Part 2

Imagine if you can, the vast plains extending from Mexico to Canada and from central Utah to the Mississippi River. To the west were high mountains in the Great Basin region of Nevada and western Utah. From these highlands flowed great sluggish streams that carried large amounts of sand and silt. Since the plains were almost flat, swamps and small lakes were probably numerous. The streams may have changed their courses from time to time as they were not confined to deep valleys. When the whole region emerged from the sea the climate became more humid. Volcanoes were active far to the west; the winds carried clouds of ejected dust eastward and deposited them on the plains. Semi-tropical conditions probably existed throughout the United States and in parts of Canada. These deposits are called the Morrison formation.

The warm humid climate provided ideal conditions for plant growth. Great forests of lush vegetation covered the land. Many of these plants have since disappeared, but some of their related species may be found today in the tropics. Most of the plants of our Temperate Zone had not yet evolved. However, there were tall stands of a type of pine, and other evergreens. There were also gingkos and curious tree ferns.

Various herb ferns formed a ground cover as thick and lush as grass on a well-watered prairie. Palmlike ferns resembling today’s cycads were common, while along the river grew horsetail rushes like those living today. Flowering plants of the Recent Epoch of geologic time (in which we are now living) had not yet made their appearance. Thus the hardwood, broadleaf forests of oak, elm, beech, maple, and similar trees were absent. So too were the flowering shrubs familiar to the Temperate Zone. Even the grasses were missing.

However, if you could picnic in this strange plant world you would soon be slapping mosquitos and cursing the ants. For even in such ancient times these insects were present; and so were a great variety of other insects as is known from the more than 1,000 species that have been discovered in Jurassic rocks. Among them were representatives of most modern orders such as grasshoppers, beetles, moths, ants, and flies. Jurassic insects probably looked much like those of today.

Among the most interesting of the strange reptiles were the pterosaurs that dominated the skies. They resembled the modern bats in some ways but their leathery wings were supported on each side by one finger instead of four, and their skins were scaly or bare instead of hairy. Some forms had long tails that were flattened at the tip and helped them balance in flight, but others were tailless. Some pterosaurs were no larger than sparrows while others had wing spans of 3 to 4 feet.

Crocodiles sunned themselves on the banks of sluggish streams and lakes. They probably looked a good deal like those that live in modern swamps and their habits were similar. Many a small dinosaur fell victim to their stealthy attack and disappeared beneath the waters of some ancient stream.

Birds have been found in Upper Jurassic rocks of Germany and may have lived here too. Their fossil remains would probably have been classed as reptiles had not feather imprints been a part of them. About the size of crows, these reptile-like birds had small conical teeth, three-clawed fingers on each wing, and a long tail instead of the fan of feathers seen on modern birds.

Small mammals were also living at the time the Morrison formation was being deposited and their remains have been found in the dinosaur quarry at Como Bluff, Wyo. The largest ones were about the size of a house cat but the majority were much smaller, probably about the size of today’s mice.

We do not know much about the habits of these early mammals but they were probably rather shy and retiring. This would be expected in the world of giants where they lived. Some of them lived in trees and there was one group whose skull characteristics resemble those of the rodents. It is likely that these primitive mammals lived a life similar to that of the rodents millions of years later.

This, then, was the setting, the stage upon which the dinosaurs played their leading roles. Although we have restricted our discussion to Morrison time in northeastern Utah, the same or similar animals lived all over the world. Worldwide humid and mild climates produced a similarity of plant and animal life during most of the Mesozoic Era whose like has not been seen in the last 60 million years. It was a strange world and ruled by strange animals, but it must have been an interesting one.

HOW DO WE KNOW?

This is a good time to stop and try to explain that this story, of plants and animals of the past, has a firm foundation in today’s facts—it is not a fantasy.

The methods by which geologists and paleontologists have established the age, climate, and life of Morrison time cannot be described for you here in detail. To attempt such a description would require too much space and would probably seem dull to most readers. Perhaps the best approach is to describe some features and explain how they contribute to our knowledge.

The rocks that were deposited here in Morrison time tell us much of the story. The sandstones were once stream sandbars or perhaps beaches around lakes. The shale, siltstone, and clay were muddy stream or lake bottoms. The discontinuous ledges of conglomerate probably represent gravel bars formed during flood stages or at places where the stream currents were very swift.

Just rocks you may say—but look closely. A piece of sandstone contains grains of sand that differ from each other in size, shape, and composition. Frequently these characteristics point to the source of the sandstone and tell something about the conditions at the time it was deposited. Chunks of black material are examined closely and prove to be charcoal—carbonized remains of plants.

Microscopic examination of clay fragments reveals shards of volcanic glass and ash that speak of active volcanoes. Sometimes these clays bear the carbonized imprints of delicate plants that long ago sank to the bottom of some lake or stream where they were buried and fossilized.

The fossils themselves are most important in reconstructing conditions of the past. We find the shells of fresh-water clams in the sandstones with dinosaur bones. Crocodile bones are also common. We are reasonably sure, then, that these deposits of sand and mud were formed in rivers and lakes when the climate was mild.

We reason by analogy. For example, fossil plants and animals have counterparts or descendants in the world of today. We assume, in the absence of contrary evidence, that the fossil animals lived like their present-day counterparts. Although no birds, mammals, or pterosaurs have been found in this quarry, they were probably living here with the dinosaurs. It is possible, in fact probable, that some modern animals and plants live in different environments than did their Morrison ancestors and relatives but we have no way of knowing which ones they were. We can only take the data available, arrange them as logically as possible, and continue the search for more. Some may scoff at such methods of reasoning yet they do provide good results. What other methods can be used when the world under investigation lies millions of years in the past?

Animals from the Quarry

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.

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.

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.

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 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