The Origin of Birds
For as long as cohesive evolutionary theories have been in place, the heated debate regarding the origin of birds and their relationship to dinosaurs has raged on. After the 1860s birds have been hypothesized as being closely related to an ample assortment of extant and extinct reptile lineages. These include a diversity of basal archosaurs and archosauromorphs, pterosaurs, crocodylomorphs (including modern crocodylians and their Mesozoic relatives), and various theropod ornithischian dinosaurs. Phylogenetic systematics is the field in which systematists use clues from the rock record to reconstruct evolutionary history and then study the patterns of relationships among organisms. With the use of this technique, systematists have been able to strongly back the view that birds evolved from theropod dinosaurs. However, opposition to this widely accepted notion still continues. The alternative hypotheses are speculative at best because they are not based on repeatable analyses of anatomical or other kinds of data. An abundance of recent evidence, including similarities in egg structures, inferable behaviors, and their integumentary structures have been amassed in support of the maniraptoran dinosaurs’ evolution into birds’ theory.(Chiappe and Dyke 306-312) This new evidence has met predictions devised long ago by supporters of the theropod ancestry of birds. There are striking resemblances between birds and dinosaurs that led scientists to classify birds as theropods.
One of the most important discoveries in this argument was the unearthing of a fossil in the Solnhofen Formation in Germany known as Archaeopteryx lithographica. This beautiful fossil turned out to be an example of a transitional form between the two vertebrate groups, traditional reptiles and birds. The half meter long fossil was chimeric because it had bird feathers co-existing with obviously “reptilian” features, such as a tail and hands with claws. Archaeopteryx is generally accepted as being the oldest known bird, being of about 150 million years of age, and is a very important link between birds and other coelurosaurs that has helped to shed light on the phylogeny of the group. Unlike all living birds, Archaeopteryx had a full set of teeth, a flat sternum, a long, bony tail, gastralia, and three claws on the wing which could have been used to grasp prey or trees. However, its feathers, wings, furcula and reduced fingers are all characteristics of modern birds. With these characteristics scientists have realized that the Archaeopteryx bears a striking resemblance to the clade Maniraptora as well as modern birds, which provides a strong phylogenetic link between the two groups.(Fastovsky and Weishampel 314-315) Dr. J.H. Ostrom’s 1969 description of Deinonychus antirrhopus and its similarities to the Archaeopteryx was a major step in the development of this theory. Ostrom’s work since the 70s has supplied the momentum for a paradigm shift in paleontologists’ visions of the origin of birds and the evolution of flight. Another scientist, Dr. Gauthier’s, cladistic work in the middle of the 1980s provided the best analytical systematic support for the theory that birds are the descendants of dinosaurs. .
One of the reptilian similarities of birds is such that, like all other reptiles, birds have scales. Feathers are produced by tissues similar to those that produce scales and birds also have scales on their feet. Birds lay eggs like other reptiles. The soft anatomy, brain, heart, musculature, and other organs, are all fairly similar except that birds are more derived due to their endothermic metabolism and their ability to fly. There are numerous skeletal resemblances between birds and reptiles that are the basis of the cladistic analyses done by Gauthier and others. The closest relatives of birds are thought to be the coelurosaurian dinosaurs, which are the diverse clade containing all theropod dinosaurs, based on Gauthier’s cladistic...
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