The ferocious dinosaurs of Mesozoic time were carnivorous, or flesh eaters, as shown by their numerous sharp teeth in relatively large heads. The largest known type is the tyrannosaur, an almost perfect skeleton of which, 40 feet long and 16 feet high, is mounted in the American Museum of Natural History in New York ([Plate 17]). So far as known, this was the largest carnivorous animal which ever walked on the earth. It is evident from the structure that it walked on its hind legs, the front ones having been much shorter and used something like arms. There were also various other smaller forms of two-legged flesh-eating dinosaurs, many of the wonderfully preserved tracks in the Triassic sandstones of the Connecticut River Valley having been made by such creatures when they walked around over soft, sandy mud flats at least eight or ten million years ago. The sandy mud with its tracks became somewhat hardened and then deeply buried under much more sediment which, through the ages, has been eroded off, thus exposing to view certain of the layers covered with tracks. Some bones of dinosaurs have also been found in the Connecticut Valley.
Fig. 62.—Skeleton (restored) of a great two-legged dinosaur of the Mesozoic era. This type of plant eater grew to be fully twenty-five feet long. (After Marsh.)
Another remarkable type of two-legged dinosaur was much like the flesh eaters just described, but they were plant eaters. The largest of these grew to be 30 feet long and 15 to 20 feet high, comparable, therefore, to the tyrannosaur in size. A wonderful collection of almost perfect skeletons may be seen in the museum in Brussels, Belgium. In mining coal 1,000 feet below the surface in Belgium, twenty-two complete skeletons and several partial skeletons were found in an ancient river deposit of Cretaceous Age. A marvelously preserved specimen of one of these two-legged plant eaters found in Wyoming, has been called a “dinosaur mummy” because the skin and much of the flesh of the creature had shriveled down upon its bones. The minutest details of the texture of its skin are almost perfectly preserved.
Fig. 63.—Skeleton of a dinosaur (triceratops) with a large remarkable head. This creature grew to be twenty-five feet long during Cretaceous time. (After Marsh.)
Another type of dinosaur, so different from the others, should be briefly described. This was triceratops, or the “three-horned face” beast, so named because of the three powerful horns which projected forward from the top of the very large, flattened skull. It grew to be twenty to twenty-five feet long. Skulls six to eight feet long have been unearthed. Just where the brain might have developed, the skull dished downward, and so one authority considers triceratops to have had the largest head and smallest brain of all the great reptiles.
It is well known that dinosaurs of many types lived during the great “Age of Reptiles,” though by no means all types ranged through the whole era. No dinosaurs are definitely known to have crossed the line into the Cenozoic era. One of the most astonishing facts in the history of animal life is the extinction of the mighty dinosaurs, but no very satisfactory explanation has yet been offered. Probably their great size was a contributing factor, for it is well known “that while very large animals spend nearly all their time in eating, small animals spend a small proportion of theirs, and most of it in other activities. Now, as long as food is abundant, the larger animals of a race have the better chances, but if a scarcity of food ensues, the larger animals may all be suddenly swept out of existence.” (Matthew.) Whatever may have been the real reason for dinosaur extinction we can at least be sure “that with the extensive changes in the elevation of land areas (Rocky Mountain Revolution) which mark the close of the Mesozoic, came the withdrawing of the great inland Cretaceous seas along the low-lying shores of which the dinosaurs had their home, and with the consequent restriction of old haunts, came the blotting out of a heroic race. Their career was not a brief one, for the duration of their recorded evolution was twice that of the subsequent mammalian (Cenozoic) age. They do not represent a futile attempt on the part of nature to people the world with creatures of insignificant moment, but are comparable in majestic rise, slow culmination, and dramatic fall to the greatest nations of antiquity.” (Schuchert.)