In the rocks of the Morrison formation at the quarry, both orders of
dinosaurs are found—_Saurischia_ and _Ornithischia_. Paleontologists
have divided the dinosaurs into these two groups on the basis of
important skeletal differences. These differences remain constant for
the orders and vary within each order only in small details.
The important structural difference in dinosaurs is found in the pelvis.
In all land vertebrates, the pelvis is made up of three pairs of bones
called the ilium, pubis, and ischium. The paired ilium is joined to each
side of the backbone and projects downward to meet the pubis and ischium
at the socket for the head of the thigh bone. The pubis forms the front
third and the ischium the rear third of this socket. In the order
_Saurischia_ the bones of the pelvis are arranged as in most reptiles
and mammals. In the order _Ornithischia_ the pubis extends backward
along the ischium as it does in the birds.
Two types of saurischian dinosaurs are found in the quarry.
_Antrodemus_, a flesh-eating type, was about the size of a horse, but
was two-footed. It had strong sharp claws on its feet. Its teeth were
about 2 inches long, flattened from side to side and with fine
serrations on front and back edges. Actually it is not known whether
_Antrodemus_ overpowered and killed the large swamp-living dinosaurs, or
merely fed on their carcasses after they had died from other causes.
However, there has been found in the quarries at Como, Wyo., a partial
skeleton of _Apatosaurus_ with grooves on the bones which suggest tooth
marks. The spacing of these grooves fit the spacing of the teeth of a
specimen of _Antrodemus_ found in the same quarry.
[Illustration: A. ORNITHISCHIAN PELVIS. B. SAURISCHIAN PELVIS.
KEY: IL—ILLIUM; IS—ISCHIUM; P—PUBIS.]
The plant-eating dinosaurs of the order _Saurischia_ which are found in
the quarry were all four-footed. They had bodies about the size of an
elephant or larger. The principal differences between the flesh- and
plant-eating dinosaurs were the length of the neck and tail, the details
of their skull structure, and other parts of their skeleton.
_Apatosaurus_ is perhaps the most familiar dinosaur to most of us. Its
hind legs were much longer than its front ones and gave the animal a
high-hipped, stooped appearance. _Apatosaurus_ was about 70 feet long
and probably weighed close to 40 tons. _Diplodocus_ was longer (one of
them 75½ feet) but was slender and lightly built. Its neck was longer
and it had a whiplash tail that looked much like the tail of the modern
whiptailed lizard. _Diplodocus_ also had long pencil-like teeth
different from those of any other known dinosaur. _Barosaurus_ has an
extremely long neck with long individual neck bones. Two members of the
genus _Camarasaurus_ are similar to each other except for size; one was
small, but the other was as large as _Apatosaurus_. _Camarasaurus_ had
longer front legs than _Apatosaurus_ and was generally better
proportioned.
[Illustration: _Antrodemus_, THE FEROCIOUS CARNIVORE OF MORRISON
TIME. (FROM A DRAWING BY CHARLES R. KNIGHT. COURTESY, AMERICAN
MUSEUM OF NATURAL HISTORY.)]
[Illustration: THE GREAT SAURISCHIAN PLANT-EATER _Apatosaurus
louisae_—ABOUT 70 FEET LONG. (FROM A DRAWING BY A. AVINOFF, CARNEGIE
MUSEUM.)]
Fossils of the saurischian plant-eaters are found much more frequently
than those of flesh-eaters and are usually in sedimentary rocks which
contain beds of clam shells. For this reason it seems probable that they
waded lagoons and streams, feeding on aquatic and bank-side vegetation.
The suggestion has been made that the larger dinosaurs could not even
walk on dry land because their weight would have crushed the bones of
their feet; they needed the buoyancy of water to help support them.
However, footprints of a huge dinosaur, much larger than any from the
quarry, have been found near Glenrose, Tex. The large footprints were
made on a sandy beach of a sea in Lower Cretaceous time. Thus we know
that they could walk on dry land if they wanted to.
All of the dinosaurs of the order _Ornithischia_ were plant-eaters, and
were of both two- and four-footed types. The two-footed types found in
the quarry are _Camptosaurus_, _Dryosaurus_, and _Laosaurus_. These
forms had well developed front legs, though much shorter than their hind
legs, which suggests that they may have dropped down on “all fours”
while feeding or resting. The teeth were small, chisel-shaped, and
fitted only for cropping vegetation. The larger specimens of
_Camptosaurus_ reached a length of 17 feet but _Laosaurus_ was only 2½
feet long.
_Stegosaurus_ is the only quadruped (four-footed) of this order found in
the quarry. It had long hind legs and very short front legs. It reached
a length of 18 to 20 feet and was 10 to 11 feet high over the hips. The
most characteristic feature of this form was the double row of bony
plates down the back and the group of spikes at the end of the tail. The
teeth were similar to those of _Camptosaurus_, but much more numerous.
Only two other groups of reptiles have been found in the quarry at
Dinosaur National Monument and their remains are rare. These are the
crocodiles and turtles. Two crocodiles are known; the larger one,
_Goniopholis_, was about the size of existing alligators and did not
differ in external appearance from present-day crocodiles. The smaller
one was less than a foot long and resembled a 2 weeks’ old alligator as
much as anything. However, we know from the texture of the surface of
the bone that it was not a young animal. The turtle, _Glyptops_, was
about the same size and general appearance as the pond turtles of today.