The Romance of Modern Geology / Describing in simple but exact language the making of the earth with some account of prehistoric animal life

by Edwin Sharpe Grew

CHAPTER XX

THE AGE OF REPTILES

(Continued.)

The Triassic period in its later stages was very like the earlier period of the era which followed it, and the reptiles which were characteristic of the close of the first were continued in some cases with only slight differentiation in the second. The Plesiosaurs and Ichthyosaurs are associated with the Trias, and we may therefore describe them now. Though some of these large aquatic creatures must have measured thirty feet from snout to tail, they do not equal in size the great aquatic mammals of to-day—the whales. In life the Plesiosaur had a body like the hull of a submarine with four great paddles attached—the fore and the hind legs. It had a long neck like a gigantic swan, and an elongated head provided with powerful jaws armed with numerous pointed teeth. It probably could swim under water as well as on the surface, and when floating could snap small lizards from the land. The paddles have a definite structure like legs, with five toes, wrist or ankle, forearm or foreleg, and upper arm or thigh. A great number of these Plesiosaurs have been found in the Lias formation of the south of England; and slabs containing whole skeletons have frequently been obtained. They and two similarly embedded and flattened skeletons of different kinds of Ichthyosaurs may be seen in quantity on the wall of the gallery of fossil reptiles in the Natural History Museum at South Kensington.

The Ichthyosaurs were much more fish-like or whale-like in form than the Plesiosaurs. "They were, indeed," says Sir E. Ray Lankester, "singularly like the porpoises and grampuses among living whales and stand in the same relation to land-living reptiles that the porpoises do to land-living mammals. Their fish-like appearance and fins are not primitive characters and do not indicate any closer blood relationship to fishes than that possessed by other reptiles. They are the offspring of four-legged terrestrial reptiles which have become specially modified and adapted to submarine life." Like many whales, they had a fin on the back devoid of bony support. The Ichthyosaur had a ring of bony plates supporting the eyeball (as birds also have), and these are often preserved in the fossil specimens.

At the end of the Triassic period some strata were laid down which have been called "Beds of passage." We have seen that the Triassic strata were probably deposited, altogether or in part, in extensive salt lakes or inland seas. At the close of the Triassic period the waters of the ocean were admitted to these areas by the sinking of the land at some point or other of their margins. With the sea-water came many living things—fishes, shells, etc.—and the very scanty life of the Triassic lake was replaced by an abundance of marine life. These beds were called the Rhætic beds because they were first found in the old Roman province of Rhætia, which occupied an Alpine district between Bavaria and Lombardy. Here they were thickest, 3000 feet of limestones and shales; but they have since been found either thicker or thinner everywhere in England, and in the United States, as well as in other parts of Europe wherever we can find the Lias lying on the Trias. They are especially interesting, because they contain the teeth of the earliest known traces of the highest division of the animal kingdom—the mammals. These early mammals belonged to the lowest of all the mammalian tribes—the Marsupials, or pouched animals, now so common in Australia. The little banded ant-eater of South America, which lives upon insects and is about the size of a rat, is probably something like the first mammal, the Microlestes, in habit and appearance.

Let us now return, however, to the reptiles of the Jurassic period. It is so called from the Jura Mountains which occupy the north-west of Switzerland, separating that country from France. They are composed of a thick series of clays, shales, and limestones, to which, in 1829, the name Jurassic was given by the French geologist Brogniart. It was soon found, however, that the lower rocks of this period were very different from the upper. The lower rocks were very shaly and clayey with thinnish layers of limestone. These were called Lias. The name Lias is derived from "layers"—pronounced broadly by the Somerset quarrymen as "lyers"—a very suitable name for the lower beds of the Lias especially, since the alternation of thin beds of limestone and of shale gives to the rock a banded or ribbon-like appearance, which may well cause the workmen to describe it as occurring in "lyers."

