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and finally, it gradually approaches zero survivors. All such curves that 1 know of have this genera] shape, and the amount of insult to give a 50% survival is called the LD50, for lethal dose, 50%. Most of the work in radiation biology has been concentrated on the lower doses, and on the small and difficult to measure cancer-producing or fatality rates at such low doses. For the purpose of understanding mass extinctions, it is important to concentrate on the right hand section of the curve, where it approaches zero survivors. If the insult is large enough to predict a survival of 10"6, then if there were 10e individuals in the original population, then that species would almost certainly be wiped out. But another species, with perhaps one hundred million individuals would have a good chance to escape extinction. And of course the details of the curve will vary from species to species, so that feature, plus the numerical size of the population can be the deciding factors in species survival or extinction. {We have all heard that if the operating insult is nuclear radiation, then the earth may be inherited by cockroaches, that have a greater tolerance for radiation than mammals.) To me, the most important thing to remember in this discussion is that we can divide environmental insults of the kind caused by a bolide impact into three classes, small, medium and large. If the insult had been small, no species would have gone extinct, the mammals would still be subordinate to the dinosaurs, and I wouldn't be giving this talk. If it had been large, all life on this planet would have ceased, so again I wouldn't be giving this talk. That tells me that the insult must have been of medium strength, and for the rather uninteresting reasons I've just mentioned, some species survived and others didn't. I think the smallness of the dinosaur population may have been an important contribution to its complete and sudden disappearance. Dale Russell, a Canadian expert on dinosaurs, reports20 that the K-T extinction wiped out virtually all land animal species weighing more than 25 kilograms. Because population magnitudes are inversely related to individual weights, it may be true that the survival of the lighter weight species depends more on their larger population than on their smaller body weights. I doubt that we will ever know the details of why certain species survived, and others went extinct, and it may even be a mistake to try to identify the difference between the two kinds. To me, that could be like trying to find out in what significant way two neighbors in California differ, that explains why one of them won 4 million dollars in the state lottery, while the other got nothing.

I've mentioned dinosaurs a few times in passing, but I'll now address that question directly. You will remember that when Walter first showed me the K-T clay, he mentioned the near coincidence of its deposit with the extinction of the dinosaurs. I will now show that the bolide impact was responsible for the extinction of the dinosaurs. If Walter and I had known then what we know now about these wonderful animals, here is what we would have said, in the form of prediction number 14: "If the boiide impact was responsible for the extinction of the dinosaurs, as it obviously was for the forams, then we should find the iridium-rich clay layer just above the 'highest dinosaur fossil.' We see the forams with nearly constant surface density on our rock faces, up to the iridium layer, where they suddenly disappear. So we will believe the bolide, which we know can cause some extinctions, caused the extinction of the dinosaurs if we find dinosaur fossils with nearly constant surface density up to the 'highest dinosaur,' with none higher." (Dale Russell, has published a graph showing that they behaved that way.)21 We obviously can't expect the iridium layer to be a fraction of a millimeter above the highest dinosaur, as it is above the highest forams. So by what measure do we judge that it is "just above" the highest dinosaur, as our prediction says it should be, and as it is "just above" the highest forams? It is easy to show mathematically, that if the dinosaurs had been flourishing before a sudden extinctions, as they had been flourishing for the previous 140 million years, and if the iridium layer really marks the time at which they underwent a sudden extinction, then the following will be true: the most probable position of the highest dinosaur fossil will be below the Ir layer by a distance exactly equal to the mean vertical spacing of such fossils. The probability that the highest fossil will be found twice as far below the layer is e~2 = 0.14. And we can use that mean spacing as a reasonable mean error on that predicted position.

