Part 3

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.