To the upper Jurassic beds, which contained much more limestone and also occasional beds of sandstone, the name of Oolite was given. The Oolitic strata have a special interest for English geologists, for it was in them that William Smith, the west of England surveyor, first made out (about the year 1790) the order of succession of the strata, and by this was led to his great discovery that "strata could be identified by their organic remains," that is by their fossils. He noticed that some of the limestone beds of the strata we are about to describe consisted of small rounded grains, which made them resemble the roe of a fish—indeed, they were called "roestone" by the workmen. Hence Smith—when seeking a name for this set of strata—bethought himself of the term "Oolite," which means "egg-stone" (Gr. oon, an egg, and lithos, a stone). Where the grains are very large the limestone is called "pea-grit" or pisolite (Lat. pisum, a pea). Some beds which contain numerous and irregularly shaped fragments of shells, corals, etc., are called rag-stones.

Plesiosaurs

Different species ranged from ten to forty feet in length.

The Jurassic strata of Great Britain were sediments laid down in warm seas surrounding an archipelago of which Dartmoor, Wales, and Cumberland formed some of the islands. The whole of Western Europe was sinking and had sunk; and the waters of the open ocean were admitted into and mingled with the salt mineral waters of the great Triassic lakes. The change was at first very like what would happen at the present day if the coast of Palestine were depressed, so that the waters of the Mediterranean flowed into the Dead Sea. The few fish of the salt lakes were killed; and as the land continued to sink, the sea at last flowed all over Central and West England, bringing with it an abundance of marine life. But the reptiles were far from being finished with; and the progress of the small mammals was extremely slow.

First, as to the reptiles. The whale-shaped Ichthyosaurs continued to develop in the seas, and grew larger and larger till some of which we have found traces reached a length of forty feet. The long-necked Plesiosaurs also advanced from strength to strength, and some types grew larger. But by this time new breeds were developing, with shorter necks and larger heads (and consequently larger brain-power), which had a better chance of surviving in the struggle for existence than the unwieldy and slow-witted reptiles which preceded them. The Ichthyosaurs became more and more fish-like, and some of them developed the habit of breeding at sea instead of having to return to the land to deposit their eggs, as do the sea-going turtles and crocodiles. Descended from quite a different stock, the Plesiosaurs adapted themselves to sea life in their own fashion. Instead of adopting the flowing lines of a fish, the body took on a form more like that of a turtle, while the lengthened neck gave rise to the description applied to him since that they had the "body of a turtle strung on a snake." At their longest their necks had as many as seventy-six bones, or vertebræ, which is more than any other animal living or extinct ever possessed. A smaller order of crocodiles appeared and flourished for a time; and the ancestors of the sea turtles, which were to enjoy so long a reign, began to make their first appearance.

Among the land animals the Dinosaurs[16] (or the "fearful" saurians) attained remarkable size and diversity, and their dominant species were easily lords of the reptile horde. They developed not only as flesh-eating monsters, but also in vegetable-eating species. Of the flesh-eaters the Ceratosaurus was the most terrific. It was only seventeen feet long, but when standing on its powerful hind legs it could have looked in at most first-floor windows, and it used its cruel fore limbs for seizing and holding prey. Imagine a kangaroo with the teeth of a crocodile, the size of an elephant, and the ferocity of a tiger and you will have a fair idea of what you would have met in a Ceratosaurus.

[16] From Gr. "deinos," fearful.

The vegetarian Dinosaurs first became known in this system, but their development was so extraordinary that they soon outranked the flesh-eaters both in size and diversity. Among these the Brontosaurus attained the extraordinary length of sixty feet, and possibly more. It walked on its four legs, and is one of the largest known of all land animals. This enormous creature in spite of all its size and bulk was yet rather weak than strong. Its general organisation was unwieldy; the head was very small, and the brain hardly bigger than a walnut. The task of providing food for such a body must have been a severe tax on so small a head. The inconvenience of its bulkiness was perhaps reduced by living in and about water; but from the excellent preservation of some of the skeletons it has been thought that its life was often ended by sinking in some quicksand or shoal, from which its own massiveness forbade that the Brontosaurus should extricate itself.

Ornitholestes Diplodoci Carnegiei

From skeletons found in Jurassic strata in Wyoming, U.S.A.

