As regards the geological distribution of Birds, there are many reasons why we should be cautious in reasoning upon merely negative evidence, and more than ordinarily careful not to infer the nonexistence of birds during any particular geological epoch, simply because we can find no positive evidence for their presence. As Sir Charles Lyell has well remarked, "the powers of flight possessed by most birds would insure them against perishing by numerous casualties to which quadrupeds are exposed during floods;" and " if they chance to be drowned, or to die when swimming on water, it will scarcely ever happen that they will be submerged so as to become preserved in sedimentary deposits," since, from the lightness of the bones, the carcass would remain long afloat, and would be liable to be devoured by predaceous animals. As, with a few utterly trivial exceptions, all the deposits in which fossils are found have been laid down in water, and more especially as they are for the most part marine, these considerations put forward by Sir Charles Lyell afford obvious ground against the anticipation that the remains of Birds should be either of frequent occurrence or of a perfect character in any of the fossiliferous rocks. In accordance with these considerations, as a matter of fact, most of the known remains of birds are either fragmentary or belong to forms which were organised to live a terrestrial life, and were not adapted for flight.
The earliest remains which have been generally referred to birds are in the form of footprints (fig. 328) impressed upon certain sandstones in the valley of the Connecticut River in the United States. These sandstones are almost certainly Triassic; and if the ornithic character of these footprints be admitted, then Birds date their existence from the commencement of the Mesozoic period, and, for anything we know to the contrary, may have existed during the Palaeozoic epoch. In the fact that these footprints are three-toed, and are certainly the tracks of bipedal animals, we have strong evidence that they were produced by birds. On the other hand, it is certain that some of the Deinosaurian Reptiles of the Triassic period walked on their hind-legs only, and it is highly probable that they were the real authors of the prints in question. If of Connecticut.
Fig. 328. - Footprint supposed to belong to a Bird. Triassic Sandstones truly ornithic, we must admit the existence in the Triassic period of a considerable number of kinds of Birds, some of which must have been of colossal dimensions, but this question does not admit of final settlement at present.
The first unmistakable remains of a bird have been found in the Solenhofen Slates of Bavaria, of the age of the Upper Oolites. A single unique specimen, consisting of bones and feathers, but unfortunately without the skull, is all that has until recently been discovered; and it has been named the Archaeopteryx macrura. The characters of this singular and aberrant bird, which alone constitutes the order Saururae, will be given in treating of the order.
In the Cretaceous rocks, not only do we find the remains of Birds of the type now existing, but we meet with the extraordinary "Toothed Birds" (Odontornithes), which seem not to have survived this period, and which will be spoken of in greater detail later on. Lastly, almost all the existing orders of Birds are represented by the time we reach the middle of the Tertiary period, and the distribution and characters of the more important fossil forms will be treated of in discussing the several orders in question.