Part 2

A greater variety of dinosaurs lived during the Jurassic Period than in the Triassic. Both two- and four-footed types were present. The flesh-eaters remained two-footed but increased in size. Antrodemus, perhaps the best known, was much bigger than a kangaroo. The larger plant-eaters weighed from 30 to 40 tons and all were four-footed. The largest land animals, they lived on dry land and in the swamps that formed an important part of the Jurassic landscape. The first of the armored plant-eating dinosaurs, Stegosaurus, inhabited the dry plains. There were also some smaller, kangaroo-size plant-eaters that were two-footed.

A wide variety of dinosaur fossils has been found in the rocks of the Cretaceous Period, the last of the Mesozoic Era. The huge swamp dwellers still thrived. The flesh-eaters had evolved much larger types and included 40-foot Tyrannosaurus, the largest that ever lived. All the flesh-eaters walked on their hind legs as did their predecessors of the Jurassic and Triassic Periods.

New and interesting dinosaurs were present among the flesh-eaters. Horned forms, somewhat similar to the rhinoceros but much larger, were common. Also common were the turtle-like ankylosaurs. Perhaps the oddest and most interesting dinosaurs of the Cretaceous were the two-footed hadrosaurs. These excellent swimmers had weird head shapes with complicated skull passages and openings. They were a very successful group and at least 15 different kinds are known from the Cretaceous rocks of North America.

WHAT THEY LOOKED LIKE

Ideas about the external appearances of dinosaurs have been developed after many years of work and study. They are a combination of the ideas of several people who had studied different specimens of a single species. Let us review briefly the materials and work necessary to arrive at a reasonably accurate picture of the body form and external appearance of these strange reptiles.

The first requirement for arriving at a good idea of the build and physical attitude of an animal is a nearly entire skeleton. We cannot have too much of the animal’s skeleton missing or we may make a serious error. But if the left hind leg is missing and we have the right, we are not seriously handicapped. However, if both hind legs are missing we must restore them according to a similar animal whose hind legs are known.

After the nearly entire skeleton has been found it must be collected with great care. This is a rather involved process and, for some of the large dinosaurs, 2 or 3 months work may be required. The specimen is first uncovered and the fossil bone is treated with a preservative such as gum arabic, shellac, or one of the plastics. An accurate diagram of the specimen as it lies in the rock is made on cross-ruled paper. A trench 2 or 3 feet wide is then dug around the specimen. The depth of the trench is determined by the width of the specimen and the nature of the rock.

If the specimen is too large to take out in one piece, as most dinosaurs are, it is divided into sections which are numbered serially as they are taken out. Each section is bandaged in strips of burlap dipped in plaster of Paris. After the plaster has set, the section is turned over and the bottom is sealed with burlap and plaster. The section is labeled with the appropriate number and the section and number are shown on the diagram.

When all of the sections have been bandaged and numbered they are packed in strong wooden boxes and shipped to the laboratory.

The work in the laboratory is more involved than that in the field, and extreme care must be exercised to be sure that the bones will be undamaged. In most cases the bones have been broken by natural causes as they lay in the rock before discovery. All the pieces of each bone must be thoroughly cleaned and securely cemented together. This is a very time-consuming task and for a large dinosaur like Apatosaurus it requires 3 men 4 or 5 years to complete the task.

After all of the bones are cleaned and cemented together the vertebral column is laid out in its proper sequence on a sand table. Special care is exercised to be sure that the vertebrae fit correctly with each other. In this way the correct curvature of the vertebral column is determined. The proper relationships of the hip bones and ribs to the vertebrae, the shoulder blade to the ribs, and elements of the limb bones to each other are determined in the same manner. All of this work is necessary to correctly fashion the steel framework which will support the skeleton when it is placed on exhibition. The results of this careful work must be the framework of an animal which could, if living, easily go through the normal activities of life such as securing food and escaping enemies.

Now that the framework of an animal has been set up so that it could move about if it had muscles, skin, and life, how do we know how large the muscles were and where they were placed? It is necessary to have a thorough knowledge of the muscles of a recent animal similar to the one we are restoring so that we will know what we are looking for in the fossil. The areas at which muscles are attached to bones are called muscle scars and are identified by their rough surfaces. Often the necessary information can be obtained from publications which usually represent the work done by graduate students for advanced degrees. At other times we must make our own investigation. Thus if we know what muscle we are looking for and the size and shape of its muscle scar, we can determine whether the muscle is a spindle-shaped mass or a broad sheet.