(These reptiles attained a length of about 80 feet.)

Not greatly removed in habit or appearance from the Brontosaurus was the Diplodocus, a magnificent specimen of which has been set up in Pittsburg, and a fine replica, owing to the generosity of Mr. Andrew Carnegie, in the Natural History Museum. The Diplodocus, a harmless placid beast, was over eighty feet from the tip of his snout to the end of his enormous tail. It has been calculated that impulses travel along the nerves to the brain at the rate of about twelve yards a second. The rate may have been less in the case of the sluggish Diplodocus, but in any case it would evidently take at least two seconds for a nerve impulse to travel the length of this reptile; so that if any enemy attacked him at the end of his tail it would be two seconds before the Diplodocus would realise the fact, and perhaps four seconds before he could begin to turn round to defend himself. Even larger than these was the tremendous Brachiosaurus, who weighed as much as a steam-engine and whose thigh-bone was nearly eight feet high. These were the largest reptiles ever known, and may be taken as reaching the point when bulk becomes a burden, and as signalising an approach to the limit of evolution in the line of size. Less bulky than these were the Stegosaurs, which were also four-footed. They were curiously armoured, and formed a group of very remarkable creatures found in England and Western America. While they were less gigantic than some of those we have just described, they found compensation in protective plates, spines, and similar modes of defence. A Stegosaurus found in Wyoming was probably the most hideous to look upon; but like his relatives he had an extraordinarily small head and brain, and was a sluggish creature depending on his ugliness and armour for protection. Very likely this small size of the brain of great extinct reptiles had to do with the fact of their ceasing to exist. Animals with bigger and ever-increasing brains outdid them in the struggle for existence.

It has already been noted that the crowding of the land may have led some reptiles to take to the sea. The same influence may have led others to take to the air and thereby escape the monsters of the swamps, jungles, and forests. Whatever the cause, the most striking and wonderful feature of this period was the development of flying reptiles. They had just been seen in the Trias. In the Jurassic they appeared fully developed. They doubtless sprang from some agile hollow-boned saurian, more or less akin to the slender leaping Dinosaurs. Between the ponderous Brontosaurs and the airy Pterodactyls was the most striking of contrasts. At first these bird-like reptiles were small, but later their wings had a spread of as much as twenty feet, veritable flying dragons. They were not adorned with feathers, but like bats had leathery membranes stretched from the fore limbs to the body and to the hind limbs. Their heads were bird-like, and their jaws at first were set with teeth. They had true powers of flight, as is shown by the discovery of their remains in places where they must have been far out at sea when they sank and were buried. Later Pterodactyls had no teeth, and were, perhaps, milder in habits.

Archæopteryx (the earliest known fossil bird), and Compsognathus (a small Dinosaur)

It seems natural to pass from the fossil reptiles to the birds. But as a matter of fact the birds are not very closely related to the Pterodactyls, and seem to have been descended from some other very special form of reptiles, so peculiar as to be considered a distinct class. It may actually have been descended from those reptiles among the Dinosaurs which walked on their hind legs and had only three toes to the foot. The first bird found belongs to Jurassic times; and its skeleton, found in some slate remains at Solenhofen in Bavaria, is now to be seen in the Natural History Museum. There is another one in Berlin. This bird, called the Archæopteryx, was of the size of a large pigeon, had a short head apparently without a beak, and its jaws were armed with teeth. Whereas living birds have the fingers of their "hands" tied together in their wings, this bird has three distinct fingers at the corner of its wings, each armed with a claw. Its legs were like those of living birds, and it had four toes. Its tail was unlike that of any living bird, and like that of a lizard. Whereas the bony part of the tail of living birds is very short and bears the tail feathers set across it fanwise, the Archæopteryx had a long bony tail made up of many bones, and the feathers were set in a series one behind the other till the tail looked like the leaf of a date palm in shape. Strange as this little creature appears it was a genuine bird, for it had these feathers well developed, as the two fossil specimens showed. There are two sets of feathers forming the wings, and the thighs were also covered with feathers.