Ichthyornis was also an aquatic, fish-eating bird, but was only about as large as a pigeon. Its wings were long and fitted for powerful flight, and its habits were apparently like those of the modern terns; but its relationship is still in doubt. The tail in both these Neorni thes had become greatly shortened as com pared with Archeeopteryx, and exhibited the condensation of bones completed in the pygostyle of modern birds.
In the succeeding epoch, the Lower Eocene, at the beginning of the Tertiary Period, or °Age of Mammals," birds had begun to fore shadow modern types with little or no refer ence to the preceding "toothed" type. That seems to have come to an end; and there is no evidence of any ancestral connection be tween them and the modern types, whose ancestors, earlier than the Eocene, remain un discovered. These earliest Tertiary birds are also aquatic, however, and are to be classed with the cormorants, cranes, etc.; but there were also gigantic, ostrich-like forms, such as Dasyornis, Gastornis and Diatryma— not real ostriches, but with affinities with wading birds. Especially notable among these was Diato?ma gigantea, of the Wahsatch formations of Utah, —a flightless bird standing 12 feet in height and the largest known fossil bird, hardly ex ceeded by the great moas (q.v.) of New Zealand. By the end of the Eocene period, a wide variety of kinds of birds occur, usually referable to existing families but not to exist ing genera. "They are all types," says Osborn,
"fitted to inhabit great warm plains, scattered with groves. . . . It is an essentially tropical assemblage [representing birds] now for the most part inhabitants of the equatorial regions of Africa and South America." In the succeed ing epoch, the Oligocene, some existing genera may be recognized, and the fauna of that time has an unmistakeable African aspect,* but in the next, or Miocene, period many forms belonging to the North Temperate Zone appear. It is in this Miocene epoch that the plains of Patagonia, then warm and bearing abundant vegetation and a crowded population of strange animals, were the home of certain great ostrich-like birds of which the most remark able, perhaps, was Phororhachos, several species of which are known from bones re covered from the Santa Cruz formation, so rich in fossils. It stood eight or nine feet high, was supported on long legs, very thick and strong, had a rather long tail fully formed wings, useless, however, for flight, and carried an immense head with a huge hooked beak shaped like an eagle's. A short-legged form of these gigantic birds, whose habits were probably raptorial, is named Brontornis.
From this time on the birds became well fixed in modern types; and they appear to have changed but little, in marked contrast to the great evolution of the Mammalia, since the Middle Tertiary.
Consult books listed under GEOLOGY ; ORNI THOLOGY ; PALAIONTOLOGY.