Alvarez, L.W., et al., Extraterrestrial Cause for the Cretaceous-Tertiary Extinction, Science, Volume 208, Number 4448, Pages 1095-1108, Jun 1980
Siegel, E., What Wiped Out the Dinosaurs?, Apr 2009
This thin layer of ash -- the boundary between light and dark in the above picture -- is the split between when dinosaurs are found and when they aren't. What's amazing about this is that this tiny, thin film of a layer contains something in great abundance that the Earth doesn't have much of at all: the element iridium. Naturally, the Earth has so little iridium that it's inconceivable that this iridium could have come from Earth. Where could it have come from?