The first appearance of birds in America, according to our present knowledge, was during the Cretaceous Period, although many announcements have been made of their existence in preceding epochs. The evidence of their presence in the Trias, based on footprints and other impressions, is, at present, as we have seen, without value; although we may confidently await their discovery there, if not in older formations. Archæopteryx, from the European Jura, the oldest bird known, and now fortunately represented by more than a single specimen, clearly indicates a much higher antiquity for the class. The earliest American forms at present known are the Odontornithes, or birds with teeth, which have been exhumed, within the last few years, from the chalk of Kansas. The two genera Hesperornis and Ichthyornis are types of distinct orders, and differ from each other and from Archæopteryx much more than do any existing birds among themselves; thus showing that Birds are now a closed type, and that the key to the history of the class must be sought for in the distant past.
In Hesperornis, we have a large aquatic bird, nearly six feet in length, with a strange combination of characters. The jaws are provided with teeth, set in grooves; the wings were rudimentary and useless; while the legs were very similar to those of modern diving birds. This last feature was merely an adaptation, as the more important characters are struthious, showing that Hesperornis was essentially a carnivorous swimming ostrich. Ichthyornis, a small flying bird, was stranger still, as the teeth were in sockets, and the vertebræ biconcave, as in fishes and a few reptiles. Apatornis and other allied forms occur in the same beds, and probably all were provided with teeth. It is strange that the companions of these ancient toothed birds should have been Pterodactyls without teeth. In the later Cretaceous beds of the Atlantic coast various remains of aquatic birds have been found, but all are apparently distinct from those of the West. The known genera of American Cretaceous birds are—Apatornis, Baptomis, Graculavus, Hesperornis, Ichthyornis, Laornis, Lestornis, Palæotringa, and Telmatomis. These are represented by some twenty species. In Europe, but two species of Cretaceous birds are known, and both are based upon fragmentary specimens.
During the Tertiary period, birds were numerous in this country, and all yet discovered appear to have belonged to modern types. The Eocene species described are mostly wading birds, but here, and in the later Tertiary deposits, some characteristic American forms make their appearance, strongly foreshadowing our present avian fauna. The extinct genera are the Eocene Uintornis, related to the woodpeckers, and Aletornis, which includes several species of waders. Among the existing genera found in our Tertiary beds are—Aquila, Bubo, Meleagris, Grus, Graculus, Puffinus, and Catarractes. The great auk (Alca impennis), which was once very abundant on our northeast coast, has become extinct within a few years.