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. [Illustration: 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.