanthus and the pectoral fin of ordinary sharks." Other authors look on these matters differently, and Dr. Traquair admits that an opposite view is almost equally probable. Cope and Dean would derive the tribasal pectoral of ordinary sharks directly from the ptychopterygium or fan-like fold of Cladoselache, while Fritsch and Woodward would look upon it as derived in turn from the Ceratodus-like fin of Pleuracanthus, itself derived from the ptychopterygium or remains of a lateral fin-fold.
If the Dipnoans are descended from the Crossopterygians, as Dollo has tried to show, the archipterygium of Pleuracanthus has had a different origin from the similar-appearing limb of the Dipnoans, Dipterus and Ceratodus.
In such case the archipterygium would not be the primitive pectoral limb, but a structure which may have been independently evolved in two different groups.
In the view of Gegenbaur, the Crossopterygians and Dipnoans with all the higher vertebrates and the bony fishes would arise from the same primitive stock, ancestors, or allies of the Ichthyotomi, which group would also furnish the ancestors of the Chimæras. In support of this view, the primitive protocercal or diphycercal tail of Pleuracanthus may be brought in evidence as against the apparently more specialized heterocercal tail of Cladoselache. But this is not conclusive, as the diphycercal tail may arise separately in different groups through degeneration, as Dollo and Boulenger have shown.
The matter is one mainly of morphological interpretation, and no final answer can be given. On page [68] a summary of the various arguments may be found. Little light is given by embryology. The evidence of Palæontology, so far as it goes, certainly favors the view of Balfour. Omitting detached fin-spines and fragments of uncertain character, the earliest identifiable remains of sharks belong to the lower Devonian. These are allies of Acanthoessus. Cladoselache comes next in the Upper Devonian. Pleuracanthus appears with the teeth and spines supposed to belong to Cestraciont sharks, in the Carboniferous Age. The primitive-looking Notidani do not appear before the Triassic. For this reason the decision as to which is the most primitive type of shark must therefore rest unsettled for the present and perhaps for a long time to come.
The weight of authority at present seems to favor the view of Balfour, Wiedersheim, Boulenger, and Dean, that the pectoral limb has arisen from a lateral fold of skin. But weight of authority is not sufficient when evidence is confessedly lacking.
For our purpose, without taking sides in this controversy, we may follow Dean in allowing Cladoselache to stand as the most primitive of known sharks, thus arranging the Elasmobranchs and rays, recent and fossil, in six orders of unequal value—Pleuropterygii, Acanthodei, Ichthyotomi, Notidani, Asterospondyli, and Tectospondyli. Of these orders the first and second are closely related, as are also the fourth and fifth, the sixth being not far remote. The true sharks form the culmination of one series, the rays of another, while from the Ichthyotomi the Crossopterygians and their descendants may be descended. But this again is very hypothetical, or perhaps impossible; while, on the other hand, the relation of the Chimæras to the sharks is still far from clearly understood.
Order Pleuropterygii.—The order of Pleuropterygii of Dean (πλεύρον, side; πτερύξ, fin), called by Parker and Haswell Cladoselachea, consists of sharks in which the pectoral and ventral fins have each a very wide horizontal base (ptychopterygium), without jointed axis and without spine. There are no spines in any of the fins. The dorsal fin is low, and there were probably two of them. The notochord is persistent, without intercalary cartilage, such as appear in the higher sharks. The caudal fin is short, broad, and strongly heterocercal. Apparently the ventral fin is without claspers. The gill-openings were probably covered by a dermal fold. The teeth are weak, being modified denticles from the asperities of the skin. The lateral line is represented by an open groove. The family of Cladoselachidæ consists of a single genus Cladoselache from the Cleveland shale or Middle Devonian of Ohio. Cladoselache fyleri is the best-known species, reaching a length of about two feet. Dean regards this as the most primitive of the sharks, and the position of the pectorals and ventrals certainly lend weight to Balfour's theory that they were originally derived from a lateral fold of skin. I am recently informed by Dr. Dean that he has considerable evidence that in Cladoselache the anus was subterminal. If this statement is verified, it would go far to establish the primitive character of Cladoselache.