Dinosaurs | Page 5

of the Age of Mammals on the other hand, are still quite extensive, especially those of its later epochs, and afford a fairly complete record in some parts of the continent of the upland fauna of those regions.
Occurrence of Dinosaur Bones. Dinosaur bones are found mostly in the great delta formations, and since those were accumulated chiefly in the early stages of great continental elevations, it follows that our acquaintance with Dinosaurs is mostly limited to those living at certain epochs during the Age of Reptiles. In point of fact so far as explorations have yet gone in this country, the Dinosaur fauna of the close of the Jurassic and beginning of the Comanchic and that of the later Cretacic are the only ones we know much about. The immense interval of time that preceded, and the no less vast stretch of time that separated them, is represented in the record of Dinosaur history by a multitude of tracks and a few imperfect skeletons assigned to the close of the Triassic period, and by a few fragments from formations which may be intermediate in age between the Jurassic-Comanchic and the late Cretacic. Consequently we cannot expect to trace among the Dinosaurs, the gradual evolution of different races, as we can do among the quadrupeds of the Age of Mammals.
Imperfection of the Geologic Record. The Age of Mammals in North America presents a moving picture of the successive stages in the evolution of modern quadrupeds; the Age of Reptiles shows (broadly considered) two photographs representing the land vertebrates of two long distant periods, as remote in time from each other as the later one is remote from the present day. Of the earlier stages in the evolution of the Dinosaurs there are but a few imperfect sketches in this country; in Europe the picture is more complete. In the course of time, as exploration progresses, we shall no doubt recover more complete records. But probably we shall never have so complete a history of the terrestrial life of the Age of Reptiles as we have of the Age of Mammals. The records are defective, a large part of them destroyed or forever inaccessible.
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.
[Illustration: 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.
[Illustration: 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