The neocatastrophists seem to be getting overly smug with their iridium-rich deposits at the Cretaceous-Tertiary boundary. Is there really incontrovertible evidence that a large asteroid or comet hit the earth at this point in history, causing widespread biological extinctions? To add some perspective, Leigh M. Van Valen has tossed 15 arguments against impact extinctions on the scales. Ten of these are reproduced below, as taken from Nature. More de-tails may be found in Paleobiology, 10: 121, 1984.
Freshwater life was unaffected;
In Montana and its vicinity, the last occurrence of dinosaurs was detectably below the crucial boundary;
Transitional floras also exist below the boundary;
Apparently extraterrestrial material exists below the boundary;
The expected effects of eliminating atmospheric ozone are missing;
The Cretaceous-Tertiary boundary is coincident with a very large marine regression, suggesting a nonextraterrestrial cause;
Marsupials but not placentals were nearly eliminated, while most aboreal multituberculates (a type of vertebrate) and birds survived;
The predicted cooling effects on the earth are absent;
The predicted effects of acid rain cannot be found; and
Assuming a marine impact, no turbidites can be found; assuming a land impact, no large terrestrial crater has been discovered.
(Van Valen, Leigh M.; "The Case against Impact Extinctions," Nature, 311:17, 1984.)