FOSSILS of birds are rare until comparatively recent times, when we see them to be closely related to the living forms. We have a few examples in the fossil record, however, from the long time that elapsed between their first appearance in the Jurassic and the appearance of the modernised kinds. The earliest and most interesting one we know is called Archce-opteryx, and is known by skeletons in London and Berlin and by a tail in Munich. Fortunately these were preserved in a limestone of such fine grain that we know many details of their structure. At first sight, it seems that this was a flying reptile rather than a bird, for it has teeth in the jaws, three-clawed fingers in the wings, and a long but not tufty tail. It differed from the reptiles however by the quality of its brain and by having feathers. The tail was formed of a series of twenty vertebra? each bearing a pair of feathers.
Of the later flying birds, those of the Cretaceous period, we have occasional evidence. From the Cretaceous deposits of Kansas, we know of a little bird called Ichlhyornis, while from the Eocene remains of Nigeria we have the breastbone of a bird that must have had a wing span of twenty feet—the largest known to us. A large swimming bird of the Cretaceous, named Hesperornis, was probably three or four feet long. The Cretaceous birds still had teeth, and some of their London Clay (Eocene) successors had tooth-like serrations on the jaws.