Archaeornithes

(paleontology) A subclass of Upper Jurassic birds comprising the oldest fossil birds.

One of the two subclasses of birds, the ancient birds, containing the single order Archaeopterygiformes, the most primitive taxon of birds. This taxon was established for the seven specimens (one feather impression and six skeletons) of the oldest known fossil bird, Archaeopteryx lithographica from the Late Jurassic limestones of Bavaria. All other fossil and living birds are placed in the second subclass, the Neornithes or modern birds.

Archaeopteryx is an outstanding example of an intermediate form demonstrating the pattern of mosaic evolution. It is pigeon-sized with a long feathered tail, a pair of feathers attached to each tail vertebra, and a fully feathered wing. The feathers are completely modern avian feathers. The skull has reptilian jaws with small sharp teeth; cranial kinesis is very similar to that of modern birds. The brain and eyes are somewhat larger than those features in most reptiles. The forelimb is modified to a wing, but with the three clawed fingers still unfused. The pectoral girdle is weak and reptilian except for the presence of a stout furcula (wishbone). A bony, keeled sternum is lacking, but ventral ribs (gastralia) exist. The body is moderately long and flexible. The sacrum is small, and the bones of the pelvic girdle are not fused strongly together. The pubis is not fully reversed. The bones of the tarsometatatsus are not fused together. The long tail is formed of a number of individual vertebrae.

Archaeopteryx probably was aboreal for part of its life, going into trees for hiding, sleeping, and nesting. It could descend to the ground with a controlled glide and most likely foraged for food on the ground. There is no evidence that Archaeopteryx had the ability of powered flight or that flight in birds evolved to enable the bird to fly up from the ground. All evidence suggests that active, powered flight in birds evolved from the gliding stage and was to permit the animal to reach the ground from an elevated position in trees. Archaeopteryx climbed up tree trunks by using the sharp claws on its hands and feet to grip the tree. It is most likely that Archaeopteryx was an obligatory homoiotherm with feathers covering its body to reduce heat loss.

Archaeopteryx has many similarities with small carnivorous dinosaurs, and many workers have argued that birds evolved from this group of dinosaurs. See also Aves; Dinosaur; Neornithes; Reptilia.