HOWEVER important these animals without backbones and plant fossils may be, scientifically or commercially, they have not the hold on the public imagination that the backboned creatures, or vertebrates, constantly exercise. The term ‘prehistoric animals,’ popular and loose though it is, is almost always synonymous with backboned animals, with the larger of the ancient fish, with giant amphibians, and especially with the enormous and grotesque extinct reptilian creatures and with their successors, the almost equally bizarre
mammals and the forerunners of man. Let us turn then to the study of these more important creatures.
The earliest of the backboned animals are known only from fragments contained in the Upper Ordovician rocks of the Western States of North America and of Russia. While the great developments of the invertebrate fauna were essentially marine, it would appear that these earliest known vertebrates developed and lived in fresh-water lakes and in the rivers, and that their excursion into wider habitats was not accomplished until probably the Devonian period, which is much later.
THE FISH ARM THEMSELVES FOR THE BATTLE OF LIFE IT is also of great interest to speculate upon the mode of life of these ancestral forms, the foundation of all succeeding backboned animals. It is very probable that at first they were speedy creatures, already well adapted in many ways, but like the earliest of the invertebrates, they were also unarmoured. Similarly, subject to some kind of attack, probably from a crustacean enemy, these primitive fish adopted armour and came to be smallish, broadened creatures, groping their way and feeding at the bottom of the water. Their descendants were, so far as we know, completely armed from the front of the head to the end of the tail in plates and scales. Curiously enough, it is known that, in some of the forms, this casing was of bone, and that therefore apparently the primitive fish were bony fish. In the development of the so-called primitive or lower fish living to-day, a cartilaginous stage precedes the bony, and for long it was considered that this was a primitive condition. Now, however, it may prove to be a secondary or degenerate state of affairs.
There were comparatively small fish, seldom more than a foot or two long, with a fish-like body with one or two fins on the back and perhaps the elements of a pair of pectoral fins behind the head. The head itself was a flattened, oval-shaped structure, completely covered above by a bony shield. In the middle of this shield, close together, were the eyes, which looked upwards. Between them was the tiny opening for the third, or pineal eye, of which we shall hear again later. The opening of the nostril was a single, unpaired slit. The mouth was without jaws and there were numerous gill-pouches. Strange as such creatures were, they bore many resemblances to such modern fish as the lampreys and the hag-fish, and it is
possible that these last are descendants of the armoured types but modified for a parasitic life.
Cephalaspis (head-shield) was typical of the primitive type of fish. Its slit-like mouth and its position on the head suggest that the fish was something of a scavenger, seeking its food on the bottom. The skull had a pair of bony horns, and it is believed that, as in certain eels, some of the bony plates were connected to the nervous system and may have served as protective electrical organs.
There were many allied forms of fish, but all were armoured. Their successors, too, were armed, and sometimes were of gigantic size, so much as thirty feet long. However, as has commonly been the fate of these heavily burdened forms of life, they did not survive. Among the primitive forms were the sharks, and they have become modified and have persisted; but few of their bone-protected contemporaries survived the Devonian period, leaving the field to other kinds of fish which were now fast developing and which were to leave their influence on both land and sea.
THE FISH THAT LEARNED TO LIVE ON LAND THE Devonian period is one of the great milestones in the study of fossils. As we have seen, in its deposits is the first definite mingling of the hitherto fresh-water fish and the marine forms of life. The primitive armed types of fish were dying out, giving way to freer swimming and faster types. The age was a time of comparative warmth, and in Old Red Sandstone days, many of the river pools were drying up, trapping yet preserving for all time the fish within them. This time has been well named the ‘The Age of Fishes,’ for many of its deposits are crowded with the remains from these dried fish pools.
While this was the end of life to many forms, it was a stimulus to others. Certain kinds living in pools which were drying up circumvented the oxygen deficiency of their element by a new device—the gulping of air. Very gradually this habit produced lung-breathing types of which we have direct descendants to-day. More than that, these lung fish produced two very important lines of development. They founded two groups, the first of which produced in time the majority of the fish we know to-day and thus secured the mastery of the seas; while the second group gave rise, though indirectly, to the partial conquerors of another element, the
land, for they produced eventually the amphibians, creatures living partly on land and partly in water. Thus, in the Devonian period, for the first time in the earth’s chequered history, a four-footed backboned creature made its bow upon the land.
This transition was responsible for many fundamental changes in the skeleton and external ornamentation of the creatures we have so far considered. In the water, a backbone composed of cartilage was quite satisfactory, and gill breathing was a necessity. For ease in swimming, it was an advantage to have the skull and shoulder girdle intimately connected. On land, however, things were very different. More solid support was necessary, and so a bony backbone came into being, while the skull and the shoulder bones ceased to be so intimately related in order to give more freedom of movement. Fins gave way to a long but somewhat compressed tail and to four simple limbs, each with five fingers or toes. Then, of course, gills disappeared in the adult, and some of the structures were absorbed into a more complicated blood circulation system, while other parts helped to form an ear capable of catching the more elusive sounds in the thinner medium of air.
The covering of scales and plates characteristic of many of the fish gave way to the moist skin we associate with modern frogs, but some of the earlier forms kept the conspicuous scales on the front or ventral side of their bodies.