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.[ToC]
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.