I think the problem is that you are not taking into account the
incredible diversity of animals, both living and in the fossil record. For example, by "birds" I assume you mean animals classified as the class called
Aves, and by "reptiles" I assume you mean animals in the class
Sauropsida.
Let's get one thing straight: you already agree that all birds could share a common ancestor, right? Well, that in itself is some pretty impressive evolution right there, if by birds you include emus, penguins, pelicans, owls, flamingos, parrots, chickens, and so on; you've got flying, flightless, running, and swimming "birds", and lots of combinations of those (e.g. penguins are great swimmers, but ducks can both swim and fly). Even among the flyers, you've just an incredible array of adaptations. Same thing with reptiles: you've got a stunning amount of diversity there as well, with all manner of crocodiles, snakes, lizards, turtles, and tortoises.
Consider a few examples from
feathered dinosaurs: was Microraptor a "bird"?
http://en.wikipedia.org/wiki/Microraptor
Quote:
Like its close relative
Cryptovolans (possibly a junior synonym of
Microraptor),
Microraptor had two sets of wings, on both its fore- and hind legs (close studies of the Berlin specimen of the primitive bird
Archaeopteryx show that it too, had flight feathers on its hind legs, albeit shortened). The long feathers on the legs of
Microraptor were true flight feathers as seen in modern
birds, with asymetrical vanes on the arm, leg, and tail feathers. As in bird wings,
Microraptor had both primary (anchored to the hand) and secondary (anchored to the arm) flight feathers. This standard wing pattern was mirrored on the hind legs, with flight feathers anchored to the upper foot bones as well as the upper and lower leg. It had been proposed by Chinese scientists that the animal glided, and probably lived in trees, pointing to the fact that wings anchored to the feet of
Microraptor would have hindered their ability to run on the ground, and suggest that all primitive dromaeosaurids may have been arboreal.
[3]
Sankar Chatterjee determined in 2005 that, in order for the creature to glide or fly, the wings must have been split-level (like a
biplane) and not overlayed (like a
dragonfly), and that the latter posture would have been anatomically impossible. Using this biplane model, Chatterjee was able to calculate possible methods of gliding, and determined that
Microraptor most likely employed a
phugoid style of gliding--launching itself from a perch, the animal would have swooped downward in a deep 'U' shaped curve and then lifted again to land on another tree. The feathers not directly employed in the biplane wing structure, like those on the tibia and the tail, could have been used to control drag and alter the flight path, trajectory, etc. The orientation of the hind wings would also have helped the animal control its gliding flight. Chatterjee also used computer algorithms that test animal flight capacity to test whether or not
Microraptor was capable of true, powered flight, in addition to passive gliding. The resulting data showed that
Microraptor did have the requirements to sustain level powered flight, so it is theoretically possible that the animal flew on occasion in addition to gliding.
[
What about
Beipiaosaurus ?
I seriously encourage you and anyone else to follow these links, look at some photos and artists' rendering, and read what the experts have found upon close, analytical comparison of modern birds and these many other species. Then judge for yourself. But just realize that words like "bird" are really referring to a list of traits....the fact is that there are all manner of combinations of animals, past and present, that have some of those traits but not others, means that there really is no fundamental barrier between a "bird" or, say, a "reptile".
There are many more similarities between birds and these bird-like things that just happen to appear in the fossil record shortly before mordern birds ....
... but you'll have to
read and judge for yourself the similarities in skeleton, lungs, heart, sleeping posture, brooding, gizzards.... there's also molecular evidence, for example
Quote:
It has been found that modern-day birds are closely related to older dinosaurs at the molecular level. Scientists have analyzed protein from a 68 million-year-old
Tyrannosaurus rex bone (a
femur). The seven
collagen types obtained from the bone fragments, compared to collagen data from living birds (specifically, a
chicken), reveal that older theropods and birds are closely related.
Here's a cool drawing of the Archeopteryx find:
http://www.daily-tangents.com/Aves/Archaeop/#berlin77
A Talk Origins article that summarizes tons of evidence for "macro" evolution, including the bird-reptile connection:
http://www.talkorigins.org/faqs/comd...ermediates_ex1
You be the judge!