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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.