Roy Plotnick has a great post about of the last day of the Mesozoic Era (or the first day of the Cenozoic Era). It would have been a very bad day, indeed!
On a day some sixty-six million years ago, life was Earth was suddenly and irreversibly altered. An enormous asteroid or comet struck the Earth next to what is now the Yucatan peninsula. After that, life on the Earth would never be the same. Dinosaurs, apart from birds (of course), died out. Even the birds, however, suffered dramatic losses. Extinctions struck plants and insects. Mammals may have lost more than 90% of their species. In the sea, the enormous marine reptiles, as well as the elegant cephalopods know as ammonites, disappeared forever. The tiny floating single celled foraminifera virtually died out. Life survived, but it would never be the same. To a large extent, the diversification of mammals and the eventual spread of humans was contingent (as Steve Gould would say) on that devastating event.
Within the past year, three papers have shown new light on that terrible day and its aftermath. They also demonstrate how modern paleontology and geosciences are multidisciplinary, using cutting edge field and analytical methods, and drawing on the expertise of large numbers of scientists.