Today, most of us would recognize a fossil bone for what it is, but in the 1790's things were different. Isolated legbones, vertebrae, and teeth of huge reptiles had been dug out of certain sedimentary rocks of Europe and North America but their scientific importance was little understood.
These specimens were found by people in all walks of life and it was natural that their curiosity was greatly aroused. The finders took the specimens to someone nearby whom they considered more competent to tell them something about these strange bones and teeth. In nearly all cases these "experts" were doctors of medicine. They studied the fossil specimens and reported on them at regular meetings of the learned societies of which they were members. It was customary to put the fossils in the collections of these societies where they could be studied by other members. In North America most reports of these early discoveries are found in the Proceedings of the American Philosophical Society in Philadelphia, Pa.
By 1842 accumulated knowledge of these large reptiles was sufficient to show that they were distinct from any group then known. This was first recognized by Sir Richard Owen of the British Museum. It was he who named the group Dinosauria. The name is made up of two Greek words: deinos (terrible) plus sauras (lizard).
As knowledge of these unusual reptiles increased through the discovery of additional types and more complete and better preserved specimens, it became evident that dinosaurs were neither a single group of reptiles nor were all of them large. Actually the dinosaurs show as much diversity in size, body form, and habits as any group of reptiles. The smallest dinosaur walked on its 2 hind legs like a chicken and was about the same size. The largest walked on all 4 legs, was about 80 feet long, and weighed 30 to 40 tons. As examples of variety in body form there are the two-footed, flesh-eating Antrodemus, the armored Stegosaurus, the turtle-like Ankylosaurus, the horned Triceratops, the huge Apatosaurus, the two-footed vegetarian Camptosaurus, and the great variety of head forms in the aquatic hadrosaurs. Although there were two distinct groups, we still retain the term "dinosaur" as a convenient name for all of them but qualify it by saying, flesh-eating dinosaur, plant-eating dinosaur, armored dinosaur, etc., to indicate the particular type we are talking about. Perhaps you are wondering how all these ancient creatures are related to reptiles in general. Where do they fit in the classification system devised to bring order to this mass of knowledge?
POSITION OF DINOSAURS AMONG REPTILES
It seems there are several orders of reptiles similar to and closely related to the dinosaurs. Remains of these reptiles are found in the sedimentary rocks which contain the earliest known dinosaurs. A number of them resembled the dinosaurs but do not quite meet the requirements as far as details of the skeleton are concerned. In the scheme of classification these orders of reptiles are grouped together into the subclass Archosauria. This subclass includes the dinosaurs, crocodiles, and the flying reptiles. The lizards, snakes, turtles, and the tuatera of New Zealand belong to other subclasses of reptiles which have been distinct from that of the dinosaurs as far back in geologic time as we can trace them. The kinship between the dinosaurs and the small lizards living in the monument today lies only in that both are reptiles. The only living relatives of the dinosaurs are the alligator and the crocodile.
The dinosaurs were so numerous, and so dominated the whole of the Mesozoic Era, that this period of earth history is frequently referred to as the Age of Reptiles.
The Mesozoic Era began some 200 million years ago and ended some 60 million years ago. Although many other animals lived during that era, the dinosaurs were the dominant forms of animal life on land. The 140 million years of the Mesozoic are divided into geologic periods named Triassic (the oldest), Jurassic, and Cretaceous (the most recent). Continental deposits representing each of these periods have been found in all parts of the world and on all continents. Dinosaur bones have been found in these depositseven in such far away places as Australia and the southern tip of South America. Only Jurassic dinosaurs have been found at Dinosaur National Monument.
The oldest known dinosaurs are found in rocks of the Triassic Period. The smaller of these were chicken-size and the largest were about as big as kangaroos. All of these Triassic dinosaurs were two-footed. They can be divided into flesh-eaters and plant-eaters, although none are believed to have been particularly specialized in their food habits. In general the flesh-eaters were small, agile, and had sharp teeth for seizing and overpowering active prey. The plant-eaters were larger with rather long front legs and small blunt teeth suited only to cropping vegetation. These plant-eaters are believed to be the Triassic ancestors of the giant marsh-dwelling dinosaurs of the Jurassic and Cretaceous Periods.
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.
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.
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 ptestosaurs, 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.