Part 8 (2/2)
Finally, we find a continuous and conspicuous advance among the fishes.
At the close of the Tria.s.sic and during the Jura.s.sic they seem to undergo profound and comparatively rapid changes. The reason will, perhaps, be apparent in the next chapter, when we describe the gigantic reptiles which feed on them in the lakes and sh.o.r.e-waters. A greater terror than the shark had appeared in their environment. The Ganoids and Dipneusts dwindle, and give birth to their few modern representatives.
The sharks with crus.h.i.+ng teeth diminish in number, and the sharp-toothed modern shark attains the supremacy in its cla.s.s, and evolves into forms far more terrible than any that we know to-day. Skates and rays of a more or less modern type, and ancestral gar-pikes and sturgeons, enter the arena. But the most interesting new departure is the first appearance, in the Jura.s.sic, of bony-framed fishes (Teleosts). Their superiority in organisation soon makes itself felt, and they enter upon the rapid evolution which will, by the next period, give them the first place in the fish world.
Over the whole Mesozoic world, therefore, we find advance and the promise of greater advance. The Permian stress has selected the fittest types to survive from the older order; the Jura.s.sic luxuriance is permitting a fresh and varied expansion of life, in preparation for the next great annihilation of the less fit and selection of the more fit.
Life pauses before another leap. The Mesozoic earth--to apply to it the phrase which a geologist has given to its opening phase--welcomes the coming and speeds the parting guest. In the depths of the ocean a new movement is preparing, but we have yet to study the highest forms of Mesozoic life before we come to the Cretaceous disturbances.
CHAPTER XII. THE AGE OF REPTILES
From one point of view the advance of life on the earth seems to proceed not with the even flow of a river, but in the successive waves of an oncoming tide. It is true that we have detected a continuous advance behind all these rising and receding waves, yet their occurrence is a fact of some interest, and not a little speculation has been expended on it. When the great procession of life first emerges out of the darkness of Archaean times, it deploys into a spreading world of strange Crustaceans, and we have the Age of Trilobites. Later there is the Age of Fishes, then of Cryptogams and Amphibia, and then of Cycads and Reptiles, and there will afterwards be an Age of Birds and Mammals, and finally an Age of Man. But there is no ground for mystic speculation on this circ.u.mstance of a group of organisms fording the earth for a few million years, and then peris.h.i.+ng or dwindling into insignificance. We shall see that a very plain and substantial process put an end to the Age of the Cycads, Ammonites, and Reptiles, and we have seen how the earlier dynasties ended.
The phrase, however, the Age of Reptiles, is a fitting and true description of the greater part of the Mesozoic Era, which lies, like a fertile valley, between the Permian and the Chalk upheavals. From the bleak heights of the Permian period, or--more probably--from its more sheltered regions, in which they have lingered with the ferns and cycads, the reptiles spread out over the earth, as the summer of the Tria.s.sic period advances. In the full warmth and luxuriance of the Jura.s.sic they become the most singular and powerful army that ever trod the earth. They include small lizard-like creatures and monsters more than a hundred feet in length. They swim like whales in the shallow seas; they shrink into the sh.e.l.l of the giant turtle; they rear themselves on towering hind limbs, like colossal kangaroos; they even rise into the air, and fill it with the dragons of the fairy tale. They spread over the whole earth from Australia to the Arctic circle. Then the earth seems to grow impatient of their dominance, and they shrink towards the south, and struggle in a diminished territory. The colossal monsters and the formidable dragons go the way of all primitive life, and a ragged regiment of crocodiles, turtles, and serpents in the tropics, with a swarm of smaller creatures in the fringes of the warm zone, is all that remains, by the Tertiary Era, of the world-conquering army of the Mesozoic reptiles.
They had appeared, as we said, in the Permian period. Probably they had been developed during the later Carboniferous, since we find them already branched into three orders, with many sub-orders, in the Permian. The stimulating and selecting disturbances which culminated in the Permian revolution had begun in the Carboniferous. Their origin is not clear, as the intermediate forms between them and the amphibia are not found. This is not surprising, if we may suppose that some of the amphibia had, in the growing struggle, pushed inland, or that, as the land rose and the waters were drained in certain regions, they had gradually adopted a purely terrestrial life, as some of the frogs have since done. In the absence of water their frames would not be preserved and fossilised. We can, therefore, understand the gap in the record between the amphibia and the reptiles. From their structure we gather that they sprang from at least two different branches of the amphibia.
