Part 4

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 quarry—fragments 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 scavengers 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 Mountain—a 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 debris—shale, sandstone, and clay—were 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 ago—running 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 ago—only 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.

Why Did Dinosaurs Become Extinct?

At Dinosaur National Monument only Morrison rocks of the upper Jurassic Period contain the fossil bones of dinosaurs. After Morrison time, the Cretaceous seas invaded this area. More than 5,000 feet of sandstone, shale, and mudstone were formed from sediments deposited in these seas.

Elsewhere in North America and the rest of the world, the diversity and numbers of dinosaurs actually increased. Entirely new groups evolved and achieved success in the battle for survival. The climax of reptile development seems to have come near the end of Cretaceous time in the Mesozoic Era. As the dinosaurs ruled the continents, so did other strange reptiles dominate the seas. Had you been able to see this ancient world, you would surely have been convinced that the dinosaurs and other reptiles would rule forever.

But it was not to be. The dinosaur hordes were wiped out and the reptiles reduced to the position of relative insignificance they occupy today. Such a profound and sudden change in the evolutionary trend of life must have had a cause, and scientists have sought it. Several theories have been proposed to explain extinction of dinosaurs, and they are most interesting.

At the end of Cretaceous time, some of our great mountain ranges were formed. It was a time of earthquakes and of volcanoes that belched forth clouds of ash and rivers of molten rock. Some people would say these catastrophic events killed all the dinosaurs. The scientist shakes his head. If these events killed dinosaurs, why not the other animals that lived with the dinosaurs. And what of those parts of the world that had no volcanoes, what killed dinosaurs there?

Changes in environments, the drainage of lakes and swamps as young mountains rose, changes in vegetation as new plants replaced old, and sudden shifts of climate occurred. These conditions could explain local extinction, but there were places where these changes did not occur and yet all dinosaurs in all places died.

A one-time favorite theory suggested that increasing numbers of small mammals ate dinosaur eggs, but there were many mammals eating dinosaur eggs during all of Cretaceous time and the dinosaur hordes increased. Many more mammals during succeeding ages have not killed off the turtles, snakes, lizards, and crocodiles that lay eggs and exist in great numbers today.

Some disease or combination of plagues may have swept the dinosaurs into extinction. If so, no evidence has been found to date that confirms or denies. However, most paleontologists do not accept this theory.

These are some of the theories that have been advanced to explain the sudden extinction of dinosaurs throughout the world. Each theory will explain the death of some dinosaurs in some places but attempts to apply any of them, or combinations of them, to worldwide extinction have failed.

This dinosaur story is like a mystery thriller with the last pages torn out. A most important part is missing. That is true and the paleontologist knows it. He also knows the riddle will probably never be solved. He might point out, however, that no one has successfully explained the extinction of the passenger pigeon which occurred quite recently, nor do we know why some other species of wildlife are on the brink of extinction today. The paleontologist is not the only one who must say, “I don’t know.”

History and Development of the Quarry

DISCOVERY AND EARLY YEARS

No one knows how long the old bones had been weathering out of the hills of what is now Dinosaur National Monument before the first man saw them. Curious Indians, wandering between the upturned ridges of Mesozoic rocks, picked up fragments and carried them off to their camps where they are now found among the arrow points, ax heads, and corn-grinding stones. In 1776, the Spaniard, Father Escalante, passed within sight of today’s dinosaur quarry, not dreaming of the antiquity hidden there. Maj. John Wesley Powell, on his second voyage down the Green River in 1871, recorded the presence of “reptilian remains” in the area, but wrote nothing more about them. Sheepherders, cattlemen, and hunters observed them and were impressed in proportion to their understanding. But, through all the years, the nature of the bones remained a mystery.

Then, in 1893, this mystery was solved. O. A. Peterson, a scientist from the American Museum of Natural History, while conducting field work in the Uinta Basin to the south of the present monument boundaries, discovered bones out-cropping from a recognized fossil-bearing stratum. The stratum was the 140,000,000 year-old Morrison formation. The bones? Peterson reported them as the remains of dinosaurs.

That report was to have an important influence, 15 years later, in directing a fellow paleontologist from the Carnegie Museum in Pittsburgh to investigate the area. Earl Douglass was the paleontologist’s name. In 1908, he and W. J. Holland, Director of the Carnegie Museum, found themselves in the region of Peterson’s discovery, searching for dinosaur remains. They extended their search to the north and thence along the Morrison hogback that flanks Split Mountain. Bone was found—not much, but enough to bring Douglass back the following summer and in company with George Goodrich, a local resident, to pursue the hunt.

The hunt came to a triumphant climax on August 17, 1909, when—to quote from Douglass’s diary—“At last in the top of the ledge where the softer overlying beds form a divide ... I saw eight of the tail bones of a Brontosaurus {Apatosaurus} in exact position.”