In 1804, as the Lewis and Clark expedition was preparing to journey into the American West, Thomas Jefferson, president at the time, pulled Merriweather Lewis aside to talk about sloths. Giant ground sloths, to be particular. And mastodons too.
You see, Jefferson was an avid naturalist in addition to being a politician. Just a few years earlier in 1797 he had presented a paper to the American Philosophical Society. In it he discussed the qualities and measurements of the bones of a bear-sized sloth that had been recently found in Virginia. Jefferson dearly wanted studies of these animals, if they should still be alive (none could be found in the East), and tasked Lewis with the job.
And perhaps it is understandable that he believed that these animals could still exist in the West. The study of prehistoric creatures and species wide extinctions was still quite new in 1804. Dinosaurs would not enter the public eye for another forty years or so. And who knew what was out beyond the pale?
Of course, Lewis never saw any ground sloths in the West. He had arrived about 10,000 years too late. Even the last populations in the Antilles probably died out around the year 1500. Jefferson had to accept being disappointed. He did get a bit of a perk in 1822 when the French scientist Anselme Demarest named one sloth species after him: Megolonyx jeffersoni. But the megafauna were nowhere to be found and Jefferson had to be content with looking at bones.