Their remains fall into two great groups, which are known as the Diapsid and the Synapsid reptiles. The former seem to be more closely related to the Microsauria, or small salamander-like amphibia of the Coal-forest; the latter are nearer to the Labyrinthodonts. It is not suggested that these were their actual ancestors, but that they came from the same early amphibian root.
We find both these groups, in patriarchal forms, in Europe, North America, and South Africa during the Permian period. They are usually moderate in size, but in places they seem to have found good conditions and prospered. A few years ago a Permian bed in Russia yielded a most interesting series of remains of Synapsid reptiles. Some of them were large vegetarian animals, more than twelve feet in length; others were carnivores with very powerful heads and teeth as formidable as those of the tiger. Another branch of the same order lived on the southern continent, Gondwana Land, and has left numerous remains in South Africa.
We shall see that they are connected by many authorities with the origin of the mammals. [*] The other branch, the Diapsids, are represented to-day by the curiously primitive lizard of New Zealand, the tuatara (Sphenodon, or Hatteria), of which I have seen specimens, nearly two feet in length, that one did not care to approach too closely. The Diapsids are chiefly interesting, however, as the reputed ancestors of the colossal reptiles of the Jura.s.sic age and the birds.
* These Synapsid reptiles are more commonly known as Pareiasauria or Theromorpha.
The purified air of the Permian world favoured the reptiles' being lung-breathers, but the cold would check their expansion for a time.
The reptile, it is important to remember' usually leaves its eggs to be hatched by the natural warmth of the ground. But as the cold of the Permian yielded to a genial climate and rich vegetation in the course of the Tria.s.sic, the reptiles entered upon their memorable development. The amphibia were now definitely ousted from their position of dominance.
The increase of the waters had at first favoured them, and we find more than twenty genera, and some very large individuals, of the amphibia in the Tria.s.sic. One of them, the Mastodonsaurus, had a head three feet long and two feet wide. But the spread of the reptiles checked them, and they shrank rapidly into the poor and defenceless tribe which we find them in nature to-day.
To follow the prolific expansion of the reptiles in the semi-tropical conditions of the Jura.s.sic age is a task that even the highest authorities approach with great diffidence. Science is not yet wholly agreed in the cla.s.sification of the vast numbers of remains which the Mesozoic rocks have yielded, and the affinities of the various groups are very uncertain. We cannot be content, however, merely to throw on the screen, as it were, a few of the more quaint and monstrous types out of the teeming Mesozoic population, and describe their proportions and peculiarities. They fall into natural and intelligible groups or orders, and their features are closely related to the differing regions of the Jura.s.sic world. While, therefore, we must abstain from drawing up settled genealogical trees, we may, as we review in succession the monsters of the land, the waters, and the air, glance at the most recent and substantial conjectures of scientific men as to their origin and connections.
The Deinosaurs (or ”terrible reptiles”), the monarchs of the land and the swamps, are the central and outstanding family of the Mesozoic reptiles. As the name implies, this group includes most of the colossal animals, such as the Diplodocus, which the ill.u.s.trated magazine has made familiar to most people. Fortunately the a.s.siduous research of American geologists and their great skill and patience in restoring the dead forms enable us to form a very fair picture of this family of medieval giants and its remarkable ramifications. [*]
* See, besides the usual authorities, a valuable paper by Dr. R. S. Lull, ”Dinosaurian Distribution” (1910).
The Diapsid reptiles of the Permian had evolved a group with h.o.r.n.y, parrot-like beaks, the Rhyncocephalia (or ”beak-headed” reptiles), of which the tuatara of New Zealand is a lingering representative. New Zealand seems to have been cut off from the southern continent at the close of the Permian or beginning of the Tria.s.sic, and so preserved for us that very interesting relic of Permian life. From some primitive level of this group, it is generally believed, the great Deinosaurs arose. Two different orders seem to have arisen independently, or diverged rapidly from each other, in different parts of the world. One group seems to have evolved on the ”lost Atlantis,” the land between Western Europe and America, whence they spread westward to America, eastward over Europe, and southward to the continent which still united Africa and Australia. We find their remains in all these regions.
