The Origin of Vertebrates

by Walter Holbrook Gaskell

The most internal layer a, by the formation of cartilage and then bone, represents the great mass of vertebrate fossils; the next layer b, by a process of calcification, as previously argued, represents the head-shield of the Osteostracan fishes; while the cuticular layer d, no longer thin, is the remnant of the Palæostracan head-carapace. Between these two layers, b and d, lies the laminated layer c. Intermediate to the Palæostracan and the Osteostracan comes the Heterostracan, with its peculiar head-shield—a head-shield whose origin is more easily conceivable as arising from something of the nature of the laminated layer than from any other structure represented in Ammocœtes.

My present suggestion, then, is this: the transition from the skeletal covering of the Palæostracan to that of the highest vertebrates was brought about by the calcification of successive layers from without inwards, all of which still remain in Ammocœtes and show how the external chitinous covering of the arthropod was gradually replaced by the deep-lying internal bony cranium of the higher vertebrates.

In Ammocœtes the layer which represents the covering of the Palæostracan has already almost disappeared. At transformation the layers representing the stage arrived at by the Heterostracan and the Osteostracan disappear; but the stage representing the higher vertebrates, far from disappearing, by the formation of cartilage reaches a higher stage and prepares the way for the ultimate stage of all—the formation of the bony cranium.

So much for the evidence as to the nature of the structure of the head-shield of the Pteraspidæ.

It suggests that these fishes were covered anteriorly with armoured plates derived from the cutis layer of the skin, a layer which was specially thickened and very vascular, apparently, to enable respiration to be very largely, if not entirely, effected by the surface of the body. It is difficult to understand how the sea-scorpions breathed, and it is easy to see how the formation of ventral and dorsal plates enclosing the mesosomatic appendages may at the outset have hindered the action of the branchiæ. The respiratory chamber, according to my view, had at first the double function of respiration and digestion. A new digestive apparatus was the pressing need at the time; it would, therefore, be of distinct advantage to remove, as much as possible, the burden of respiration from this incipient alimentary canal.

What can be said as to the shape of these ancient forms of fishes? Certain parts of them are absolutely known, other parts are guesswork. They are known to have possessed a dorsal shield, a ventral shield formerly looked upon as belonging to a separate species, called Scaphaspis, and a spine attached to the dorsal shield. The rest of their configuration, as given in Smith Woodward's restoration (Fig. [142]) is guesswork; the fish-like body with its scales, the heterocercal tail, is based on the most insufficient evidence of something of the nature of scales having being found near the head-plates.

The dorsal shield is characterized by a pair of lateral eyes situated on the edge of the shield, not as in Cephalaspis near the middle line. In the middle line, where the rostrum meets the large dorsal plate, median eyes were situated. But the slightest sign of any median single nasal opening, such as is so characteristic of the head-shield of the Osteostraci and of Ammocœtes has never been discovered. The olfactory organ must have been situated on the ventral side as in the larval stage of Ammocœtes, or in the Palæostraca. Many of these head-shields are remarkably well preserved, and it is difficult to believe that an olfactory opening would not be seen if any such had existed, as it does in Thyestes.

Fig. 142.—Restoration of Pteraspis. (After Smith Woodward.)

The difficulty of interpreting these types is the difficulty of understanding their method of locomotion; that is largely the reason why the spine has been placed as if projecting from the back, and a fish-like body with a heterocercal tail-fin added. If, on the contrary, the spine is a terminal tail-spine, then, as far as the fossilized remains indicate, the animal consisted of a dorsal shield, a ventral shield, and a tail-spine, to which must be added two apparently lateral pieces and a few scales. If the animal did not possess a flexible body with a tail-fin, but terminated in a rigid spike after the fashion of a Limulus-like animal, then it must have moved by means of appendages. At present we have not sufficient evidence to decide this question.