DINOSAURIA, a sub-class or order of Mesozoic reptiles. The term dinosaur is in current use for the members of two groups of extinct reptiles, most members of which are character ized by their gigantic size.
The first group, the Saurischia, first appears in the Trias, the earliest known remains being found in the Middle Trias, although the evidence of foot-prints suggests that they already existed in the Lower Trias. These Triassic forms are very variable in size, ranging from animals no bigger than a hen to creatures some 15ft. or more in length. All of them are bipedal, walking on the elongated hind legs and scarcely ever if at all touching the ground with their small hands during ordinary progress. In the main these animals were carnivorous, and some of their descendants contin ued to feed on flesh until their disappearance at the top of the Cretaceous. One other group, however, of very heavily built, more quadri-pedal animals, apparently adopted herbivorous habits and developed into the great group of Sauropoda. The truly car nivorous Jurassic and Cretaceous forms fall into many families , differing very considerably from one another. The most impor tant end-forms, i.e., those which represent the climax of the evolu tion of a line, are Struthiomimus and Tyrannosaurus.
Struthiomimus is a bipedal animal probably capable of running at a great speed, its legs and particularly its feet, strongly recalling those of such struthious birds as the emu and rhea. There are three toes, the middle much the longest, the others forming a symmetrical pair; and they correspond to the second, third and fourth of the full pentadactyl foot. The fore-limb is also three toed, but is peculiar in structure. Its bones and the muscles which move them are slender, whilst the digits are of nearly the same length, each of the three ending in a large curved and sharp pointed claw. These fingers could not be opposed to one another, the hand could merely be shut and opened like that of a sloth : it can have served only as an organ for grasping boughs. The neck is long, slender and freely movable, whilst the head is relatively very small and the jaws are covered with a horny beak like that of an ostrich. The eye is large. It seems most probable that the animal used its slender arms for pulling down the boughs of trees, in order that their leaves might be plucked by the beak, but it is possible that the animal was capable of digging up shrubs with its hands. It is about I aft. in length and stands about 4ft. high at the pelvis.
Tyrannosaurus, which is of slightly more recent date, presents a remarkable contrast to Struthiomimus. The animal stands about I 8f t. high and has a length of some 38f t. ; the skull alone is more than 4ft. in length. The animal walked entirely on its hind legs, only the toes resting on the ground ; the fore limbs are absurdly small and feeble, and can scarcely have been of much use to the animal in feeding. The body is narrow from side to side, but very deep and must have been immensely heavy; in order to balance its weight the thick tail is nearly twice as long as the anterior part of the body. Tyrannosaurus has teeth nearly 'ft. in length and is by far the largest land-living carnivorous animal known. It is a descendant of the somewhat smaller Gorgonosaurus from the Middle Cretaceous of Alberta, Canada, in which the fore-legs are somewhat larger and the tail shorter; not dissimilar are Megalosaurus from the Lower Jurassic of England and Allo saurus from somewhat later rocks in North America. In these forms, however, the neck is relatively longer and the fore limbs are still more so.
A great contrast to Tyrannosaurus is afforded by Compsogna thus. This animal was no bigger than a pigeon, and had a foot with a grasping big toe very like that of a bird. Its very delicate skull was provided with small very sharp teeth. The only known specimen, which comes from the Solenhofen slate of Bavaria, contains within its ribs the remains of a skeleton; this has some times been interpreted as that of an unborn individual of the same form, but it is more probably the remains of a small lizard-like reptile which was its last meal.
The other great group of Saurischia, the Sauropoda, is composed of large reptiles, many of which attained gigantic dimensions. The smallest and one of the most recent, Titanosaurus, is little more than 5ft. high at the shoulder and about 25ft. in length, more than half of it being tail. The largest form, Brachiosaurus, has a humerus about 6f t. in length. These animals walked on all four legs and the fore-leg was not disproportionately shorter than the hind one. The fore-foot usually bears a single large claw on the thumb, the second and third fingers retain some phalanges, and the fourth and fifth are usually reduced to mere stumps. The hind-foot is similar, but usually the first three digits have claws. The animal walks on the ends of its metacarpals and tarsals, that is, the palm of the hand and the sole of the foot were not placed on the ground ; the limbs were rather elephant-like. The trunk is short and the neck long and flexible. The tail is extraordinarily long, and often ends in a whip-like extremity. The head is very small with the nostrils opening on its upper surface; the teeth are never large, and form a uniform series which may be restricted to the front of the mouth. In Diplodocus (which has a total length of 65ft.), the dentition is reduced to some 20 teeth in each jaw, each tooth about as big as the stump of a thin pencil. It is clear that these animals were not carnivorous, and it is exceedingly difficult to discover any food soft enough to be eaten with such a dentition, and sufficiently at undant to maintain an animal weigh ing some 35 tons. As the remains of these animals are not uncom monly found in shallow water marine deposits, it is probable that they were mainly aquatic in their habits, walking on the bottom of shallow estuaries, reaching the air by raising their long necks, and feeding on water plants. The skin, in the only case in which it is known, was covered with small horny scales about half an inch in diameter. The oldest known Sauropod is Cetiosaurus from the Great Oolite of Oxf ord, England, but the best known forms are Dip/odocus and Apatosaurus from the top of the Jurassic of Colorado. The most widely distributed was Titanosaurus found in the Cretaceous rocks of England, Hungary, India, Madagascar and Patagonia.
The other great division of the dinosaurs, the Ornithischia, contains only herbivorous animals. The oldest member of the group comes from the Upper Trias of South Africa and the group becomes extinct at the extreme top of the Cretaceous. The most familiar forms are animals with the shape of a gigantic kangaroo, the hind legs being very long and the animal balanced about them by the great tail. The fore-limbs are shorter than the hind ones and may perhaps have been used for grasping food. Iguanodon (about 28ft. long and i4ft. high) is the most familiar European form, whilst the Cretaceous of North America and of central Asia yields the remains of many animals similar in appearance, but in many cases diversified by the presence of a crest rising from the top of the head. These animals, the Trachodonts, are sometimes so perfectly preserved that the impression of the skin still covers the entire skeleton, and shows that during life the animals were covered with small horny scales often arranged in little groups forming a pattern. These animals appear to have been predominantly aquatic.
Another very different group, is represented by the armoured dinosaurs: they are the oldest known forms, and are characterized by the possession of at least two rows of bony scutes set in the skin of the back. These scutes may be relatively small, but in larger forms such as Stegosaurus from the Jurassic of North America and England, they are enlarged so as to stand up as a wall of bony plates 3ft. high by 3ft. wide, on each side of the mid-line of the back of the animal, the plates in the two rows alternating with one another. The largest plates are over the base of the tail, whose extremity bears two pairs of sharp pointed spines, each one sometimes 3ft. in length, projecting laterally. Stegosaurus is flat from side to side and its legs are straight and rather elephant-like.
A widely different form is Palaeoscincus and its allies, from the Middle Cretaceous of Alberta, Canada. This animal, with a total length of some 5ft. was only 4ft. high, but had a width across the middle of its back of some 6 feet. The head is short and rounded, the neck is also short and is covered with pairs of bony plates lying in the skin; in the region of the shoulder these plates are tall, very sharp-pointed spines, and a similar group of them lies on the elbow. In the middle of the back the spines disappear and are replaced by flat but keeled small bony plates, forming four rows along the animal's back; these extend over the hip region and swell out on the tail so as to form nearly com plete rings. Finally, in some cases at any rate, there is an unar moured but quite stiff region terminating at the end of the tail in an enormous mass of bone composed of numerous bony scutes.