Archaeopteryx

The genus Archaeopteryx shows a mosaic of (for birds) primitive, i.e. reptilian characteristics, which were later discarded by modern birds (Neornithes), and derived, i.e. bird-typical characteristics (which, however, according to the present state of knowledge are only partly considered to be solely characteristic of birds).

Primitive features include the presence of teeth and abdominal ribs (gastralia), a long caudal spine, a relatively small number of unfused pelvic vertebrae (sacralia), unfused metacarpal, metatarsal, tarsal, and pelvic bones, the three finger claws, and the absence of a bony sternum.

To the bird-typical characteristics, one can count the modern seeming asymmetrical swing-feathers, furthermore the clavicles merged to a fork-leg and the backward or laterally-backward oriented first toe (Hallux) of the foot (anisodactyler bird-foot).

Archaeopteryx reached about the size of ravens (about 50 cm body length) and a weight of 0.8 to 1 kg.

The avian characteristics of the proto-bird are also attested for some feathered dinosaurs or, as in the case of the reverted first toe of Archaeopteryx, are not unchallenged. Therefore, some paleontologists do not consider the proto-bird to be significantly more bird-like than some theropod dinosaurs that are not classified as birds (e.g. Microraptor).

Within the last 20 years, a large number of fossils of primitive birds and bird-like dinosaurs have been discovered, especially in sedimentary rocks of the Lower Cretaceous of Northeast China (the Jehol Group). Thus, Archaeopteryx does not stand alone as a mosaic form, but can be placed in a (morphological, not temporal) sequence of dinosaurs that successively became more similar to birds.

Proponents of the hypothesis that the flapping flight of birds evolved from gliding down from an elevated point interpret the claws of Archaeopteryx as those of a tree climber gliding down from the branches. During palecological investigations of the find horizons, however, some researchers came to the conclusion that a hot and dry climate must have prevailed at the place of formation of the Solnhofen plate limestones and that trees probably did not occur. In turn, however, they pointed to high cliffs on the coast of the Jurassic Sea as a possible starting point for the first attempts at flight. Burgers and Chiappe showed that Archaeopteryx may also have been able to take off from the ground.

Research examining the eleventh fossil known to date with the best-preserved feathers to date shows that Archaeopteryx was probably feathered over its entire body. Long feathers on the legs were arranged in rows, and by their shape they probably had no function in flying (as is thought in the distantly related Microraptor), but they may have been useful in landing, similar to the feathered trousers of recent birds of prey. The plumage probably served, similar to the related groups, not only for flying but also for thermal insulation, probably it also had functions as a signaling device to conspecifics or for camouflage. A special feature of the flight feathers used for flying is their asymmetrical plume.