The Fishes have advanced much beyond those of the Trias. The Sharks have attained practically their modern condition, and the broad, flattened Rays are a new type of the order. The Chimceroids were much more numerous and relatively important than they are at present, when only a few are left. The Dipnoans had become very scarce and are hardly represented in the northern hemisphere, save for the persistence of Ceratodus. The Crossopterygians were greatly reduced, though a few exceedingly curious forms, like Undina, still linger. Of the Teleostome fishes the Ganoids were still the dominant type, as they had been since the Devonian. Some of these Jurassic forms are evidently the forerunners of the Sturgeons, but most of them resemble the Gar-pike of our Western rivers (Lepidosteus), and are covered with a heavy armour of thick, shining, rhomboidal scales.
Fig. 290. - Dapediuspolitus. (Smith Woodward).
Fig. 291. - Aspidorhynchus acutirostris Ag. (Smith Woodward).
Fig. 292. - Hypsocortnus insignis Wagner. (Smith Woodward).
Many of these Ganoids are of small or moderate size, such as Dapedius (Fig. 290) and Aspidorhynchus (Fig. 291), while others, like the superb Lepidotus, were very large, evidently the kings of their race. Some of the Jurassic fishes approximate the Teleosts so closely that it seems arbitrary to call them Ganoids. Caturus, Leptolepis, Hypsocormus (Fig. 292), and Megalurus are much like what the ancestral Teleosts, must have been.
No Amphibia are certainly known from the Jurassic.
The Reptiles have attained a higher and more diversified plane of existence than in the Trias. Most of the Triassic genera and several entire orders have become extinct, but new and more advanced forms come in to take their places. The Rhynchocepha-lians abound and give rise to many diversified types of terrestrial, semi-aquatic and fully aquatic reptiles, and the first of the true Lizards (Lacertilia) appear. Turtles have grown much more numerous than in the Trias and have distributed themselves over the world. The Ichthyosauria are a highly characteristic Jurassic group; for though they are found in both the Trias and the Cretaceous, the Jura, and especially the Lias, is the time of their principal expansion. Certain localities in the Lias of England and Germany have yielded an incredible number of skeletons, and some of the specimens have preserved the impressions of the outline of the body and limbs, showing recognizably the nature of the skin. These reptiles were entirely marine in their habits and preyed upon fishes, and their limbs were converted into swimming paddles; there is a dorsal fin and a large tail-fin, the principal organ of propulsion (see Fig. 293). The muzzle is drawn out into an elongate slender snout, armed with numerous sharp teeth, which were set in a continuous groove, not in separate sockets.
The eye is very large and protected by a number of bony plates, which are often preserved in the fossil state. The neck is very short and hardly distinguished from the porpoiselike body. The skin was smooth, having neither horny scales nor bony scutes, which was of advantage in lessening the friction of the water. In length, these reptiles sometimes exceeded 25 feet, and in appearance they must have been very like the modern porpoises and dolphins, but the resemblance is entirely superficial, for porpoises and dolphins are warm-blooded Mammals. Bap-tanodon, found in Wyoming, is an Ichthyosaur without teeth and must have fed upon small and soft marine invertebrates, as do the toothless whales. Another group of carnivorous marine reptiles is that of the Plesiosauria, which began in the Trias and culminated in the Jura, and which forms a curious contrast to the Ichthyosaurs. In the typical genus Plesiosaurus (Fig. 294) the head is relatively very small, and the jaws are provided with large, sharp teeth, set in distinct sockets. The neck is exceedingly long, slender, and serpentlike, and marked off distinctly from the small body.
The swimming paddles are much larger than in the Ichthyosaurs and probably had more to do with locomotion; the skeleton of the paddle departs much less widely from the structure of a terrestrial reptile's foot than does that of an Ichthyosaur. With their long necks, the Plesiosaurs could lie motionless far below the surface, occasionally raising their heads above the water to breathe, or darting them to the bottom after their prey, which consisted chiefly of fish. The Jurassic species of Plesiosaurus do not much exceed a length of 20 feet, but Pliosaurus of the same group was gigantic, a single paddle sometimes measuring 6 feet in length; the reptiles of the latter genus had, however, proportionately larger heads and shorter necks.
Fig. 293. - Ichthyosaurus quadriscissus Quenst., Lias. Restoration by C. R. Knight under the direction of Prof. H. F. Osborn. (Copyright, American Museum of Natural History, N.Y).
