The Jurassic Period
The Jurassic period, the second great division of the Mesozoic era, receives its name from the Jura Mountains lying between France and Switzerland, a locality rich in rock formations of the period. Jurassic rocks are widely distributed throughout much of Europe, extending from the extreme north of Scotland, across England and France, to the Alps and Apennines, to Spain and northern Germany, and to central and eastern Russia. They are also found in Tibet, Kashmir, Nepal and in South Africa, New Zealand, Australia and South America.
In North America there are few Jurassic rocks. During the period most of the land in America was high above the water. In the East there are no rocks which were formed during the period, but in parts of the West where the land sank we find Jurassic rocks.
Alaska was the first land to disappear beneath the waters, and then Vancouver Island and much of California and Oregon. Later an inland sea covered most of British Columbia and much of Alberta, and extended over Montana, Idaho, Nevada, Utah and Wyoming, and a part of Colorado. This has been named the Logan Sea. In Wyoming the rocks are 3,500 feet thick. The rocks of the Jurassic in America are sandstones, shales, limestones and mans.
Towards the close of the Jurassic period parts of the world which had been under the sea began to rise. Volcanic disturbances occurred in North and South America. In North America the Pacific Mountain system was formed. Mountain ranges rose from Alaska to Mexico, among them the Sierra Nevadas.
As far as animal and plant life are concerned, there was no great gap between the Triassic and Jurassic periods. In each the plants and animals were of the same general character, although in the Jurassic there were many more species of animals. The reptiles increased so much in size and numbers that the period is often called the Age of Reptiles.
The vegetation of the Jurassic period was probably luxuriant, but not greatly varied in character. It greatly resembled that of the Triassic. Cycads and cone-bearing trees, ferns and horsetails were the most common types. The Ginkgo, or maiden-hair tree, also flourished and was the first broad-leaved tree. It still exists to-day in China and Japan, having undergone no changes for more than a hundred million years.
The animal life of the period varied greatly and is full of interest. Both land and water forms abounded; many strange forms appeared. The ammonites were the most characteristic shell-fishes of the Jurassic, although they showed signs of dying out. The belemnites, ancestors of the squids, more than held their own. There is no lack of sponges among the fossils of this period, and the remains of starfishes and molluscs are plentiful. Crinoids were common, the largest form reaching a height of fifty feet with a crown a yard long. The ancestors of the modern crab and lobster appeared. Corals were particularly abundant, and during this period most of Europe seems to have been submerged under a sea full of coral islands and coral reefs. These reefs can be traced in parts of England and from Normandy in France to the Mediterranean. They are found, too, over the east of France along the Jura Mountains and in parts of the Alps. A very different Europe it must have been, with coral reefs in England and on the mountains.
As many as a thousand species of Jurassic insects are known, including beetles, dragonflies, grasshoppers, crickets, earwigs, walking-sticks, cockroaches and termites. The social ants made their appearance in the early years of the Jurassic period. Fishes were also numerous, and sharks and rays were well represented in the seas. Most of the Paleozoic fishes about which we have already read had disappeared.
In the Triassic rocks the first mammals appeared, and in the Jurassic rocks a few more have been found. Mammals, however, were still an unimportant-seeming form of animal life. Both the European and American mammals of this early day were very much alike.
A COUNTRY GIRL DISCOVERS A THIRTY-FOOT MONSTER
The most interesting animals of the time were undoubtedly the great lizards. One of the most amazing of these was the ichthyosaurus, or fish-lizard, which originated in the Triassic and continued to develop during the Jurassic period. Certain localities in England and Germany have furnished great numbers of their remains. In a quarry at Lyme Regis, England, the first fossil ichthyosaurus was found by a country girl, Mary Anning, who made a living by collecting and selling fossils. She was hammering away at the rock one day when she noticed some big bones sticking out. When she had cleared away the rubbish and rubble round about, she found the skeleton of a huge animal and hired workmen to dig out the whole block of stone. It proved to be a monster thirty feet long, with six-foot jaws and huge eyes like saucers.
The plesiosaurus, another lizard having its origin in Triassic times, was still abundant in the Jurassic seas. However, the plesiosaurus of the Jurassic period did not reach as great a length as did his Triassic ancestor.
Crocodiles resembling the modern gavial of India swarmed in the seas and rivers. They probably spent less of their lives in the water than do their modern descendants.
PTERODACTYLS, REPTILES WITH THE POWER OF FLIGHT
Stranger in some ways, though not so monstrous, were the pterodactyls, or flying reptiles. Judging from their remains, they must have been quite common and of many different kinds. Some were as large as an albatross, and others grew no bigger than a sparrow. In appearance they were more or less bird-like. The head was usually fairly long and thin and had slender jaws equipped with curved teeth well suited for holding and tearing their prey. The bones of the forelegs, or arms, were very long and supported the leathery wing membrane. Upon the ground the pterodactyls walked about readily on their hind legs. In flight they either glided or flapped their wings just as do our modern birds.
THE FIRST TRUE BIRD MAKES ITS APPEARANCE
In the Jurassic rocks of Germany the first true bird was discovered—the Archaeopteryx. The name is Greek for “ancient wing.” Archaeopteryx was rather smaller than a modern crow. He had no beak; his jaws were lined with small, sharp teeth. His tail was longer than the rest of his body and was set with feathers on either side, giving it a fern-like appearance. In modern birds the tail is merely a stump, and the feathers spread from it like a fan. The wings of Archaeopteryx were equipped with three wing fingers. These early birds were probably better at gliding than at true flying.
Big as were the Jurassic sea lizards, there were still larger reptiles on the land. The dinosaurs which appeared in the Triassic period now attained gigantic proportions. The brontosaurus was fully sixty-five feet long and weighed about thirty-seven tons. He had a barrel-like body, at one end of which was a long, snake-like neck and at the other a slender, tapering tail. Each of his foot-prints covered about a yard. The diplodocus was still larger, reaching a length of eighty feet. The gigantosaurus, the largest land animal known, was eighty feet long, thirty-six feet of which was neck. He weighed something like forty tons.
STEGOSAURUS, A DINOSAUR CLAD IN BONY ARMOR
Not all Jurassic dinosaurs attained the tremendous proportions of the diplodocus and the gigantosaurus. Some of the most curious of them were the smaller armored reptiles which appeared quite late in the period. The strangest of these was stegosaurus. His name means “covered lizard.” Along his back ran a double row of great bony plates, which reached a height of more than two feet over his hips. Near the tip of his tail were several long spines
