One may well inquire why it is that no such gigantic carnivora have evolved among the mammalian land animals. The largest predaceous quadrupeds living today are the lion and tiger. The bears although some of them are much larger, are not generally carnivorous, except for the polar bear, which is partly aquatic, preying chiefly upon seals and fish. There are indeed carnivorous whales of gigantic size, but no very large land carnivore. There were, it is true, during the Tertiary and Pleistocene, lions and other carnivores considerably larger than the living species. But none of them attained the size of their largest herbivorous contemporaries, or even approached it. Among the dinosaurs on the other hand we find that—setting aside Brontosaurus and its allies as aquatic—the predaceous kinds equalled or exceeded the largest of the herbivorous sorts. The difference is striking, and it does not seem likely that it is merely accidental.
The explanation lies probably in the fact that the large herbivorous mammals are much more intelligent and active, and would be able to use their weapons of defense so as to defy the attacks of relatively slow moving giant beasts of prey, as they do also the more active but less powerful assaults of smaller ones. The elephant or the rhinoceros is in fact practically immune from the attacks of carnivora, and would still be so were the carnivora to increase in size. The large modern carnivora prey upon herbivores of medium or smaller size, which they are active enough to surprise or run down. Carnivora of much larger size would be too slow and heavy in movements to catch small prey, while the larger herbivores by intelligent use of their defensive weapons could still fend them off successfully. In consequence giant carnivores would find no field for action in the Cenozoic world, and hence they have not been evolved.
But the giant herbivorous dinosaurs, well armed or well defended though they were, had not the intelligence to use those weapons effectively under all circumstances. Thus they might be successfully attacked, at least sometimes, by the powerful although slow moving Megalosaurians.
The suggestion has also been made that these giant carnivores were carrion-eaters rather than truly predaceous. The hypothesis can hardly be effectively supported nor attacked. It is presented as a possible alternate.
Albertosaurus. Closely allied to the Tyrannosaurus but smaller, about equal in size to Allosaurus, was the Albertosaurus of the Edmonton formation in Canada. It is somewhat older than the Tyrannosaur although still of the late Cretacic period, and may have been ancestral to it. A fine series of limbs and feet as also skull, tail, etc., are in the Museum's collections. At or about this time carnivorous dinosaurs of slightly smaller size are known to have inhabited New Jersey; a fragmentary skeleton of one secured by Professor Cope in 1869 was described as Laelaps (=Dryptosaurus).[10]
Ornitholestes. In contrast with the Allosaurus and Tyrannosaurus this skeleton represents the smaller and more agile carnivorous dinosaurs which preyed upon the lesser herbivorous reptiles of the period. These little dinosaurs were probably common during all the Age of Reptiles, much as the smaller quadrupeds are today, but skulls or skeletons are rarely found in the formations known to us. The Anchisaurus, Podokesaurus and other genera of the Triassic Period have left innumerable tracks upon the sandy shales of the Newark formation, but only two or three skeletons are known. A cast of one of them is exhibited here. The original is preserved in the Yale Museum. In the succeeding Jurassic Period we have the Compsognathus, smallest of known dinosaurs, and this Ornitholestes some six feet long. A cast of the Compsognathus skeleton is shown, the original found in the lithographic limestone of Solenhofen is preserved in the Munich Museum. The Ornitholestes is from the Bone-Cabin Quarry in Wyoming. The forefoot with its long slender digits is supposed to have been adapted for grasping an active and elusive prey, and the name (Ornitho-lestes = bird-robber) indicates that that prey may sometimes have been the primitive birds which were its contemporaries. In the Cretacic Period, there were also small and medium sized carnivorous dinosaurs, contemporary with the gigantic kinds; a complete skeleton of Ornithomimus at the entrance to the Dinosaur Hall finely illustrates this group. In appearance most of these small dinosaurs must have suggested long-legged bipedal lizards, running and walking on their hind limbs, with the long tail stretched out behind to balance the body. From what we know of their tracks it seems that they walked or ran with a narrow treadway, the footsteps almost in the middle line of progress. They did not hop like perching birds, nor did they waddle like most living reptiles. Occasionally the tail or fore feet touched the ground as they walked; and when they sat down, they rested on the end of the pubic bones and on the tail. So much we can infer from the footprint impressions. The general appearance is shown in the restorations of Ornitholestes, Compsognathus and Anchisaurus by Charles Knight.
Fig. 17.—Skeleton of Ornitholestes a small carnivorous dinosaur of the Jurassic period. American Museum No. 619.
After Osborn