Not enough evidence. It’s one of the most important, and difficult to evaluate, criticisms in science. In his blog “In terra veritas” post 41 Angry Scientists, Bryan (a geologist) takes on the Science article The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary. He has two main complaints: The ‘panel of 41’ that authored the report is presenting essentially rehash of information loosely accepted since 1980, and in any case, there is insufficient evidence to draw such a hard-edged conclusion for any mass extinction event.
The report [SciTechStory: Science panel: Chicxulub did it] attempts to settle the arguments over whether the sudden disappearance of many species (including virtually all dinosaurs) 65 million years ago was caused by the impact of an asteroid at Chicxulub, Mexico, or by something else (primarily volcanoes in India). The consensus was that, given the evidence, the Chicxulub impact was responsible.
Science being skepticism incarnate, there was bound to be dissent. This voice – trying hard to keep the distinction between being denialism (no counter evidence) and skepticism (not good enough evidence) – is a more general dissent than some will be:
…Here is the evidence/advances that I want before I am willing to begin to draw conclusions regarding causal mechanisms in an extinction event:
1) Standardization of the terms “Catastrophic” and “Gradual”. It seems like a little thing, but nobody has adequately defined these terms in this context. For example, ask yourself if “Catastrophic” refers to a rate (if a rate, where is the cutoff? Is this cutoff based on anything or is it arbitrary?) or does it refer to a magnitude (same questions, where is the cutoff and what is the cutoff based on?)? I’ve seen both usages in the literature. This is actually one of the side projects I have been working on. But if someone beats me to it, all the more power to them.
2) We are able to surmount the Signor-Lipps effect. This is the biggest obstacle to overcome in extinction studies. Sadly, I have seen very few studies that turn the Signor-Lipps effect inward to examine itself. Though there are some, and I remember writing about one. They make for good reads too.
3) We are able to distinguish between an extirpation event and an extinction event. Did the taxa really die out where we find the highest in situ fossil? Or did the taxa just shift their geographical extent, and settle down in an environment that isn’t conducive to preservation?
[Source: In Terra Veritas]
The Signor-Lipps effect, as developed in a paper Sampling bias, gradual extinction patterns, and catastrophes in the fossil record] essentially says that unless the disappearance of many species (“taxa”) before the catastrophic event can be explained, then the assumption of mass extinction after the event is questionable. Bryan is also expressing the problem that many species may have survived the catastrophe but moved somewhere else that (later) became unfit and they died out.
Of such criticism is scientific knowledge tested and retested. Of course, sometimes it’s like dog eating its tail. The criticism in this blog doesn’t actually cite what the authors claim is new evidence, no does it deal with reasons the panel thought it might be worthwhile to issue the paper. Somehow I don’t think the ‘gang of 41’ will agree that nothing has been learned about Chicxulub or the Deccan Traps since the 1980 report.