There's a fun story lurking in the vaults at the Smithsonian magazine, titled The Ten Most Disturbing Scientific Discoveries. Couple of interesting entries in those ten.
Sample quote:
"The concept of extinction took a while to sink in. Thomas Jefferson saw mastodon bones from Kentucky, for example, and concluded that the giant animals must still be living somewhere in the interior of the continent. He asked Lewis and Clark to keep an eye out for them."