Although many lines of evidence indicate
that birds evolved from ground-dwelling
theropod dinosaurs, some scientists remain
Lineunconvinced. They argue that theropods
(5)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, where-
as the fossil remains of various nonavian
(10)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
(15)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
(20)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
(25)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
(30)lungs are preserved in the paleontological
record.
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