Dinosaurs | Page 5

America (after Schuchert).]
These last, however, fragmentary and loose and overlying the rest, were
the first to be swept away by erosion during the periods of elevation;
and of such formations in the Age of Reptiles very little, if anything,
seems to have been preserved to our day. Consequently we know very
little about the upland animals of those times, if as seems very probable,
they were more or less different from the animals of the coast-forests
and swamps. The river-plain deposits 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