long heavy tail, where the hinder part of the body overbalances by far the forward portion, as in Brontosaurus, Diplodocus, Scelidosaurus, and Stegosaurus, the neural ganglion in the sacrum is many times larger than the cephalic ganglion or brain, a condition necessitated by the great mass of the body that must be innervated from the sacral ganglion in these animals.
|
| SKULL OF CERATOSAURUS. |
|
Skull of Ceratosaurus nesicornis: a, nasal opening; b, horn core; c, anteorbital opening (filled with matrix stone); d, orbit (ditto); e, lower temporal fossa (ditto); t, transverse bone. |
|
| CEPHALIC AND SACRAL GANGLIA. |
|
Drawings of casts showing the expansions of the spinal cord of Stegosaurus ungulatus. (1) Cast of neural cavity of sacrum: a, anterior end; p, posterior end at exit of neural canal in last sacral vertebra; f, f′, f″, foramina between the sacral vertebræ. (2) Cast of brain-cavity of same animal: c, cerebral hemispheres; cb, cerebellum; m, medulla; ol, olfactory lobes; on, optic nerve; op, optic lobes. |
In habits the dinosaurs were terrestrial, and often amphibious, and the structure of the tail in some genera indicates its use as a swimming organ. Some were of graceful, bird-like action, walking, running, or leaping on their three-toed hind limbs. Others were heavy, clumsy beasts, walking or crawling on their solidly built four legs. Dinosaurs vary greatly in size. The smallest are of the size of a chicken. The largest are the greatest land animals ever known to have existed, with lengths of 60 to 70 feet, heights of 10 to 20 feet, and weights estimated to have varied from 20 to 25 tons. Dinosaurs appeared in the early Triassic time, and during the Jurassic and Cretaceous periods they ruled the land, but toward the end of the Cretaceous they began to decline, and they finally gave way to the early mammals. The remains of the latest members of the order are found in the uppermost formations of the Cretaceous, namely the Laramie group of western America. Their remains have been found in the Mesozoic rocks of Europe, southern Asia, South Africa, North Australia, North and South America, and the most noted localities whence they have been obtained are those of Bernissart in Belgium, and the Rocky Mountain region of North America. The order Dinosauria is divided into three suborders, some of the peculiarities of which are here given. They are the Sauropoda, Theropoda, and Predentata.
Sauropoda. This is the most primitive group of dinosaurs, resembling the crocodiles and rhynchocephalians in the structure of its skeleton and the proportions of its parts. It includes the gigantic herbivorous genera: Atlantosaurus, Cetiosaurus, Brontosaurus (q.v.), and Diplodocus (q.v.), with very small head, long neck, heavy trunk supported on strong plantigrade live-toed fore and hind limbs of equal size, and with a heavy long tail. The teeth are long, spatulate, and spreading. The bones are solid and heavy, and the pubis is simple. This group is mostly of Triassic and Jurassic age.
Theropoda. Carnivorous dinosaurs with cutting teeth in sockets. Their skeletons are of rather delicate construction and their vertebræ and limb-bones are hollow. Their five-toed fore limbs are of small size and apparently of little use as locomotory organs, but their three-toed hind limbs are strong and the toes are furnished with prehensile claws. These dinosaurs were digitigrade and they walked as do birds or leaped as do the kangaroos. The pelvic bones are elongated. Examples of this suborder are Anchisaurus and Ceratosaurus (qq.v.). Hallopus, a leaping dinosaur from the Jurassic of Colorado, and the bird-like Compsognathus, one of the smallest dinosaurs, from the Jurassic lithographic limestones of Solenhofen, Bavaria, also belong in this division.
Predentata. Here are included the most specialized and hence the most extravagantly formed of dinosaurs. They were all herbivorous animals that walked on either four or two feet. Some of them undoubtedly ran with great rapidity on their hind legs, using the tail as a balance after the manner of certain modern lizards. In all of them the pelvis is provided with a postpubis, a character found in no other reptiles, but constant in the birds; and the ilium and ischium are elongated and variously modified. Another distinctive character is the presence of a predentary bone on the anterior end of the mandible which has a beak-like tip. Accordingly, the teeth are absent from the front of the jaws and are restricted to the posterior portions. The teeth are compressed, have crenulated edges, and they are frequently replaced by successional teeth. The Predentata comprise three divergent lines of descent—Stegosauria, Ceratopsia, Ornithopoda.
Stegosauria. In form these dinosaurs resemble most closely the Sauropoda. Their vertebræ are all biconcave, their heads are very small, and the hinder parts of their bodies are enormously developed. Many of them were armored. Omosaurus, from the Upper Jurassic of England, has strong dorsal spines; Polacanthus, of the English Wealdon, has the lumbar-sacral region inclosed in solid armor-plate; Scelidosaurus, of the English Lower Lias, has a row of small vertical plates along the dorsal line. The extreme of ornamental armor is seen in Stegosaurus (q.v.) from the Upper Jurassic of Colorado and Wyoming, which has a row of heavy large triangular bony plates extending from the back of the head along the back to the tip of the tail.
Ceratopsia. This group includes the horned dinosaurs, land reptiles of gigantic size and formidable appearance, which were evolved in late Mesozoic time after the dinosaur race had begun to decline. They are probably highly specialized descendants from the Stegosaurian race. Their remains have been found in the Upper
