Dismissed hypotheses suggested to explain it include the existence of a twin star to our sun and a tenth planet.  
Dismissed hypotheses suggested to explain it include the existence of a twin star to our sun and a tenth planet.  The Signor-Lipps effect may explain an apparent contradiction of fossil evidence to this theory, though the Chicxulub crater on the Yucatan Peninsula supports it.  First proposed based on the discovery of high levels of iridium near Gubbio, Italy, by Walter and Luis Alvarez, FTP, identify this hypothesized mass extinction named for the two geologic periods between which it occurred.
Dismissed hypotheses suggested to explain it include the existence of a twin star to our sun and a tenth planet.  
KT event or Cretaceous-Tertiary mass extinction event