1 Purbeckian from the “Isle” of Purbeck. Aquilonien from Aquilo (Nord). Bononien from Bononia (Boulogne). Virgulien from Exogyra virgula. Pteroceran from Pteroceras oceani. Astartien from Astarte supracorollina. Rauracien from Rauracia (Jura). Argovien from Argovie (Switzerland). Neuvizien from Neuvizy (Ardennes). Divesien from Dives (Calvados). Bathonien from Bath (England). Bajocien from Bayeux (Calvados). Toarcien from Toarcium (Tours). Charmouthien from Charmouth (England). Sinemourien from Sinemurum, Semur (Côte d’Or). Hettangien from Hettange (Lorraine).
Life in the Jurassic Period.—The expansion of the sea during this period, with the formation of broad sheets of shallow and probably warmish water, appears to have been favourable to many forms of marine life. Under these conditions several groups of organisms developed rapidly along new directions, so that the Jurassic period as a whole came to have a fauna differing clearly and distinctly from the preceding Palaeozoic or succeeding Tertiary faunas. In the seas, all the main groups were represented as they are to-day. Corals were abundant, and in later portions of the period covered large areas in Europe; the modern type of coral became dominant; besides reef-building forms such as Thamnastrea, Isastrea, Thecosmilia, there were numerous single forms like Montivaltia. Crinoids existed in great numbers in some of the shallow seas; compared with Palaeozoic forms there is a marked reduction in the size of the calyx with a great extension in the number of arms and pinnules; Pentacrinus, Eugeniacrinus, Apiocrinus are all well known; Antedon was a stalkless genus. Echinoids (urchins) were gradually developing the so-called “irregular” type, Echinobrissus, Holectypus, Collyrites, Clypeus, but the “regular” forms prevailed, Cidaris, Hemicidaris, Acrosalenia. Sponges were important rock-builders in Upper Jurassic times (Spongiten Kalk); they include lithistids such as Cnemediastrum, Hyalotragus, Peronidella; hexactinellids, Tremadictyon, Craticularia; and horny sponges have been found in the Lias and Middle Jurassic.
Polyzoa are found abundantly in some of the beds, Stomatopora, Berenicia, &c. Brachiopods were represented principally by terebratulids (Terebratula, Waldheimia, Megerlea), and by rhynchonellids; Thecae, Lingula and Crania were also present. The Palaeozoic spirifirids and athyrids still lingered into the Lias. More important than the brachiopods were the pelecypods; Ostrea, Exogyra, Gryphaea were very abundant (Gryphite limestone, Gryphite grit); the genus Trigonia, now restricted to Australian waters, was present in great variety; Aucella, Lima, Pecten, Pseudomonotis Gervillia, Astarte, Diceras, Isocardia, Pleuromya may be mentioned out of many others. Amongst the gasteropods the Pleurotomariidae and Turbinidae reached their maximum development; the Palaeozoic Conularia lived to see the beginning of this period (Pleurotomaria, Nerinea, Pteroceras, Cerithium, Turritella).
Cephalopods flourished everywhere; first in importance were the ammonites; the Triassic genera Phylloceras and Lytoceras were still found in the Jurassic waters, but all the other numerous genera were new, and their shells are found with every variation of size and ornamentation. Some are characteristic of the older Jurassic rocks, Arietites, Aegoceras, Amaltheus, Harpoceras, Oxynoticeras, Stepheoceras, and the two genera mentioned above; in the middle stages are found Cosmoceras, Perisphinctes, Cardioceras, Kepplerites Aspidoceras; in the upper stages Olcostephanus, Perisphinctes, Reineckia, Oppelia. So regularly do certain forms characterize definite horizons in the rocks that some thirty zones have been distinguished in Europe, and many of them can be traced even as far as India. Another cephalopod group, the belemnites, that had been dimly outlined in the preceding Trias, now advanced rapidly in numbers and in variety of form, and they, like the ammonites, have proved of great value as zone-indicators. The Sepioids or cuttlefish made their first appearance in this period (Beloteuthis, Geoteuthis,) and their ink-bags can still be traced in examples from the Lias and lithographic limestone. Nautiloids existed but they were somewhat rare.