The seas and rivers of Jurassic times were swarming with Crocodiles, the most ancient yet known, Teleosaurus being the commonest genus of the period. In appearance these reptiles much resembled the modern Gavial of India and had a similar elongate and slender snout. The fore legs were much smaller than the hind, and these animals were doubtless of more exclusively aquatic habits than the crocodiles and alligators of the present day. One suborder of the Crocodiles, the Thalattosuchia, was almost entirely marine in habits, the skin being smooth and without scales and the fore limbs converted into paddles, while the very long tail ended in a large fin. In the Jurassic of South Africa has been found the other extreme of crocodilian development, a little reptile which was terrestrial and had long, running legs.
Fig. 294. - Plesiosaurus macrocephalus, 1/20. (Owen).
Fig. 295. - Allosaurus agills Marsh, a carnivorous Dinosaur from the Morrison. Restoration by C. R. Knight under the direction of Prof. H. F. Osborn. (Copyright, American Museum of Natural History, N.Y).
The Dinosauria became much larger, more numerous and diversified than they had been in the Trias, though, as the footprints in the Newark sandstones teach us, only a small fraction of the Triassic Dinosaurs has yet been recovered. Making all due allowance for this, it seems, nevertheless, to be true that the group had made notable progress in the Jurassic. The known American Jurassic Dinosaurs are from the Morrison, and so some of those mentioned below may be Cretaceous. The group of Dinosauria is a greatly varied one, comprising reptiles of very different size, appearance, structure, and habits of life. Some were heavy, slow-moving quadrupeds, having fore and hind legs of not very unequal length, with hoof-like toes, and usually with very small heads. Dinosaurs of this type were mostly plant-feeders and had rows of grinding teeth adapted for such a diet. Brontosaurus, from the Morrison, is an example of this kind of Dinosaur, which attained a length of 60 feet, and Diplodocus was a not very dissimilar and even larger reptile. Stegosaurus was another herbivorous reptile, but with such short fore legs that the gait must have been bipedal, or else the back must have been arched upward very strongly to the hind quarters.
This animal, and its European allies, Sceli-dosaurus and Omosaurus, were provided with an armour of bony plates and spines covering the back and tail. Ceratosaurus, Allosaurus (Fig. 295), and the very similar European genus, Megalosaurus, on the other hand, were gigantic carnivorous Dinosaurs, having terrible, sharp pointed teeth, while the toes were armed with sharp, curved claws. These creatures walked upon their elongated hind legs and were the most formidable beasts of prey that scourged the Jurassic lands. Not all of the Jurassic Dinosaurs were gigantic; very small ones also ranged through the forests, or may even have been arboreal in their habits. Compsogna-thus, for example, was a bipedal, carnivorous Dinosaur hardly larger than a house cat.
Fig. 296. - Restoration of Pterosaurian, Rham-phorhynchus. (Zittel).
Another very remarkable order of reptiles, the Pterosauria, the earliest known appearance of which is in the Rhaetic, became important and characteristic in the Jurassic (Fig. 296). These animals were provided with wings, and were true fliers, thus realizing the old myth of flying dragons. The head is relatively large, but very lightly constructed, and set at right angles with the neck, as in birds. In the Jurassic species, the jaws are more or less completely armed with teeth, which by their form show the carnivorous propensities of the animal. The joints of the external or little finger of the hand are much thickened and elongated, this finger being longer than the body and legs together. A membrane, or patagium, was stretched between the elongate finger on one side, and the body and leg on the other, thus forming the wing, which rather resembled the wing of the bat than that of a bird, though differing from the former in being supported by one finger instead of four. A few exceptionally well-preserved specimens found in the Solenhofen limestones have retained the clearly-marked impressions of these wing membranes. The legs, like those of bats, were small and weak, and the tail was very short in some species, very long in others.
Some, at least, of the latter had a membranous, oar-like expansion at the tip of the tail. That the Pterosaurs had the power of true flight, and did not merely take great leaps like the flying squirrels, is shown by the hollow, pneumatic bones (like those of birds), and by the keel on the breastbone for the attachment of the great muscles of flight. This keel is found in both birds and bats. The skin was naked, having neither scales nor feathers. The Jurassic Pterosaurs were small, the spread of wings not exceeding three feet.