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. [Illustration: DINOSAUR FOSSIL WITH SKIN AND LIGAMENTS PRESERVED. (NOT FROM DINOSAUR QUARRY. COURTESY, AMERICAN MUSEUM OF NATURAL HISTORY.)] [Illustration: RARE FOSSIL OF DINOSAUR SKIN. (COURTESY, AMERICAN MUSEUM OF NATURAL HISTORY.)] 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.