birds

birds Birds, together with bats and pterosaurs, form the three groups of terrestrial vertebrates that developed flight independently. Each of them met the structural requirements for flight in a slightly different way. In the birds the development of feathers for use as a flight membrane is the major unique derived character (autapomorphy).

The most famous bird, and probably the most famous fossil, is Archaeopteryx. This was first discovered in the Late Jurassic Solnhofen Limestone of Bavaria in 1860, and since then a total of only six specimens have been found. Archaeopteryx is generally recognized as being the earliest fossil bird and, as such, it provides a considerable amount of information towards an understanding of the origin of birds and the development of flight. Birds have a number of specializations of the skeleton that are related to flight, particularly enlargement of the sternum (breastbone) for the attachment of flight muscles and the fusion of the clavicles (collar bones) to form a V-shaped furcula (wishbone). Archaeopteryx possesses a furcula, but its sternum is not enlarged, which suggests a limited flight ability. In birds the fingers have become fused and act only as supports for the wing feathers, whereas in Archaeopteryx the fingers are still distinct and bear functional claws. All birds, both fossil and modern, have reduced the tail to a mass of fused vertebrae termed the pygostyle. Archaeopteryx, however, retained a long tail. It also retained socketed teeth and a pelvis that is structured like that of carnivorous dinosaurs. In fact Archaeopteryx has so many characters in common with carnivorous dinosaurs that without its flight feathers (preserved as impressions in the fine-grained Solnhofen Limestone) it would be identified as one. The relationship between birds and dinosaurs was identified more than a century ago by T. H. Huxley, and this view has been strongly substantiated in recent years by cladistic analysis. Such analyses have shown that some features considered to be typically avian, such as the furcula, first appeared in carnivorous dinosaurs. Feathers, although a major character of birds, probably developed initially from epidermal scales as insulation and were only later adapted for flight, a view substantiated by recent Chinese discoveries of dinosaurs with feathers.

There are two main views about the way in which flight developed in birds. The arboreal theory is the traditionally favoured view and holds that birds started by gliding from trees and then progressed to flapping flight. A more recent cursorial theory suggests that flight developed in small terrestrial animals that ran and jumped into the air, gaining flight ability gradually as their wings developed. Although it is certainly easier to gain speed by dropping from a tree than running along the ground, the cursorial theory does fit better with the dinosaurian ancestry of birds. More evidence is needed to provide a resolution of these conflicting theories.

Little is known of Cretaceous birds, although Early Cretaceous birds discovered in Spain, South America, and China and called enantiornithines show a full complement of avian features such as a pygostyle and enlarged sternum. Late Cretaceous birds are represented by open-water forms such as Hesperornis, which was loon-like, and the tern-like Ichthyornis. None of these are members of the modern bird orders, most of which appear in the Eocene or later and led to the numerous and highly adapted organisms that we know today.

David K. Elliott