Dinosaurs and the origin of birds

We have the considerable evidence that birds are theropod’s dinosaurs, because they are the members of Maniraptora, a group of theropods which includes dromaeosaurs and oviraptorids, among others. As scientists discover more non avian theropods that are closely related to birds, the previously clear distinction between non-birds and birds has become blurred. The oldest known bird, the Late Jurassic Archaeopteryx, is well-known as one of the first transitional fossils to be found in support of evolution in the late 19th century, though it is not considered a direct ancestor of modern birds. Protoavis texensis may be even older, although the fossil’s fragmentation leaves considerable doubt regarding whether it was a bird ancestor. The dromaeosaurids Cryptovolans and Micro raptor may have been capable of powered flight to a similar or greater extent than that of Archaeopteryx. Cryptovolans had a sternal keel, and ribs with uncinate processes, and in fact makes a better "bird" than Archaeopteryx, which lacks some of these modern bird features. Because of this, some paleontologists have suggested that dromaeosaurs are actually basal birds, and that the larger members of the family are secondarily flightless. 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. An alternate theory to the dinosaurian origin of birds, proposed by a few scientists, states that birds evolved from early archosaurs like Longisquama; this theory is contested by most paleontologists.