Part 9 (1/2)
The next order of the Deinosaurs is that of the biped vegetarians, the Ornithopods, which gradually became heavily armoured and quadrupedal.
The familiar Iguanodon is the chief representative of this order in Europe. Walking on its three-toed hind limbs, its head would be fourteen or fifteen feet from the ground. The front part of its jaws was toothless and covered with horn. It had, in fact, a kind of beak, and it also approached the primitive bird in the structure of its pelvis and in having five toes on its small front limbs. Some of the Ornithopods, such as the Laosaur, were small (three or four feet in height) and active, but many of the American specimens attained a great size. The Camptosaur, which was closely related to the Iguanodon in structure, was thirty feet from the snout to the end of the tail, and the head probably stood eighteen feet from the ground. One of the last great representatives of the group in America, the Trachodon, about thirty feet in length, had a most extraordinary head. It was about three and a half feet in length, and had no less than 2000 teeth lining the mouth cavity. It is conjectured that it fed on vegetation containing a large proportion of silica.
In the course of the Jura.s.sic, as we saw, a branch of these biped, bird-footed vegetarians developed heavy armour, and returned to the quadrupedal habit. We find them both in Europe and America, and must suppose that the highway across the North Atlantic still existed.
The Stegosaur is one of the most singular and most familiar representatives of the group in the Jura.s.sic. It ran to a length of thirty feet, and had a row of bony plates, from two to three feet in height, standing up vertically along the ridge of its back, while its tail was armed with formidable spikes. The Scleidosaur, an earlier and smaller (twelve-foot) specimen, also had spines and bony plates to protect it. The Polacanthus and Ankylosaur developed a most effective armour-plating over the rear. As we regard their powerful armour, we seem to see the fierce-toothed Theropods springing from the rear upon the poor-mouthed vegetarians. The carnivores selected the vegetarians, and fitted them to survive. Before the end of the Mesozoic, in fact, the Ornithopods became aggressive as well as armoured. The Triceratops had not only an enormous skull with a great ridged collar round the neck, but a sharp beak, a stout horn on the nose, and two large and sharp horns on the top of the head. We will see something later of the development of horns. The skulls of members of the Ceratops family sometimes measured eight feet from the snout to the ridge of the collar.
They were, however, sluggish and stupid monsters, with smaller brains even than the Sauropods.
Such, in broad outline, was the singular and powerful family of the Mesozoic Deinosaurs. Further geological research in all parts of the world will, no doubt, increase our knowledge of them, until we can fully understand them as a great family throwing out special branches to meet the different conditions of the crowded Jura.s.sic age. Even now they afford a most interesting page in the story of evolution, and their total disappearance from the face of the earth in the next geological period will not be unintelligible. We turn from them to the remaining orders of the Jura.s.sic reptiles.
In the popular mind, perhaps, the Ichthyosaur and Plesiosaur are the typical representatives of that extinct race. The two animals, however, belong to very different branches of the reptile world, and are by no means the most formidable of the Mesozoic reptiles. Many orders of the land reptiles sent a branch into the waters in an age which, we saw, was predominantly one of water-surface. The Ichthyosauria (”fish-reptiles”) and Thalattosauria (”sea-reptiles”) invaded the waters at their first expansion in the later Tria.s.sic. The latter groups soon became extinct, but the former continued for some millions of years, and became remarkably adapted to marine life, like the whale at a later period.
The Ichthyosaur of the Jura.s.sic is a remarkably fish-like animal. Its long tapering frame--sometimes forty feet in length, but generally less than half that length--ends in a dip of the vertebral column and an expansion of the flesh into a strong tail-fin. The terminal bones of the limbs depart more and more from the quadruped type, until at last they are merely rows of circular bony plates embedded in the broad paddle into which the limb has been converted. The head is drawn out, sometimes to a length of five feet, and the long narrow jaws are set with two formidable rows of teeth; one specimen has about two hundred teeth. In some genera the teeth degenerate in the course of time, but this merely indicates a change of diet. One fossilised Ichthyosaur of the weaker-toothed variety has been found with the remains of two hundred Belemnites in its stomach. It is a flash of light on the fierce struggle and carnage which some recent writers have vainly striven to attenuate.
The eyes, again, which may in the larger animals be fifteen inches in diameter, are protected by a circle of radiating bony plates. In fine, the discovery of young developed skeletons inside the adult frames has taught us that the Ichthgosaur had become viviparous, like the mammal.
Cutting its last connection with the land, on which it originated it ceased to lay eggs, and developed the young within its body.
The Ichthyosaur came of the reptile group which we have called the Diapsids. The Plesiosaur seems to belong to the Synapsid branch. In the earlier Mesozoic we find partially aquatic representatives of the line, like the Nothosaur, and in the later Plesiosaur the adaptation to a marine life is complete. The skin has lost its scales, and the front limbs are developed into powerful paddles, sometimes six feet in length.
