The Cretaceous / Tertiary (K / T) Boundary (65 MYBP) is a period of massive extinction of large terrestrial fauna. The horseshoe shape of the Chicxulub Crater in Yucatan, Mexico, which dates to this time, indicates a glancing impact from the southeast. Ejecta and the explosive force of the impact may have killed most terrestrial life in North America almost instantaneously. Subsequent environmental changes, including prolonged twilight and sharp temperature drops lasting many years (the "nuclear winter" scenario), may have had world-wide impact. Alternative theories suggest that the Asteroid Holocaust was only one of a series of events leading to gradual extinctions over a period of millions of years.
Text material © 2005 by Steven M. Carr