<P> Although ornithischian (bird - hipped) dinosaurs share the same hip structure as birds, birds actually originated from the saurischian (lizard - hipped) dinosaurs if the dinosaurian origin theory is correct . They thus arrived at their hip structure condition independently . In fact, a bird - like hip structure also developed a third time among a peculiar group of theropods, the Therizinosauridae . </P> <P> An alternate theory to the dinosaurian origin of birds, espoused by a few scientists, notably Larry Martin and Alan Feduccia, states that birds (including maniraptoran "dinosaurs") evolved from early archosaurs like Longisquama . This theory is contested by most other paleontologists and experts in feather development and evolution . </P> <P> The basal bird Archaeopteryx, from the Jurassic, is well known as one of the first "missing links" to be found in support of evolution in the late 19th century . Though it is not considered a direct ancestor of modern birds, it gives a fair representation of how flight evolved and how the very first bird might have looked . It may be predated by Protoavis texensis, though the fragmentary nature of this fossil leaves it open to considerable doubt whether this was a bird ancestor . The skeleton of all early bird candidates is basically that of a small theropod dinosaur with long, clawed hands, though the exquisite preservation of the Solnhofen Plattenkalk shows Archaeopteryx was covered in feathers and had wings . While Archaeopteryx and its relatives may not have been very good fliers, they would at least have been competent gliders, setting the stage for the evolution of life on the wing . </P> <P> The evolutionary trend among birds has been the reduction of anatomical elements to save weight . The first element to disappear was the bony tail, being reduced to a pygostyle and the tail function taken over by feathers . Confuciusornis is an example of their trend . While keeping the clawed fingers, perhaps for climbing, it had a pygostyle tail, though longer than in modern birds . A large group of birds, the Enantiornithes, evolved into ecological niches similar to those of modern birds and flourished throughout the Mesozoic . Though their wings resembled those of many modern bird groups, they retained the clawed wings and a snout with teeth rather than a beak in most forms . The loss of a long tail was followed by a rapid evolution of their legs which evolved to become highly versatile and adaptable tools that opened up new ecological niches . </P>

The most ancient of the living birds is