BIRDS, Fossil. Birds are rare as fos sils, compared with other vertebrates, and little is latown about their early evolution. Four or five hundred extinct species have been de scribed, as against 12,000 living, and most of them are from very fragmentary remains, found in widely scattered places. The explana tion 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 presented 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 evolu tion are not lcnown.
The oldest fossil bird known is the Archtropteryx, of which three specimens,. one in marvelously complete condition, have been re covered from the upper Jurassic lithographic slates at Solenhofen, Bavana; 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 latown 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 remarlcable feature of Archceopteryx, 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 fos sil, are many pairs of broad quill-feathers: and it is probable that each caudal bone sup ported a pair of these, arranged horizontally into a flat series of tail-feathers. What was the coveting 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. Freder ick 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 ap pears in the rocks of the Upper Cretaceous Age, when the dinosaurs had about disap peared, 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 Hes perornis and Ichthyornis, both of which are found fossil in the Cretaceous rocks of west ern Kansas. The true affinities of both are still in discussion. They were first put to gether in a °toothed-bird° group called Odon tornithes; then rearranged into two groups, on account of a difference in dentition: (1) Neornithes Odontolcce (Hesperornis, etc.), in which the teeth are set in a continuous groove and (2) Neornithes Odontotornue (Ich thyornis, etc.) having teeth set in separate sock ets. 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, re sembling 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 contem porary with it.