The Encyclopedia Americana (1920)/Birds, Fossil
BIRDS, Fossil. Birds are rare as fossils, compared with other vertebrates, and little is known about their early evolution. Four or five hundred extinct species have been described, as against 12,000 living, and most of them are from very fragmentary remains, found in widely scattered places. The explanation of this is found in their small size, their liability to be eaten, dead as well as alive, and the slight construction of their skeletons, which makes their bones less likely to be buried in sediments and preserved as fossils. At a few favorable places, however, as in the Oligocene Strata of the department of Allier in France, and the Pleistocene deposits of Fossil Lake in Oregon, they occur abundantly. Birds have been found as far back in geological time as the Jurassic Period of the Age of Reptiles. The supposed bird-tracks of the more ancient Triassic sandstones of the Connecticut Valley are now believed to be mostly, if not all, tracks of dinosaurs (q.v.), a group of reptiles having many bird-like characters. From some ancient offshoot of this group the birds are probably descended, but the early stages of their evolution are not known.
The oldest fossil bird known is the Archæopteryx, of which three specimens, one in marvelously complete condition, have been recovered from the upper Jarassic lithographic slates at Solenhofen, Bavaria; and it is a true bird although its skeleton presents many reptilian features. It was about the size of a crow, and had a rather elongated, narrow body, with a small, somewhat flattened head, and very large eye-sockets. The jaws protruded in a beak-like form; but there was no horny beak, and the upper jaw, and probably the lower also, was armed with many slender lizard-like teeth, set in a groove. The legs were of normal length, and had four bird-like toes; but the two bones of the shank (tibia and fibula) were separate, as in most reptiles. The wings were short and rounded, “but unlike all known birds there were three long, slender fingers on each wing, which was armed with a hooked, sharp-edged claw.” The wing-quills were large and strong. These feet and the claw-armed wings indicate arboreal habits; but great powers of flight are doubtful, mainly because the breast-bone is poorly preserved, so that its adaptation to large flight-muscles cannot be determined. The bird probably took short flights, and scrambled about in tree-tops by aid of its wing-fingers. Its food can only be guessed at. The most remarkable feature of Archæopteryx, however, was its tail, which was as long as its body and head together, and consisted of 23 free bones, as in lizards. Beside it, in the fossil, are many pairs of broad quill-feathers; and it is probable that each caudal bone supported a pair of these, arranged horizontally into a flat series of tail-fathers. What was the covering of the body is not known; but there are indications of feathering on the legs, and around the neck, and it is certain that the body was not coated with scales. Dr. Frederick A. Lucas said of it: “It was, on the whole, much nearer to the birds than to the reptiles. It is clearly a connecting link between the two classes, and yet we are undoubtedly still very far front the original point where the branch was made from the reptilian stem. . . . It must have taken a very long period of time for the development of such distinctly bird-like feet and feathers.”
In point of time the next bird known appears in the rocks of the Upper Cretaceous Age, when the dinosaurs had about disappeared, and the earliest known mammals are faintly discerned as precursors of their class. These are the toothed birds of the subclass Neornithes, in two typical forms named Hesperomis and Ichthyornis, both of which are found fossil in the Cretaceous rocks of western Kansas. The true affinities of both are still in discussion. They were first put together in a "toothed-bird" group called Odontornithes; then rearranged into two groups, on account of a difference in dentition: (1) Neornithes Odontolcæ (Hesperornis, etc.), in which the teeth are set in a continuous groove and (2) Neornithes Odontotormæ (Ichthyornis, etc.) having teeth set in separate sockets. Hesperornis was a flightless, swimming and diving sea-bird, nearly four feet long, with a long neck and strong legs ending in four lobed toes, and set far back at right angles to the spine. It no doubt caught fish as food, and rarely came ashore except to nest, resembling in this respect the habits of a modern penguin. Its structure, however, was very primitive, and its race soon became extinct, without leaving any line of developing descent, although several related forms were contemporary with it.
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 Neornithes had become greatly shortened as compared with Archæopteryx, 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 foreshadow modern types with little or no reference to the preceding “toothed” type. That seems to have come to an end; and there is no evidence of any ancestral connection between them and the modern types, whose ancestors, earlier than the Eocene, remain undiscovered. 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 Diatryma gigantea, of the Wahsatch formations of Utah. — a flightless bird standing 12 feet in height and the largest known fossil bird, hardly exceeded 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 existing 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 succeeding 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 remarkable, perhaps, was Phororhachos, several species of which are known from bones recovered 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.