Keller is full of shit and has been for many years.
Geologists reject Occam's razor. And I am not kidding.

It should be obvious to anyone that gradual mass extinctions are inherently unlikely. First realize that natural selection can operate far more rapidly than we see in the fossil record. Traits in higher organisms can evolve at several percent per century in nature - which means that that it can easily keep up with what a geologist would consider a 'gradual' change.
Look, a frog can evolve much greater tolerance of acidification in 40 years. Insects evolve faster than that. In order for something to cause a mass extinction, it must build up more rapidly than organisms can adapt to it. The simplest way in which this can happen is if the rise time is short compared to the time required for evolutionary response. This is not true of any change that a geologist woud call gradual, but it sure is true of an asteroid strike.

gcochran-

I agree.

You know what bothers me about debates like this one? The same thing that seems to show up when any scientific dogma is challenged or even minor conflicting evidence is presented. The guys invested in the status quo immediately go nuts and try to destroy the reputation of whoever is presenting the contrary evidence.

Why not simply take the new evidence and apply it to the existing evidence and come up with new theories to explain the whole?

Dinos are an interest of mine, I've read a good bit about the extinction event. Its proven that the impact occurred, that it was a massive global event, and that the global extinction event happened at the same time. But ever since the theory emerged over the last 20 or so years people have been attacking it as "bunk science". As the evidence has mounted they have become more and more shrill (and dumb) in their attacks. "Everything on the planet got the flu or a virus and died." "Wait! Frogs and Turtles (now plankton) lived! This proves the impact didn't cause the extinction." Huh?

Please. In order to say the impact wasn't responsible or didn't affect the event to a large degree you have to be worshipping a dogma. Watching these guys twist themselves into pretzels to deny this early on I was dismayed, now I am simply amused. I don't see how people that do this can call themselves scientists. These guys act more like politicians or religious nuts than scientists. Priests of the Holy Theory if you ask me.

Precisely, Katy. Also note, that the article's author picks the most extreme examples to color the debate.

Wasn't it Kuhn who pointed out that scientists don't actually change theories, just that the scientists with the earlier theory eventually die and the formerly young turk theory becomes the status quo?

It was actually the great physicist Planck who made that quip. That theory has actually been tested against historical records by the philosopher of science Larry Laudan, and found wanting.

That said, scientists naturally can behave like just as complete prats as any other member of the human species- as the story illustrates.

I'm not flying any flag for gradualism (not in this context, anyway), but I don't see any absurdity in the idea of a gradual mass extinction. Suppose a world growing warmer; cold-loving species would be pushed towards the poles or onto higher ground, but as the carrying capacity of their enivironment reduced, species would go extinct one by one (top predators first). It is no answer to say that they could adapt to warmer conditions, because they would be competing with species already much better adapted to those conditions.

Also, no need to be contemptuous about plankton: they are sensitive indicators of oceanic conditions, and the supposed extinction of many plankton species at the K/T boundary has been cited by the asteroid camp as a strong point in their favour. If in fact they survived the asteroid, it is a real point against the asteroid theory.

The kind of scenario you suggest could never produce anything like the K-T extinction. Look, there are plenty of ways in which you can wipe out a fair number of species - competition can do it. Look at what happened to the South Ameican mammalisn fauna after the isthmus of Panama formed; toast.
But we don't use the phase 'mass extinction' casually - the K-T extinction is something dramatically different; not one large land animal survived anywhere in the world. A big fraction of _plant_ species disappeared - that's not easy. The best guess is that only mammals survivng anywhere in the world were burrowing insectivores.
Keller is an expert at misclassifying the nannoplankton when it serves her screwy purpose. There is no question that most of the forams disappeared. I'm not sure what paleontological information one would be expected to get from Chixculub samples anyhow - you want to look at uninterrupted deposits, undisturbed strata, and a 180-kilometer diameter impact crater might not quite fit the bill. Impacts can turn strata upside down as the edges peel back - that has been seen in the Barringer crater in Arizona.
If you turn up the intensity enough, a gradual geological process could cause a mass extinction. But it has to be more intense, the effect has to be far stronger, things must go past where _anything_ can adapt. And it's harder to find a
refuge, since that refuge has to last hundrds of thousands or millions of years. THere could have been short-term refuges that allowed _some_ higher life to survive an asteroid mpact. For example, living underground could shield small mammals during the hours after the impact in which re-entering debris raised the entire surface of the Earth to a few hundred degrees.
I think a forest buried in one of the landslides caused by the asteroid-caused earthquakes, a millionfold stronger than any we have ever seen, could have been a good refuge for insects and burrowing insectivores. Lakes in karst country could have shielded some freshwater fauna from the temperatures and from the acid rain.

gcochran: you make fair points.

Having Googled a bit on Keller, I agree she seems rather fanatical. But I get most of my 'feel' for the mass extinction issues from Anthony Hallam and Steven M. Stanley, who are hardly dyed-in-the-wool gradualists.

I'm not sure how surprising it is that all large land animals (do crocs count?) went extinct. They are inherently vulnerable to ecological changes, due to their relatively small populations and slow breeding rates.