Animals from the Quarry
In the rocks of the Morrison formation at the quarry, both orders of
dinosaurs are foundSaurischia 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.
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A. ORNITHISCHIAN PELVIS. B. SAURISCHIAN PELVIS.
KEY: ILILLIUM; ISISCHIUM;
PPUBIS.
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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.
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-caring 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 75-1/2 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.
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Antrodemus. THE FEROCIOUS CARNIVORE OF
MORRISON TIME. (from a drawing by Charles R. Knight. Courtesy, American Museum of
Natural History.)
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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 bouyancy 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 2-1/2 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.
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THE GREAT SAURISCHIAN PLANT-EATER Apatasaurus louisaeABOUT
70 FEET LONG.(from a drawing by A. Avinoff, Carnegie Museum.)
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WHY SO MANY?
The partial skeletons of more than 20 individual dinosaurs and the
scattered bones of about 300 more have been discovered in the Dinosaur
Quarry. Many of the best specimens may be seen today at museums of
natural history in the larger cities of the United States and Canada.
The quarry is easily the largest and best preserved deposit of Jurassic
dinosaurs known today.
How and why did so many dinosaur skeletons accumulate here?
How were they preserved? These are among the common questions asked
of park rangers and naturalists at Dinosaur. The answer is a combination
of circumstances and luck.
Many people get the impression from the mass of bones in the quarry
wall that some catastrophe such as a volcanic explosion or a sudden
flood killed a whole herd of dinosaurs in this area. True enough this
could have happened, but it probably did not. The main reasons for
thinking otherwise are the scattered bones and the thickness of the
deposit. In other deposits where the animals were thought to have died
together, the skeletons were usually complete and often all the bones
were in their proper positions, or articulated. In a mass killing the
bones would have been deposited on the stream or lake bottom together at
the same level, but in this deposit the bones occur throughout a zone of
sandstone about 12 feet thick. The mixture of swamp dwellers and
dry-land types also seems to indicate that the deposit is a mixture
derived from different sources. Rounded fragments of fossil bone have
been discovered in the quarryfragments that attained their
pebblelike shape by rolling along the stream bottom.
If the mass of bones was not the result of catastrophe what did
happen? The quarry area is a dinosaur graveyard, not a place where they
died. A majority of the remains probably floated down an eastward
flowing river until they were stranded on a shallow sandbar. Some of
them, such as the stegosaurs, may have come from far-away dryland areas
to the west. Perhaps they drowned trying to ford a tributary stream or
were washed away during floods. Some of the swamp dwellers may have
mired down on the very sandbar that became their grave while others may
have floated for miles before being stranded.
Even today similar events take place. When floods come in the spring,
sheep, cattle, and deer are often trapped by rising waters and
frequently drown. Their bloated carcasses float downstream until the
flood recedes and leaves them stranded on a bar or shore where they lie,
frequently half buried in the sand, until they decompose. Early
travelers on the Missouri River reported that shores and bars were
frequently lined with the decomposing bodies of bison that had perished
during spring floods.
In Dinosaur National Monument, the positions in which partial
skeletons of the dinosaurs lie suggest that they decomposed on a
sandbar. The bones on the underside of a skeleton are often arranged as
they were when the animal was alive, while those on the upper or exposed
side may be scattered. Such scattering would be expected as the
ligaments and muscles holding the bones together decomposed; stream
currents and scavangers could then disperse them. Stream currents are
suggested by the position of the long, flexible tails and necks of the
large plant feeders. These, like streaming water plants in a river,
trail downstream to the east.
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CamptosaurusAN ARNITHISCHIAN PLANT-EATER.
(drawn by J. G. German. Corutesy, American Museum of Natural History.)
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HOW WERE THEY PRESERVED?
The concentration and burial of dinosaur bones is only the beginning
of the fossil story. The combination of circumstances which operated
here was a common one and yet fossil quarries are rare. Why? The bones
have to be preserved and this seldom happens. The bones that are buried
in one flood are frequently unearthed and scattered by the next. Those
that are exposed to the weather usually disintegrate completely in a few
years. The bones in the Dinosaur Quarry did not.
Sometime after they were buried, the organic minerals of the bones
were more or less completely replaced by minerals of inorganic origin
such as silica. No one knows exactly why or how this happened, but it
did. Most geologists think this replacement process occurs when
subsurface or ground water containing soluble and colloidal minerals
dissolves a molecule of the bone and immediately replaces it with a new
mineral. Roughly such a process is like removing red bricks from a house
and substituting yellow. When the substitution is complete, the house
still has the same dimensions but it is composed of different materials.
The replacement was a faithful one, too, because microscopic structure
of the original bone was faithfully reproduced by the replacing
minerals.
Following Morrison time, thousands of feet of younger sediments were
deposited on the sandbar that contained the dinosaur bones. The whole
sequence of sediments was compacted into rock and some bones were
crushed and distorted.
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Stegosaurus. AN ARMORED DINOSAUR OF THE JURASSIC PERIOD. (from a
drawing by Charles R. Knight. Courtesy, American Museum of Natural History.)
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HOW WERE THEY EXPOSED?
After the sediments became rock and the bones had probably been
replaced by stone (fossilized), this part of the world, which lay near
or below sea level for millions of years, began to rise. Great forces
acted upon the earth's crust. These forces created faults, or fractures,
in the rock crust along which movement occurred. And what had once been
sea bottom was moved upward and became lofty mountains. This titanic
change has been called the Laramide Revolution; it closed the Mesozoic
Era with the formation of the Rocky Mountains.
Although the effects of the Laramide Revolution were not as profound
at Dinosaur as they were east of it, they were quite important. The
rocks were lifted to form the southwest flank of Split Mountaina
small arch, or anticline, on the south side of the Uinta Mountains. This
mountain building explains the pronounced southward tilt of the Dinosaur
Ledge and other rock layers visible in the quarry area. As the land
rose, streams flowed more rapidly, cutting deeper into the rocks and
carrying away the debris. Gradually thousands of feet of this
debrisshale, sandstone, and claywere stripped away through
erosion.
Finally all the material on top of the Morrison sandbar weathered
away. Some 140 million years after burial the fossil bones were exposed
by the agent that had buried them so long agorunning water! All
that remained was for them to be found, and that was the luckiest chance
of all. Just suppose they had been uncovered a million years
agoonly a second in geologic time. No one would have been present
to discover them, and through the years they could well have crumbled
into dust and been blown away.
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