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