LORDS OF THE AIR
In the early Triassic (225-200 million years ago) small pseudosuchians such as Saltoposuchus showed the essential characteristics of birds including bipedalism. There are no fossils detailing the change from the ectothermicbipedalreptiles into endothermic flying birds except for five fossil specimens of the upper Jurassic (about 150 million years ago) found in the lithographic slates of Solnhofen, Bavaria. These Archaeopteryx lithographica probably achieved some degree of gliding, and are certainly the earliest known animal to possess feathers. Anatomically these animals are much less specialized than the modern birds but does represent the earliest animal classified as a member of Aves and is in its own subclass Archaeornithes. All other birds were extinct or living belong to the subclass Neornithes. In 1860 the first fossilized feather was found, and a year later the first Archaeopteryx was found. The whole body axis was elongated, the dorsalvertebrae were not fixed and only five were fused to form the sacrum. There was a long tail, with feathers arranged in parallel rows along its sides. The fore-limbs ended in three clawed digits, with separate metacarpals and carpals. This limb was used as a wing since feathers were attached to the ulna and hand, but the wing was small and the shape rounded. The pelvicgirdle and hind-limb resembled that of the archosaurs. In the skull there were sharp teeth in both jaws, and the eyes and brain were considerably smaller than modern birds. The bones were not hollowed and since the sternum bone (keel) was not well developed, it could not have had muscles that could achieved flapping flight. It has been suggested that it used its feathers which probably originally evolved as some form of insulation, as a kind of net to trap insects while running fast across land. Alternatively it was suggested that it was arboreal and the feathers which were originally derived from reptilianscales, enabled Archaeopteryx to glide short distances much as gliding lizards do today (e.g. Draco volans). Thus the two theories that flight evolved 'from ground up' and 'from trees down' have been proposed. The descendants of Archaeopteryx and other ancient birds underwent a dramatic adaptive radiation during the Cretaceous period when both aquatic and terrestrial habitats were invaded. Hesperornis was a loon-like diver that possessed teeth, and had already lost its power of flight since the wings had become functionless and is the only other birdspecies known to have teeth.
A model of Archaeopteryx lithographica
on display at the Oxford University Museum