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.
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.
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.
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.
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.
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.