Discuss how birds evolved from their non-avian ancestors

Speculation about the evolution of birds began in the eighteenth century when de Maillet claimed that they had arisen from flying fish (Stahl, 1974). The earliest known member of Aves is Archaeopteryx, first found in the Late Jurassic rocks of Bavaria, and it is now known that birds are the second most direct (surpassed only by the more obvious reptiles) descendants of the dinosaurs but as yet there has been no universally accepted theory of their exact ancestry. Currently two major hypotheses on the evolution of birds still stand in conflict, with one other slightly more obscure theory still hanging on by a thread.

To start with the least well accepted hypothesis, in 1972 A.D. Walker claimed that there were close relations between extant birds and extinct crocodylomorphs (ie Sphenosuchus) based mainly on his detailed studies of the morphology of the braincase and ear region of both. Other proposed evidence included similarities in features of the tarsus, skull and teeth. Howev...