Although many lines of evidence indicate that birds evolved from ground-dwelling theropod dinosaurs, some scientists remain unconvinced. They argue that theropods appeared too late to have given rise to birds, noting that Archaeopteryx lithographica – the oldest known bird – appears in the fossil record about 150 million years ago, whereas the fossil remains of various nonavian maniraptor theropods – the closest known relatives of birds – date only to about 115 million years ago. But investigators have now uncovered bones that evidently belong to nonavian maniraptors dating to the time of Archaeopteryx. In any case, failure to find fossils of a predicted kind does not rule out their existence in an undiscovered deposit. Skeptics also argue that the fused clavicles (the “wishbone”) of birds differ from the unfused clavicles of theropods. This objection was reasonable when only early theropod clavicles had been discovered, but fossilized theropod clavicles that look just like the wishbone of Archaeopteryx have now been unearthed. Finally, some scientists argue that the complex lungs of birds could not have evolved from theropod lungs, an assertion that cannot be supported or falsified at the moment, because no fossil lungs are preserved in the paleontological record.
The primary purpose of the passage is to
compare the development of two hypotheses concerning the evolutionary origin of birds
suggest revisions to the standard theory of the evolutionary history of birds
evaluate the usefulness of fossil evidence in determining the evolutionary history of birds
challenge the theory that birds evolved from ground-dwelling theropod dinosaurs
respond to criticisms of the theory that birds evolved from ground-dwelling theropod dinosaurs
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