After we have determined the size and position of the muscles which operate the limbs, head, and neck, we have a reasonably accurate idea of the external form of the animal, but we still know nothing of the nature of the skin which covered the body. Since dinosaurs were reptiles, we are obliged to assume that they were covered with a scaly skin in order to preserve the body moisture. None of the modern reptiles possess sweat glands in the skin. If they did not possess a waterproof covering of scales they would die in a few hours as a result of the loss of body moisture by evaporation through the skin. It is possible that some of the marsh dwellers like Apatosaurus had naked skin which was, as in the elephant, nearly an inch thick. The elephant does not possess sweat glands but the outer half of its skin is composed entirely of dead cells which form a covering as waterproof as the scales of today’s reptiles.

There have been only a few lucky finds of mummified dinosaurs which show the impressions of the scales. We know that all lizards do not possess the same type of scales, and therefore, by analogy, we cannot assume that the dinosaurs did. Eventually, we will probably find that the dinosaurs exhibited as great a variety of scale-types as do today’s lizards. As yet we have found nothing in the fossil record which indicates the color of the dinosaurs. Again, we can only assume that they exhibited as great a variety of colors as do our lizards. So also, we assume their body functions were somewhat similar to the reptiles and other related animals we know today.

TEMPERATURE TOLERANCE

We know the body temperatures of reptiles vary with that of the air or water in which they live, as they have no means of internal temperature control. They are very sluggish when their body temperatures are low and become more active as these temperatures rise, but only to a certain point. If the body temperatures of reptiles become too high, they die in a few minutes.

A group of physiologists from Columbia University spent nearly 2 months in southern Florida experimenting on reptiles. They determined the rate of rise of body temperatures of large lizards and alligators of all sizes during exposure to the midday sun. As was expected, the smaller the reptile the more rapid the rise in body temperature. Dinosaurs were reptiles so we can make two assumptions: That their physiology was very similar to that of living reptiles; and that the rate of rise of their body temperatures from exposure to the sun would follow the principles found for living reptiles.

By applying these principles to the dinosaurs, this group of scientists calculated that if the great bulk of an Apatosaurus were exposed to the direct rays of the sun at an air temperature of 110° F. for 36 to 40 hours, its body temperature would rise only 1° F. Therefore, if these calculations are correct, it is probable that the very size of the huge dinosaurs operated to maintain a fairly constant body temperature. Consequently, daily and seasonal temperature changes probably did not affect the activities of the large dinosaurs. However, the activities of the small ones may have been affected by the daily range in temperature.

GIZZARD STONES

For many years rounded stones with a very high polish have been found in the sedimentary rocks which contain bones of extinct reptiles. The polish on these stones is very much higher than could have been applied by the action of water or wind. Some look as though they had been polished by a jeweler. Since we cannot attribute this very high polish to wind or water action, we must seek another agent.

Just as chickens swallow fine gravel for their gizzards to aid digestion, so it is thought that some large dinosaurs swallowed stones for the same purpose. There is some evidence to support this idea. Several specimens of a group of swimming reptiles, called plesiosaurs, which swarmed the Jurassic and Cretaceous seas, have been found with highly polished stones inside the rib basket. Also a mass of highly polished stones was found similarly associated with one dinosaur, Protiguanodon, in the Lower Cretaceous rocks of Mongolia.

On the other hand, no highly polished stones have been found associated with the specimens in the Dinosaur Quarry or anywhere in the quarry. A search of the many publications on dinosaurs has not turned up any mention of highly polished stones being associated with any of the many specimens found in North America. Thus the evidence which we have does not permit us to say that the dinosaurs found in the quarry did or did not possess gizzard stones.

The Climate, Life, and Landscape of Jurassic Time

The geologists who attempt to reconstruct the geography and climate of the Jurassic Period first gather all possible facts and try to fit them together to form a logical pattern. The results are then examined for weak points and an attempt is made to find field evidence throwing light on these weak points. The final result represents the sum of our knowledge at the time but is subject to change as new facts are obtained. Thus the following outline represents present thinking that may be changed somewhat by future studies.

The land for miles around the Dinosaur Quarry was a low-lying desert in early Jurassic time. The mountains you see now had not yet been formed, and the whole desert area lay close to sea level. Great restless sand dunes drifted across this level land to form a blanket 700 feet thick. As the earth’s crust sank, these dunes were covered by a long arm of an arctic sea that extended southward along the present trend of the Rocky Mountains across Canada, Montana, Wyoming, and Utah. Millions of years later, in late Jurassic time, when the Morrison formation was deposited, the area rose again and the stage was set for the dinosaurs.

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.