CHAPTER III.

KINDS OF DINOSAURS.

COMMON CHARACTERS AND DIFFERENCES BETWEEN THE VARIOUS GROUPS.

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.

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.

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.

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.

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 } " Plateosauridæ } Triassic, Europe.* 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 } " Morosauridæ } Jurassic and Comanchic. " Diplodocidæ } Order ORNITHISCHIA Seeley (=Orthopoda Cope, Predentata Marsh.) Suborder _Ornithopoda_ Marsh (Iguanodontia Dollo) Fam. Nanosauridæ Jurassic, Colorado. " Camptosauridæ } " Iguanodontidæ } Jurassic and Comanchic. " 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.

FOOTNOTES:

[Footnote 2: If some vast catastrophe should today blot out all the mammalian races including man, and the birds, but leave the lizards and other reptiles still surviving, with the lower animals and plants, we might well expect the lizards in the course of geologic periods to evolve into a great and varied land fauna like the Dinosaurs of the Mesozoic Era.]

[Footnote 3: The ancestral types have four complete toes, but in the true Theropoda the inner digit is reduced to a small incomplete remnant, its claw reversed and projecting at the back of the foot, as in birds.]