(Image courtesy of National Geographic)
Few subjects in evolutionary theory have posed such intriguing puzzles for so long as the origin of birds. Evidence of avian beginnings has been elusive in the fossil record because birds' light, hollow bones rapidly decompose. So far, the oldest-known bird fossil is the famous Archaeopteryx lithographica, discovered in 1861 just two years after the publication of Darwin's On the Origin of Species, but Archaeopteryx leaves many questions unanswered.
(Information courtesy of PBS Evolution Library-- http://www.pbs.org/wgbh/evolution/library/03/4/l_034_01.html )