A great change had come over the crustaceans; in place of the Palaeozoic trilobites we find long-tailed lobster-like forms, Penaeus, Eryon, Magila, and the broad crab-like type first appeared in Prosopon. Isopods were represented by Archaeoniscus and others. Insects have left fairly abundant remains in the Lias of England, Schambelen (Switzerland) and Dobbertin (Mecklenburg), and also in the English Purbeck. Neuropterous forms predominate, but hemiptera occur from the Lias upwards; the earliest known flies (Diptera) and ants (Hymenoptera) appeared; orthoptera, cockroaches, crickets, beetles, &c., are found in the Lias, Stonesfield slate and Purbeck beds.
Fishes were approaching the modern forms during this period, heterocercal ganoids becoming scarce (the Coelacanthidae reached their maximum development), while the homocercal forms were abundant (Gyrodus, Microdon, Lepidosteus, Lepidotus, Dapedius). The Chimaeridae, sea-cats, made their appearance (Squaloraja). The ancestors of the modern sturgeons, garpikes and selachians, Hybodus, Acrodus were numerous. Bony-fish were represented by the small Leptolepis.
So important a place was occupied by reptiles during this period that it has been well described as the “age of reptiles.” In the seas the fish-shaped Ichthyosaurs and long-necked Plesiosaurs dwelt in great numbers and reached their maximum development; the latter ranged in size from 6 to 40 ft. in length. The Pterosaurs, with bat-like wings and pneumatic bones and keeled breast-bone, flew over the land; Pterodactyl with short tail and Rhamphorhyncus with long tail are the best known. Curiously modified crocodilians appeared late in the period (Mystriosaurus, Geosaurus, Steneosaurus, Teleosaurus). But even more striking than any of the above were the Dinosaurs; these ranged in size from a creature no larger than a rabbit up to the gigantic Atlantosaurus, 100 ft. long, in the Jurassic of Wyoming. Both herbivorous and carnivorous forms were present; Brontosaurus, Megalosaurus, Stegosaurus, Cetiosaurus, Diplodocus, Ceratosaurus and Campsognathus are a few of the genera. By comparison with the Dinosaurs the mammals took a very subordinate position in Jurassic times; only a few jaws have been found, belonging to quite small creatures; they appear to have been marsupials and were probably insectivorous (Plagiaulax Bolodon, Triconodon, Phascolotherium, Stylacodon). Of great interest are the remains of the earliest known bird (Archaeopteryx) from the Solenhofen slates of Bavaria. Although this was a great advance beyond the Pterodactyls in avian characters, yet many reptilian features were retained.
Comparatively little change took place in the vegetation in the time that elapsed between the close of the Triassic and the middle of the Jurassic periods. Cycads, Zamites, Podozamites, &c., appeared to reach their maximum; Equisetums were still found growing to a great size and Ginkgos occupied a prominent place; ferns were common; so too were pines, yews, cypresses and other conifers, which while they outwardly resembled their modern representatives, were quite distinct in species. No flowering plants had yet appeared, although a primitive form of angiosperm has been reported from the Upper Jurassic of Portugal.
The economic products of the Jurassic system are of considerable importance; the valuable coals have already been noticed; the well-known iron ores of the Cleveland district in Yorkshire and those of the Northampton sands occur respectively in the Lias and Inferior Oolites. Oil shales are found in Germany, and several of the Jurassic formations in England contain some petroleum. Building stones of great value are obtained from the Great Oolite, the Portlandian and the Inferior Oolite; large quantities of hydraulic cement and lime have been made from the Lias. The celebrated lithographic stone of Solenhofen in Bavaria belongs to the upper portion of this system.