Based on fossil and biological evidence, most scientists accept that birds are a specialized subgroup of theropod dinosaurs. More specifically, they are members of Maniraptora, a group of theropods which includes dromaeosaurs and oviraptorids, among others. As scientists have discovered more nonavian theropods closely related to birds, the previously clear distinction between nonbirds and birds has become blurred. Recent discoveries in the Liaoning Province of northeast China, which demonstrate many small theropod dinosaurs ...
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Based on fossil and biological evidence, most scientists accept that birds are a specialized subgroup of theropod dinosaurs. More specifically, they are members of Maniraptora, a group of theropods which includes dromaeosaurs and oviraptorids, among others. As scientists have discovered more nonavian theropods closely related to birds, the previously clear distinction between nonbirds and birds has become blurred. Recent discoveries in the Liaoning Province of northeast China, which demonstrate many small theropod dinosaurs had feathers, contribute to this ambiguity. The consensus view in contemporary paleontology is that the birds, or avialans, are the closest relatives of the deinonychosaurs, which include dromaeosaurids and troodontids. Together, these form a group called Paraves. Some basal members of this group, such as Microraptor, have features which may have enabled them to glide or fly. The most basal deinonychosaurs were very small. This evidence raises the possibility that the ancestor of all paravians may have been arboreal, have been able to glide, or both. Unlike Archaeopteryx and the non-avian feathered dinosaurs, who primarily ate meat, recent studies suggest that the first birds were herbivores. The Late Jurassic Archaeopteryx is well known as one of the first transitional fossils to be found, and it provided support for the theory of evolution in the late 19th century. Archaeopteryx was the first fossil to display both clearly traditional reptilian characteristics: teeth, clawed fingers, and a long, lizard-like tail, as well as wings with flight feathers identical to those of modern birds. It is not considered a direct ancestor of modern birds, though it is possibly closely related to the real ancestor. Early disagreements on the origins of birds included whether birds evolved from dinosaurs or more primitive archosaurs. Within the dinosaur camp, there were disagreements as to whether ornithischian or theropod dinosaurs were the more likely ancestors. Although ornithischian (bird-hipped) dinosaurs share the hip structure of modern birds, birds are thought to have originated from the saurischian (lizard-hipped) dinosaurs, and therefore evolved their hip structure independently. In fact, a bird-like hip structure evolved a third time among a peculiar group of theropods known as the Therizinosauridae. A small minority of researchers, oppose the majority view, contending that birds are not dinosaurs, but evolved from early reptiles like Longisquama. The earliest known bird (avialan) fossils currently known hail from the Tiaojishan Formation of China, which has been dated to the late Jurassic period (Oxfordian stage), about 160 million years ago. The avialan species from this time period include Anchiornis huxleyi, Xiaotingia zhengi, and Aurornis xui. The well-known early avialan, Archaeopteryx, dates from slightly later Jurassic rocks (about 155 million years old) from Germany. Many of these early avialans shared unusual anatomical features that may be ancestral to modern birds, but were later lost during bird evolution. These features include enlarged claws on the second toe which may have been held clear of the ground in life, and long feathers or "hind wings" covering the hind limbs and feet, which may have been used in aerial maneuvering. Avialans diversified into a wide variety of forms during the Cretaceous Period. Many groups retained primitive characteristics, such as clawed wings and teeth, though the latter were lost independently in a number of bird groups, including modern birds (Neornithes). While the earliest forms, such as Archaeopteryx and Jeholornis, retained the long bony tails of their ancestors, the tails of more advanced birds were shortened with the advent of the pygostyle bone in the clade Pygostylia. In the late Cretaceous, around 95 million years ago, the ancestor of all modern birds also evolved better olfactory senses.
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