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.)