Among the most extraordinary animals not only of the Mesozoic, but also of all time, were the flying reptiles or literal dragons of the air. Some were very small, while others were the largest creatures which ever flew, with a spread of wing of twenty to twenty-five feet—twice that of any modern bird. Unlike birds they had no feathers, but the two wings consisted of large membranes (batlike) supported by one enormously elongated finger of each front limb. The other fingers were armed with sharp claws. The early Mesozoic flying reptiles had sharp teeth, while the later ones were mostly entirely toothless, but all were carnivorous. Their short bodies were supplied with tails of varying lengths, one long-tailed species having a rudder at the end. Their heads were fairly large, but of light build. The creature called “pteranodon” was not only the largest of the flying reptiles, but also probably the most highly specialized creature which ever lived, everything possible apparently having been sacrificed to facilitate flight ([Plate 18]). The hollow bones were so wonderfully light and strong that it has been estimated that the living animal, with twenty-five foot spread of wing, and head four feet long, could not have weighed more than twenty-five pounds! The rear portions of the body and hind limbs were very weak.
Fig. 64.—A small carnivorous flying reptile of Mesozoic time. Spread of wings about two feet. (Restored by Marsh.)
It should not be thought that the above-described groups of reptiles were the only ones which existed during Mesozoic time. There were also certain groups still living, like turtles, lizards, and crocodiles, but they were doubtless mostly completely under the dominance of certain of the now long-extinct types above described. The oldest-known fossil snakes are from very late Mesozoic rocks, where they are small and comparatively rare. More than likely they evolved from lizards by deterioration of the legs. Poisonous snakes were not evolved until early in the next (Cenozoic) era.
We shall now turn our attention to next to the highest class of Vertebrate animals—the birds. They and the mammals are the only warm-blooded animals. What is their ancestry? From what original stock did they branch off? The oldest-known bird lived during the Jurassic period, and it was so decidedly reptilian in character as to render it practically certain that birds are specialized descendants of certain Mesozoic reptiles, though not, as might be supposed, of the flying reptiles. The few known specimens of the Jurassic birds were found in the famous lithographic limestone quarries of Bavaria. At least two of the specimens are in a marvelous state of preservation, with practically the whole skeleton intact and almost perfect impressions of the feathers on the rock. That the creature was really a bird is proved not only by its feathers, but also its beak, brain, limb bones, and feet. Among the reptilian characters are its long, vertebrated tail, teeth set in sockets, and long claws on the wings. This reptilian bird was about the size of a small crow.
By late Cretaceous time the birds made notable evolutionary progress and they became diversified, more than thirty species being known from Cretaceous rocks. These were distinctly more modern in structure and appearance than the Jurassic bird. The only important reptilian characteristic still retained was the possession of teeth. The tail had become much shortened and the brain was still relatively smaller than in modern birds. One type, about nine inches high, was a powerful flier, as shown by the strong keel and wing bones. Another important Cretaceous type was almost wholly a water dweller, with powerfully developed legs used in swimming. Its teeth were set in grooves instead of in sockets, thus indicating degeneration of tooth structure. This type was notable for its size—five to six feet in length.
Fig. 65.—An early type of bird with teeth. This bird grew to a height of about nine inches in Cretaceous time, millions of years ago. (Restored by Marsh.)