Extinction of the Old, Evolution of the New: What really happened to the dinosaurs?

I refer, of course to the realization that a giant object from outer space can land on the earth with catastrophic consequences, perhaps causing a mass extinction, and that this could for the first time nicely explain a strange pattern in the fossil record which, in turn, had only just then been discovered: that long periods of boring stasis were occasionally (maybe even periodically) punctuated by mass extinction events.

These realizations ... this fundamental, holistic and richly explanatory rewrite of paleontology ... happened more or less spontaneously from about 1970 to 1990. Oh, I should note: In geology, 20 years is less than an instant. (And I'm not talking about the fossil record here, but rather, the often glacial pace at which ideas set in stone are either buried or eroded into dust. Often at no great loess.)

It has been understood for a very long time that the history of life on earth can be divided into great ages during which time one or another fauna and flora dominated. The Devonian was the age of fish. We are at present in the age of mammals. The age of dinosaurs spans the Triassic, Jurassic and the (K)Cretaceous (ignore that "K" for now) which together make up the Mesozoic Era (225 to 65 million years ago). Of course, all these ages together are really the Age of Bacteria. But that is another discussion for another time.

What was not appreciated for all this geologist time (the past couple of centuries) is that very stark boundaries separated many -- maybe all -- of these time periods. The realization that this is so was famously made official by Niles Eldridge and Stephen Gould in a 1972 paper on what they called punctuated equilibira (not equilibrium). To make a long story short, during the 1970s and into the 1980s paleontologists and geologists, as well as evolutionary biologists, made an important transition; At the end of this period of intellectual history, most people accepted as very likely the idea that species evolution is rare and slow and unspectacular for long periods of time, and that these periods of relative stasis (equilibrium) are punctuated by very short periods ... geological instants ... during which there was lots of extinction. The extinction presumably caused open niches, and possibly the causes of extinction caused changes in conditions, so extinctions would be followed by lots and lots of speciation. Because something bad happened, and that bad thing may well have been a large object hitting the earth, nasty volcanism, dramatic sea level change causing the chemistry of the oceans to shift dangerously, or whatever.

In the late 1970s a large impact crater was discovered in the Yucatan. The crater became known as Chicxulub. Later on in time, a team of physicists and geologists, the core members being Luis and Walter Alvarez, made the startling observation that here and there around the earth one can find a layer different from whatever is below it, and different from whatever is above it, and that itself contains an anomalously high concentration of iridium, an element that usually shows up from outer space, and that is otherwise quite rare on earth.

So over several years, the pattern of the fossil record, based on data that had been built up over centuries, was reinterpreted to be based on a framework of sudden, major, occasional events, a giant impact crater that dated to the time of the dinosaur impact was discovered, and convincing geological evidence of a global event marking the Cretaceous-Tertiary transition ... called the KT boundary (K for Cretaceous) was identified.

Theory, data, smoking gun. Case closed.

The exact order of discovery, realization, integration, and synthesis was actually somewhat complex. My personal recolleciton is that many geologists took a long time to get on board with each of these elements. I'm pretty sure that the Alvarez Hypothesis was extant, and being taken pretty seriously, before the argument that the Chicxulub crater in the Yucatan was the impact site for this event fully took hold, but that eventually did happen.

I remember well one moment in the history of this punctuation in thinking among geologists and paleontologists. It was at a meeting of the American Association for the Advancement of Science (probably in New York). It was something that Stephen Jay Gould Said:

"Get that thing closed or tell those people to shut up ... I'm trying to give a lecture here...."

Oh, no wait, that's not it. yes, he did say that, but I was thinking of something different. Let's see (.... accessing memory banks ....) maybe this:

"No, Bernard, I do NOT give you permission to answer that question for me. I said I was not answering it and that's the end of it! It was a stupid question when asked of me and asking if of you does not change that situation one iota!"

No, wait, that wasn't it either. Let me think about this for a secon..... Oh, OK, I got it! He said:

"It is a remarkable observation of the history of science and intellectual discourse generally that what may well be the most important single fact to come to light relevant to the human endeavor, indeed to the survival of humanity itself, was gleaned from the close scrutiny and dedicated study of of the dull gray fossil record by earnest but mostly boring paleontologist working in the dusty attics and store houses of largely neglected natural history museums the world over." ... or words to that effect.

