Extraterrestrial cause for the Cretaceous-Tertiary extinction

Direct physical evidence is presented for an unusual event at exactly the time of extinctions in the planktonic realm. Deep-sea limestones exposed in Italy, Denmark, and New Zealand indicate iridium increases of about 30, 160, and 20 times, respectively, above the background level at precisely the time of the Cretaceous-Tertiary extinctions, 65 million years ago. Reasons are given that this iridium is of extraterrestrial origin, but did not come from a nearby supernova. A hypothesis is set forth which accounts for the extinctions and the iridium observations. One prediction of this hypothesis is verified, that the chemical composition of the boundary clay, which is thought to come from the stratospheric dust, is markedly different from that of clay mixed with the chemically similar Cretaceous and Tertiary limestones.