From the given evidence.............i would say something big hit the earth, 66 million years ago..........and to describe how big that rock was.......i will use the same analogy the white man i met at the museum 2 days ago here in dc did..............6 miles up, from us............are commercial airlines.........the big planes..............the 777s, so a rock that, if u rested it on the surface of the earth, would be in the middle of international airspace..............that is a big freaking rock, and if it did travel 40,000 miles an hour..................well, as a cartoon character would say...........it fell down, and went Booooom!!!!!!!!!!!!!!!

In the past 540 million years there have been five major events when over 50% of animal species died. Mass extinctions seem to be a Phanerozoic phenomenon, with extinction rates low before large complex organisms arose.

Estimates of the number of major mass extinctions in the last 540 million years range from as few as five to more than twenty. These differences stem from the threshold chosen for describing an extinction event as "major", and the data chosen to measure past diversity.