Another stock is believed to have arisen in America.
Both these groups seem to have been more or less biped, rearing themselves on large and powerful hind limbs, and (in some cases, at least) probably using their small front limbs to hold or grasp their food. The first group was carnivorous, the second herbivorous; and, as the reptiles of the first group had four or five toes on each foot, they are known as the Theropods (or ”beast-footed” ), while those of the second order, which had three toes, are called the Ornithopods (or ”bird-footed”). Each of them then gave birth to an order of quadrupeds.
In the spreading waters and rich swamps of the later Tria.s.sic some of the Theropods were attracted to return to an amphibious life, and became the vast, sprawling, ponderous Sauropods, the giants in a world of giants. On the other hand, a branch of the vegetarian Ornithopods developed heavy armour, for defence against the carnivores, and became, under the burden of its weight, the quadrupedal and monstrous Stegosauria and Ceratopsia. Taking this instructive general view of the spread of the Deinosaurs as the best interpretation of the material we have, we may now glance at each of the orders in succession.
The Theropods varied considerably in size and agility. The Compsognathus was a small, active, rabbit-like creature, standing about two feet high on its hind limbs, while the Megalosaurs stretched to a length of thirty feet, and had huge jaws armed with rows of formidable teeth. The Ceratosaur, a seventeen-foot-long reptile, had hollow bones, and we find this combination of lightness and strength in several members of the group. In many respects the group points more or less significantly toward the birds. The brain is relatively large, the neck long, and the fore limbs might be used for grasping, but had apparently ceased to serve as legs. Many of the Theropods were evidently leaping reptiles, like colossal kangaroos, twenty or more feet in length when they were erect. It is the general belief that the bird began its career as a leaping reptile, and the feathers, or expanded scales, on the front limbs helped at first to increase the leap. Some recent authorities hold, however, that the ancestor of the bird was an arboreal reptile.
To the order of the Sauropods belong most of the monsters whose discovery has attracted general attention in recent years. Feeding on vegetal matter in the luscious swamps, and having their vast bulk lightened by their aquatic life, they soon attained the most formidable proportions. The admirer of the enormous skeleton of Diplodocus (which ran to eighty feet) in the British Museum must wonder how even such ma.s.sive limbs could sustain the mountain of flesh that must have covered those bones. It probably did not walk so firmly as the skeleton suggests, but sprawled in the swamps or swam like a hippopotamus. But the Diplodocus is neither the largest nor heaviest of its family. The Brontosaur, though only sixty feet long, probably weighed twenty tons.
We have its footprints in the rocks to-day, each impression measuring about a square yard. Generally, it is the huge thigh-bones of these monsters that have survived, and give us an idea of their size. The largest living elephant has a femur scarcely four feet long, but the femur of the Atlantosaur measures more than seventy inches, and the femur of the Brachiosaur more than eighty. Many of these Deinosaurs must have measured more than a hundred feet from the tip of the snout to the end of the tail, and stood about thirty feet high from the ground. The European Sauropods did not, apparently, reach the size of their American cousins--so early did the inferiority of Europe begin--but our Ceteosaur seems to have been about fifty feet long and ten feet in height.
Its thigh-bone was sixty-four inches long and twenty-seven inches in circ.u.mference at the shaft. And in this order of reptiles, it must be remembered, the bones are solid.
To complete the picture of the Sauropods, we must add that the whole cla.s.s is characterised by the extraordinary smallness of the brain.
The twenty-ton Brontosaur had a brain no larger than that of a new-born human infant. Quite commonly the brain of one of these enormous animals is no larger than a man's fist. It is true that, as far as the muscular and s.e.xual labour was concerned, the brain was supplemented by a great enlargement of the spinal cord in the sacral region (at the top of the thighs). This inferior ”brain” was from ten to twenty times as large as the brain in the skull. It would, however, be fully occupied with the movement of the monstrous limbs and tail, and the s.e.x-life, and does not add in the least to the ”mental” power of the Sauropods. They were stupid, sluggish, unwieldy creatures, swollen parasites upon a luxuriant vegetation, and we shall easily understand their disappearance at the end of the Mesozoic Era, when the age of brawn will yield to an age of brain.
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