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.
[Illustration: 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.