Critics of this theory have suggested that birds have evolved from some other ancestors that had lost the first and fifth digits. Studies of developing limbs of chicken, including a polydactylous variety and mouse have confirmed that the wing digits are actually 1, 2 and 3 and support the hypothesis of theropod descent of birds. (Vargas; Fallon, 87)
Conclusion:
The origin of birds from dinosaurs is a fascinating study. The discovery of the first fossil protobird, the Archaeopteryx started most of the studies on the dinosaurian origins of birds. Most paleontologists now agree that birds have descended from a particular line of dinosaurs, the theropods, more specifically the coelurosaurs who had features that were remarkably similar to birds. The study of the skeletal remains of the Archaeopteryx, Anchiornis, Mononykus, Alvarezsaurus and many other fossils have confirmed the hypothesis that birds have evolved from feathered dinosaurs through small evolutionary steps which included small leaps into the air to catch prey followed by swoops which later evolved into steered swoops, glides and finally into full-fledged flight. This process was helped by numerous small skeletal and physiological adaptations that helped in flight as well as ensured survivality of that primitive dinosaur which later evolved into birds. The origins of flight, however, are still debatable with experts differing over cursorial, arboreal and "pouncing proavis theory." There is also considerable debate over the issue of the origin and purpose of the evolution of feathers since feathers have been found in many non-avian dinosaurs as well. Despite all these debates, one cannot discount the fact that there are many characteristics of more than 120 that are shared by both dinosaurs and birds.
References
Alexander, David E; Vogel, Steven (FRW). Nature's flyers.
JHU Press, 2004.
Davies, Gareth Huw. Evolution....
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