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.
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