The first flying vertebrates were true reptiles in which one of the fingers of the front limbs became very elongated, providing support for a flap of stretched skin that served as a wing. These were the pterosaurs, literally the “winged lizards.” The earliest pterosaurs arose near the end of the Triassic period of the Mesozoic Era, some 70 million years before the first known fossils of true birds occur, and they presumably dominated the skies until they were eventually displaced by birds. Like the dinosaurs, some the pterosaurs became gigantic; the largest fossil discovered is of an individual that had a wingspan of 50 feet or more, larger than many airplanes. These flying reptiles had large, tooth-filled jaws, but their bodies were small and probably without the necessary powerful muscles for sustained wing movement. They must have been expert gliders, not skillful fliers, relying on wind power for their locomotion.
Birds, despite sharing common reptilian ancestors with pterosaurs, evolved quite separately and have been much more successful in their dominance of the air. They are an example of a common theme in evolution, the more or less parallel development of different types of body structure and function for the same reason — in this case, for flight. Although the fossil record, as always, is not complete enough to determine definitively the evolutionary lineage of the birds or in as much detail as one would like, it is better in this case than for many other animal groups. That is because of the unusual preservation in a limestone quarry in southern Germany of Archaeopteryx, a fossil that many have called the link between dinosaurs and birds. Indeed, had it not been for the superb preservation of these fossils, they might well have been classified as dinosaurs. They have the skull and teeth of a reptile as well as a bony tail, but in the line-grained limestone in which these fossils occur there are delicate impressions of feathers and fine details of bone structure that make it clear that Archaeopteryx was a bird. All birds living today, from the great condors of the Andes to the tiniest wrens, trace their origin back to the Mesozoic dinosaurs.
鸟类虽然与翼龙共享共同的爬行动物祖先，但它们分开演变并且在空气优势方面取得了更大的成功。它们是进化中共同主题的一个例子，不同类型的身体结构和功能或多或少并行发展出于同样的原因 - 在这种情况下，对于飞行。虽然化石记录一如既往地不够完整，无法确定鸟类的进化谱系，也没有尽可能详细地确定，但在这种情况下，它比许多其他动物群体更好。这是因为在德国南部的始祖鸟（Archeopteryx）的一个石灰石采石场中保存得非同寻常，这是一种许多人称之为恐龙与鸟类之间联系的化石。事实上，如果不是对这些化石的精湛保存，它们很可能被归类为恐龙。他们有爬行动物的头骨和牙齿以及骨头尾巴，但是在这些化石出现的线粒状石灰岩中，有精致的羽毛印记和骨骼结构的精细细节，这使得始祖鸟是一只鸟。今天生活的所有鸟类，从安第斯山脉的大秃鹰到最小的一群，都追溯到中生代的恐龙。