(I must say ... I actually got a headache writing that last paragraph.)

But as you probably know by now, because a lot of people have been talking about it, the case is not as closed as was once thought. After two decades of transformation of geologists and paleontologists the world over, who tend to be very conservative about their ideas (the utterly obvious idea of plate tectonics took about a half a century to fully take hold ... the first use of stable isotopes to document glacial change was in the late 1960s but the method did not become normal and accepted by everyone until the mid 1980s ... and so on), there is now evidence that the Chicxulub impact occurred 300,000 years before the KT boundary indicated by the iridium.

The evidence is actually pretty straight foward. It consists of a layer of sediment that post dates the impact and that seems to have been laid down by normal processes, over a reasonably well estimated interval of 300,000 years. The KT layer sits atop this layer.

Here is a description of this finding by the discoverer of this evidence, Gerta Keller:

The newest research ... uses evidence from Mexico to suggest that the Chicxulub impact predates the K-T boundary by as many as 300,000 years. "From El Penon and other localities in Mexico we know that between 4 and 9 metres of sediments were deposited at about 2-3 centimetres per thousand years after the impact. The mass extinction level can be seen in the sediments above this interval" says Keller.

Advocates of the Chicxulub impact theory suggest that the impact crater and the mass extinction event only appear far apart in the sedimentary record because of earthquake or tsunami disturbance that resulted from the impact of the asteroid.

'The problem with the tsunami interpretation' says Dr Keller, 'is that this sandstone complex was not deposited over hours or days by a tsunami; deposition occurred over a very long time period'.

The study found that the sediments separating the two events were characteristic of normal sedimentation, with burrows formed by creatures colonising the ocean floor, erosion and transportation of sediments, and no evidence of structural disturbance.

As well as this, they found evidence that the Chicxulub impact had nothing like the dramatic impact on species diversity that has been suggested. At one site at El Peon, the researchers found 52 species present in sediments below the impact spherule layer, and counted all 52 still present in layers above the spherules. In contrast, at a site at La Sierrita which records the K-T boundary, 31 out of 44 species disappeared from the fossil record.

"We found that not a single species went extinct as a result of the Chicxulub impact...these are astonishing results that have been confirmed by more studies in Texas" says Keller.

There are two elements in this description: 1) The sepration of the Chicxulub impact and the KT event; and 2) the relative ho-hum nature of the KT boundary itself. A third element not mentioned is that Keller suggests that a major volcanic event in India may have been the actual cause of extinction that we associated with the end of the Cretaceous.

So is this correct? I shall explain.

The separation of the Chicxulub impact and the KT boundary needs to be very firmly established, and maybe it is so established at this point. This would rule out Chicxulub as the cause of the KT event. However, this does not rule out an impact. The iridium is still there.

It might be that the 300,000 year intervening time period is not really there. This hinges mainly on a small number of geological sections that are interpreted using techniqes that are a) well established but b) not necessarily tested under all the necessary circumstances to be truly bullet proof. I myself was involved in an excavation where such evidence was used to argue for a certain sedimentary process, but under careful examination we turned out to be (probably) wrong. Things are not always what they seem in the geological record. (See: The Greg Layer.) Having said all that, I'm currently liking the 300,000 year time interval, but that is subject to revision.

Again, separating a particular crater from the KT boundary does not eliminate the impact idea, it just complicates things (in important ways).

Regarding the lack of extinction across the boundary in some places: That is interesting. This has been noted before in other areas, and in some cases the seeming lack of extinction went way when it was realized that the fossils above the boundary were actually from below the boundary, but were washed into the upper sediment (from raised-up fossil beds). Also, we expect the boundary to be imperfect ... there should be groups of organisms surviving past the boundary then going extinct later (and some, obviously, not going extinct at all). These particular cases, if verified, are not hypothesis killers, but rather, they are just reality complexifiers.

So, I conclude: The next couple of years studying this is going to be interesting. Don't you think?

The research summary quoted above can be found here.

Kim Hannula gives her perspective here.
Ethan Siegal discusses this issue here.
Suvrat Kher provides his analysis here.

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This is an open access paper published a while back that will give you an excellent orientation to this work:

Keller, G. (2004). From The Cover: Chicxulub impact predates the K-T boundary mass extinction Proceedings of the National Academy of Sciences, 101 (11), 3753-3758 DOI: 10.1073/pnas.0400396101