The neck is drawn out until, in some specimens, it is found to consist of seventy-six vertebrae: the longest neck in the animal world. It is now doubted, however, if the neck was very flexible, and, as the jaws were imperfectly joined, the common picture of the Plesiosaur darting its snake-like neck in all directions to seize its prey is probably wrong. It seems to have lived on small food, and been itself a rich diet to the larger carnivores. We find it in all the seas of the Mesozoic world, varying in length from six to forty feet, but it is one of the sluggish and unwieldy forms that are destined to perish in the coming crisis.
The last, and perhaps the most interesting, of the doomed monsters of the Mesozoic was the Pterosaur, or ”flying reptile.” It is not surprising that in the fierce struggle which is reflected in the arms and armour of the great reptiles, a branch of the family escaped into the upper region. We have seen that there were leaping reptiles with hollow bones, and although the intermediate forms are missing, there is little doubt that the Pterosaur developed from one or more of these leaping Deinosaurs. As it is at first small, when it appears in the early Jura.s.sic--it is disputed in the late Tria.s.sic--it probably came from a small and agile Deinosaur, hunted by the carnivores, which relied on its leaping powers for escape. A flapperlike broadening of the fore limbs would help to lengthen the leap, and we must suppose that this membrane increased until the animal could sail through the air, like the flying-fish, and eventually sustain its weight in the air. The wing is, of course, not a feathery frame, as in the bird, but a special skin spreading between the fore limb and the side of the body. In the bat this skin is supported by four elongated fingers of the hand, but in the Pterosaur the fifth (or fourth) finger alone--which is enormously elongated and strengthened--forms its outer frame. It is as if, in flying experiments, a man were to have a web of silk stretching from his arm and an extension of his little finger to the side of his body.
From the small early specimens in the early Jura.s.sic the flying reptiles grow larger and larger until the time of their extinction in the stresses of the Chalk upheaval. Small Pterosaurs continue throughout the period, but from these bat-like creatures we rise until we come to such dragons as the American Pteranodon, with a stretch of twenty-two feet between its extended wings and jaws about four feet long. There were long-tailed Pterosaurs (Ramphorhyncus), sometimes with a rudder-like expansion of the end of the tail, and short-tailed Pterosaurs (Pterodactyl), with compact bodies and keeled b.r.e.a.s.t.s, like the bird. In the earlier part of the period they all have the heavy jaws and numerous teeth of the reptile, with four or five well-developed fingers on the front limbs. In the course of time they lose the teeth--an advantage in the distribution of the weight of the body while flying--and develop h.o.r.n.y beaks. In the gradual shaping of the breast-bone and head, also, they ill.u.s.trate the evolution of the bird-form.
But the birds were meantime developing from a quite different stock, and would replace the Pterosaurs at the first change in the environment.
There is ground for thinking that these flying reptiles were warm-blooded like the birds. Their hollow bones seem to point to the effective breathing of a warm-blooded animal, and the great vitality they would need in flying points toward the same conclusion. Their brain, too, approached that of the bird, and was much superior to that of the other reptiles. But they had no warm coats to retain their heat, no clavicle to give strength to the wing machinery, and, especially in the later period, they became very weak in the hind limbs (and therefore weak or slow in starting their flight). The coming selection will therefore dismiss them from the scene, with the Deinosaurs and Ammonites, and retain the better organised bird as the lord of the air.
There remain one or two groups of the Mesozoic reptiles which are still represented in nature. The turtle-group (Chelonia) makes its appearance in the Tria.s.sic and thrives in the Jura.s.sic. Its members are extinct and primitive forms of the thick-sh.e.l.led reptiles, but true turtles, both of marine and fresh water, abound before the close of the Mesozoic.
The sea-turtles attain an enormous size. Archelon, one of the primitive types, measured about twelve feet across the sh.e.l.l. Another was thirteen feet long and fifteen feet from one outstretched flipper to the other.
In the Chalk period they form more than a third of the reptile remains in some regions. They are extremely interesting in that they show, to some extent, the evolution of their characteristic sh.e.l.l. In some of the larger specimens the ribs have not yet entirely coalesced.
The Crocodilians also appear in the later Tria.s.sic, abound in the Jura.s.sic, and give way before the later types, the true Crocodiles, in the Cretaceous. They were marine animals with naked skin, a head and neck something like that of the Ichthyosaur, and paddles like those of the Plesiosaur. Their back limbs, however, were not much changed after their adaptation to life in the sea, and it is concluded that they visited the land to lay their eggs. The Teleosaur was a formidable narrow-spouted reptile, somewhat resembling the crocodiles of the Ganges in the external form of the jaws. The modern crocodiles, which replaced this ancient race of sea-crocodiles, have a great advantage over them in the fact that their nostrils open into the mouth in its lower depths.
