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In the preceding chapter we have attempted to point out the place in nature that the Dinosaurs occupied and the conditions under which they lived. They were the dominant land animals of their time, just as the quadrupeds were during the Age of Mammals. Their sway endured for a long era, estimated at nine millions of years, and about three times as long as the period which has elapsed since their disappearance. They survived vast changes in geography and climate, and became extinct through a combination of causes not fully understood as yet; probably the great changes in physical conditions at the end of the Cretacic period, and the development of mammals and birds, more intelligent, more active, and better adapted to the new conditions of life, were the most important factors in their extinction.

The Dinosaurs originated, so far as we can judge, as lizard-like reptiles with comparatively long limbs, long tails, five toes on each foot, tipped with sharp claws, and with a complete series of sharp pointed teeth. It would seem probable that these ancestors were more or less bipedal, and adapted to live on dry land. They were probably much like the modern lizards in size, appearance and habitat:[2]

From this ancestral type the Dinosaurs evolved into a great variety of different kinds, many of them of gigantic size, some herbivorous, some carnivorous; some bipedal, others quadrupedal; many of them protected by various kinds of bony armor-plates, or provided with horns or spines; some with sharp claws, others with blunted claws or hoofs.

Fig. 6.—Outline Restorations of Dinosaurs. Scale about nineteen feet to the inch.

These various kinds of Dinosaurs are customarily grouped as follows:

I. Carnivorous Dinosaurs or Theropoda. With sharp pointed teeth, sharp claws; bipedal, with bird-like hind feet, generally three-toed;[3] the fore-limbs adapted for grasping or tearing, but not for support of the body. The head is large, neck of moderate length, body unarmored. The principal Dinosaurs of this group in America are

Allosaurus, Ornitholestes—Upper Jurassic period.

Tyrannosaurus, Deinodon, Albertosaurus, Ornithomimus—Upper Cretacic period.

Fig. 7.—Skulls of Dinosaurs, illustrating the principal types—Anchisaurus after Marsh, the others from American Museum specimens.

II. Amphibious Dinosaurs or Sauropoda. With blunt-pointed teeth and blunt claws, quadrupedal, with elephant-like limbs and feet, long neck and small head. Unarmored. Principal dinosaurs of this group in America are Brontosaurus, Diplodocus, Camarasaurus (Morosaurus) and Brachiosaurus, all of the Upper Jurassic and Comanchic periods.

III. Beaked Dinosaurs or Predentates. With a horny beak on the front of the jaw, cutting or grinding teeth behind it. All herbivorous, with pelvis of peculiar type, with hoofs instead of claws, and many genera heavily armored. Mostly three short toes on the hind foot, four or five on the fore foot. This group comprises animals of very different proportions as follows:

1. Iguanodonts. Bipedal, unarmored, with a single row of serrated cutting teeth, three-toed hind feet. Upper Jurassic, Comanchic and Cretacic. Camptosaurus is the best known American genus.

2. Trachodonts or Duck-billed Dinosaurs. Like the Iguanodonts but with numerous rows of small teeth set close together to form a grinding surface. Cretacic period. Trachodon, Hadrosaurus, Claosaurus, Saurolophus, Corythosaurus, etc.

3. Stegosaurs or Armored Dinosaurs. Quadrupedal dinosaurs with elephantine feet, short neck, small head, body and tail armored with massive bony plates and often with large bony spines. Teeth in a single row, like those of Iguanodonts. Stegosaurus of the Upper Jurassic, Ankylosaurus of the Upper Cretacic.

Fig. 8.—Hind Feet of Dinosaurs, to show the three chief types (Theropoda, Orthopoda, Sauropoda).

4. Ceratopsian or Horned Dinosaurs. Quadrupedal with elephantine feet, short neck, very large head enlarged by an enormous bony frill covering the neck, with a pair of horns over the eyes and a single horn in front. Teeth in a single row, but broadened out and adapted for grinding the food. No body armor. Triceratops is the best known type. Monoclonius, Ceratops, Torosaurus and Anchiceratops are also of this group. All from the Cretacic period.

Classification of Dinosaurs. It is probable that the Dinosaurs are not really a natural group or order of reptiles, although they have been generally so considered. The Carnivorous and Amphibious Dinosaurs in spite of their diverse appearance and habits, are rather nearly related, while the Beaked Dinosaurs form a group apart, and may be descendants of a different group of primitive reptiles. These relations are most clearly seen in the construction of the pelvis (see fig. 9). In the first two groups the pubis projects downward and forward as it does in the majority of reptiles, and the ilium is a high rounded plate; while in the others the pelvis is of a wholly different type, strongly suggesting the pelvis of birds.

Fig. 9.—Pelves of Dinosaurs illustrating the two chief types (Saurischia, Ornithischia) and their variations.

Recent researches upon Triassic dinosaurs, especially by the distinguished German savants, Friedrich von Huene, Otto Jaekel and the late Eberhard Fraas, and the discovery of more complete specimens of these animals, also clear up the true relationships of these primitive dinosaurs which have mostly been referred hitherto to the Theropoda or Megalosaurians. The following classification is somewhat more conservative than the arrangement recently proposed by von Huene.

Order Saurischia Seeley.
Suborder Coelurosauria von Huene (=Compsognatha Huxley, Symphypoda Cope.)
Fam. Podokesauridæ	Triassic, Connecticut.
" Hallopodidæ	Jurassic, Colorado.
" Coeluridæ	Jurassic and Comanchic, North America.
" Compsognathidæ	Jurassic, Europe.
Suborder Pachypodosauria von Huene.
Fam. Anchisauridæ	Triassic, North America and Europe.
" Zanclodontidæ	Triassic, Europe.*
" Plateosauridæ
Suborder Theropoda Marsh (=Goniopoda Cope)
Fam. Megalosauridæ	Jurassic and Comanchic.
" Deinodontidæ	Cretacic.
" Ornithomimidæ	Cretacic, North America.
Suborder Sauropoda Marsh (=Opisthocoelia Owen, Cetiosauria Seeley.)
Fam. Cetiosauridæ	Jurassic and Comanchic.
" Morosauridæ
" Diplodocidæ
Order Ornithischia Seeley (=Orthopoda Cope, Predentata Marsh.)
Suborder Ornithopoda Marsh (Iguanodontia Dollo)
Fam. Nanosauridæ	Jurassic, Colorado.
" Camptosauridæ	Jurassic and Comanchic.
" Iguanodontidæ
" Trachodontidæ	(=Hadrosauridæ), Cretacic.
Suborder Stegosauria Marsh.
Fam. Scelidosauridæ	Jurassic and Comanchic.
" Stegosauridæ
" Ankylosauridæ	(=Nodosauridæ), Cretacic.
Suborder Ceratopsia Marsh.
Fam. Ceratopsidæ	Cretacic.
* Regarded by Dr. von Huene as ancestral respectively to the Theropoda and Sauropoda.