Jurassic System

The Jurassic has been termed the "Age of Reptiles." The most striking forms of life during the period were the dinosaurs, which ranged in size from animals no larger than a rabbit to the gigantic Atlantosaurus, roo ft. long, in the Jurassic of Wyoming. They included many of the carnivorous Theropoda, e.g., Megalo saurus, Tyrannosaurus, Streptospondylus and herbivorous Sauro poda, e.g., Cetiosaurus; also Ornithischia (rarer, e.g., Scelido saurus. Other land reptiles were the mammal-like theromorpha and lizards. The rivers were stocked with crocodiles (e.g., Teleo saurus). In the seas swam innumerable turtles, and the huge extinct ichthyosaurs and plesiosaurs. Even in the air there flew reptiles, the Pterosauria or pterodactyls. By Kimeridgian times (the Solenhofen slates of Germany) the first bird, Archaeopteryx, was evolved. It had a long tail with steering feathers growing out at right angles on both sides, claws on its wings and reptilian jaws and teeth. Of mammals there were very few, and all smaller than a rat. Throughout the long Mesozoic era they underwent little change, and must be regarded as eking out an existence in an unfavourable environment, in a world dominated by the Reptilia. They all belong to the Marsupials, and were probably insectivor ous. Phascolotherium, Amphilestes, Stereognathus and Amphi therium have been found in the Stonesfield slate, while Amblo therium, Plagiaulax and Triconodon occur in the Purbeck beds. Fish were approaching the modern forms; heterocercal ganoids were becoming scarce, while the homocercal forms abounded (Gyrodus, Microdon, Lepidosteus, Lepidotus, Dapedius). The Chimaeridae or cat-fish here made their appearance (Squaloraja). The ancestors of the modern sturgeons and selachians (Hybodus, Acrodus) were numerous. Bony fish were represented by the small Leptolepis.

A great change had come over the Crustaceans ; in place of the Palaeozoic trilobites we find long-tailed lobster-like forms (Eryon, Eryma, Glyphaea) and the broad crab-like type (Prosopon). Isopods and Ostracods were common at certain stages (particu larly in the Purbeckian). All the orders of insects were repre sented, but the Neuroptera still predominated.

Mollusca and Molluscoidea (Brachiopods) were extremely abundant, but the most characteristic were the Cephalopods, especially the ammonites, of which an almost endless variety abounded. Crinoids and Echinoids were also abundant, while the

corals were important rock-builders (especially in the Corallian period). Sponges, too, occasionally formed rocks, such as the Spongiten Kalk, and the Rhaxella Chert of England.

Vegetation.

The Jurassic vegetation was in the main a con tinuation of that of Triassic times. In the estuarine and fresh water beds numerous Gymnosperms abound, completely dominat ing the Jurassic flora. They included Benettitales, Ginkgoales (Ginkgos) and Coniferales in abundance. Until recently there were thought to have been no Jurassic Angiosperms, but Angiospermous fruits have recently been described by H. Thomas from the Estuarine series of Yorkshire (Bajocian) and a Dicotyledonous leaf has been described by Seward from the Stonesfield slate (Bathonian).

Economic Products.

These are very various. Coal occurs principally in the Lias; in Hungary there are 25 workable beds; also in Persia, Turkestan, Caucasus, South Siberia, China, Japan, Farther India, New Zealand and the Pacific Islands. Thin seams of coal occur in the Estuarine series of Yorkshire and in the Oxfordian of Scotland. Iron ores occur at many horizons. Those in Britain will be described under Domerian, Toarcian, Corallian and Kimeridgian; the richest and most plentiful ores are those of Lorraine and Cleveland, in Yorkshire, of Northamptonshire, Lincolnshire and Oxfordshire. Oil shales are found in Germany, and the Kimeridgian and Lower Lias of England are sometimes highly bituminous. Good building stones are obtained from the Aalenian, Bathonian, Portlandian and Purbeckian. Hydraulic cement is made from the Lower Lias, and most of the calcareous beds are burnt for lime. The clays all make excellent bricks, but the most famous are the Oxford clay bricks of Peterborough. In Bavaria the Kimeridgian produces the celebrated lithographic slates of Solenhofen, which are of the highest interest to palaeon tologists, as containing the very perfect impression of Archaeop teryx (q.v.) the oldest known fossil bird, with marked reptilian characteristics.