Birds (Encyclopedia of Science)
Birds are warm-blooded vertebrate (having a backbone) animals whose bodies are covered with feathers and whose forelimbs are modified into wings. Most can fly. Birds are in the class Aves, which contains over 9,500 species divided among 31 living orders. One order, the Passeriformes or perching birds, accounts for more than one-half of all living species of birds.
Most scientists believe that birds evolved from saurischian dinosaurs about 145 million years ago. The first truly birdlike animal, they point out, was Archaeopteryx lithographica, which lived during the Jurassic period. Fossils from this animal were found in Germany in the nineteenth century. This 3-foot (1-meter) long animal is considered to be an evolutionary link between the birds and the dinosaurs. It had teeth and other dinosaurian characteristics, but it also had a feathered body and could fly.
A fossil discovery by scientists in 2000, however, threw into doubt the theory of birds' evolution. The fossils in question were excavated in 1969 in Kyrgyzstan, a former Soviet republic, but were not correctly identified until some thirty years later. The animal, Longisquama insignis, lived in Central Asia 220 million years ago, not long after the time of the first dinosaurs. From impressions left in stone, it had four legs and what appeared to be feathers on its body. Scientists who analyzed the...
(The entire section is 1126 words.)
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