They can therefore close their teeth on their prey under water and breathe through the nose.
Snakes are not found until the close of the Mesozoic, and do not figure in its characteristic reptile population. We will consider them later.
But there was a large group of reptiles in the later Mesozoic seas which more or less correspond to the legendary idea of a sea-serpent. These Dolichosaurs (”long reptiles”) appear at the beginning of the Chalk period, and develop into a group, the Mososaurians, which must have added considerably to the terrors of the sh.o.r.e-waters. Their slender scale-covered bodies were commonly twenty to thirty feet in length. The supreme representative of the order, the Mososaur, of which about forty species are known, was sometimes seventy-five feet long. It had two pairs of paddles--so that the name of sea-serpent is very imperfectly applicable--and four rows of formidable teeth on the roof of its mouth.
Like the Deinosaurs and Pterosaurs, the order was doomed to be entirely extinguished after a brief supremacy in its environment.
From this short and summary catalogue the reader will be able to form some conception of the living inhabitants of the Mesozoic world. It is a.s.suredly the Age of Reptiles. Worms, snails, and spiders were, we may a.s.sume, abundant enough, and a great variety of insects flitted from tree to tree or sheltered in the fern brakes. But the characteristic life, in water and on land, was the vast and diversified family of the reptiles. In the western and the eastern continent, and along the narrowing bridge that still united them, in the northern hemisphere and the southern, and along every ridge of land that connected them, these sluggish but formidable monsters filled the stage. Every conceivable device in the way of arms and armour, brute strength and means of escape, seemed to be adopted in their development, as if they were the final and indestructible outcome of the life-principle. And within a single geological period the overwhelming majority of them, especially the larger and more formidable of them, were ruthlessly slain, leaving not a single descendant on the earth. Let us see what types of animals were thus preferred to them in the next great application of selective processes.
CHAPTER XIII. THE BIRD AND THE MAMMAL
In one of his finest stories, Sur La Pierre Blanche, Anatole France has imagined a group of Roman patricians discussing the future of their Empire. The Christians, who are about to rise to power on their ruin, they dismiss with amiable indifference as one of the little pa.s.sing eccentricities of the religious life of their time. They have not the dimmest prevision, even as the dream of a possibility, that in a century or two the Empire of Rome will lie in the dust, and the cross will tower above all its cities from York to Jerusalem. If we might for a moment endow the animals of the Mesozoic world with AEsopian wisdom, we could imagine some such discussion taking place between a group of Deinosaur patricians. They would reflect with pride on the unshakable empire of the reptiles, and perhaps glance with disdain at two types of animals which hid in the recesses or fled to the hills of the Jura.s.sic world.
And before another era of the earth's story opened, the reptile race would be dethroned, and these hunted and despised and feeble eccentricities of Mesozoic life would become the masters of the globe.
These two types of organisms were the bird and the mammal. Both existed in the Jura.s.sic, and the mammals at least had many representatives in the Tria.s.sic. In other words, they existed, with all their higher organisation, during several million years without attaining power. The mammals remained, during at least 3,000,000 years, a small and obscure caste, immensely overshadowed by the small-brained reptiles. The birds, while making more progress, apparently, than the mammals, were far outnumbered by the flying reptiles until the last part of the Mesozoic.
Then there was another momentous turn of the wheel of fate, and they emerged from their obscurity to a.s.sume the lords.h.i.+p of the globe.
In earlier years, when some serious hesitation was felt by many to accept the new doctrine of evolution, a grave difficulty was found in the circ.u.mstance that new types--not merely new species and new genera, but new orders and even sub-cla.s.ses--appeared in the geological record quite suddenly. Was it not a singular coincidence that in ALL cases the intermediate organisms between one type and another should have wholly escaped preservation? The difficulty was generally due to an imperfect acquaintance with the conditions of the problem. The fossil population of a period is only that fraction of its living population which happened to be buried in a certain kind of deposit under water of a certain depth. We shall read later of insects being preserved in resin (amber), and we have animals (and even bacteria) preserved in trees from the Coal-forests. Generally speaking, however, the earth has buried only a very minute fraction of its land-population. Moreover, only a fraction of the earth's cemeteries have yet been opened. When we further reflect that the new type of organism, when it first appears, is a small and local group, we see what the chances are of our finding specimens of it in a few scattered pages of a very fragmentary record of the earth's life. We shall see that we have discovered only about ten skeletons or fragments of skeletons of the men who lived on the earth before the Neolithic period; a stretch of some hundreds of thousands of years, recorded in the upper strata of the earth.