So with that as a prediction, Wc a.sli our paleontologist colleague, Bill Clemens, to collect rock samples above the highest dinosaur in his research area, in Montana, so we can test them for iridium. He tells us that the highest 17 articulated fossils in his section had an average vertical spacing of almost exactly one meter, and despite what many dinosaur experts have said, there is no indication in his data of a gradual decrease in fossil surface density with height. (Articulated fossils are ones in which two or more bones are in their expected relationship, and are nearly the only ones considered seriously by careful paleontologists; very large single bones are also acceptable. The bone fragments that are frequently found above the iridium layer are of no interest, since they could have been moved by running water, or by animals that lived much later.) On a larger linear scale, we find that the articulated fossils behave just as the forams did, with a sudden cut off at the iridium layer. So we predict that the iridium layer (not yet seen) will be close to 1 ± 1 meter above the highest observed fossil. After Frank and Helen found the iridium layer 2 meters above Bill's highest fossil, we felt that our prediction had been well confirmed. (We have seen that iridium layers are exceedingly rare in the fossil record.) But Bill still faults the impact theory because the iridium layer wasn't at the level of the highest fossil, and continues to talk about the existence of the "2 meter gap" as having disproved the impact theory. We know the most probable gap would be 1 meter, but Bill's desired gap of zero meters, and the observed gap of 2 meters are both consistent with the predicted value of 1 ± 1 meters. So we consider prediction 14 to be satisfied, and we don't doubt that the unique bolide impact (figure 9) was responsible for the unique event that was discovered more than a century ago, by paleontologists—the sudden, and until recently, mysterious extinction of the dinosaurs.

We continue to hear that paleontologists have recently disproved the more than a century-old scenario of the sudden extinction of the dinosaurs on land, in the sea, and in the air (after 140 million years of adaptations to serious temperature and sea level changes in their environment.) The latest such story was published in the New York Times,22 just a month ago, and says that dinosaur fossils were found 500 feet above the K-T horizon, corresponding to a million years into the Tertiary period. But then we are told that the investigator (Robert E. Sloan) "said the identity of most of the post-Cretaceous dinosaurs could not be determined because the bones were too fragmentary". (Emphasis added.) That indicates to me that non-articulated fossils were used to arrive at an invalid conclusion. Walter says he finds it strange to hear that our theory is wrong because paleontologists have shown that the dinosaur extinction did not occur at the time the iridium was deposited—Clemens has shown that the dinosaurs died out before the iridium event, and Sloan has shown that they lived for a million years afterwards! I continue to accept the conventional paleontological wisdom that the dinosaurs finally did meet an environmental crisis that they couldn't tolerate, and I've given my reasons for believing that that crisis was initiated by the "expected" ten kilometer diameter bolide.

Occam's Razor tells us that when we find two extraordinary events that can be tied together, as cause and effect, by a "reasonable" theory, we shouldn't take seriously other theories, particularly when there is no valid evidence to support such theories.

At the present time, those who don't accept the impact theory of extinctions appear to have only two suggested alternatives. One is a volcanic eruption, which I've mentioned several times as being in strong disagreement with three observations—the worldwide distribution of sanidine spherules, the greatly enhanced iridium concentration in the K-T boundary layer, and the widely observed presence of shocked quartz in the layer. So I see no way in which the volcanic origin of the K-T layer and the extinctions can be taken seriously. Before a theory can begin to make predictions, it must first agree with all the known observations, and volcanic theories are in serious discord with such known observations. But we can't dismiss the idea that volcanic eruptions were triggered by the bolide impact, so the K-T extinction could easily have associated volcanic eruptions.

An alternative theory that appeals to many paleontologists is that the dinosaurs went extinct as the result of wide excursions of temperature, or sudden changes in sea level. We can rule out sea level changes as being of any consequence, by noting that in the 140 million years during which the dinosaurs existed, the sea level had dropped suddenly by about 150 meters on two occasions,33 the same amount it dropped suddenly at the K-T horizon, and the dinosaurs didn't react adversely to those earlier changes. But one should expect sudden drops in sea level to follow bolide impacts, as Don Morris and Rich Muller have shown,24 in their theory to explain the magnetic reversals that frequently coincide with extinctions. The sudden drop in temperature can deposit ice on the continents, thereby lowering the sea level.

Returning to the dinosaurs, we see no evidence that the many sharp drops in sea level were threats to the existence of the land-based dinosaurs—and even if they were, why would they cause the nearly (or exactly) simultaneous extinctions of marine dinosaurs or of the flying pterosaurs? (One can even ask the question, "How could a marine reptile tell that the sea level had gone down by one hundred meters, and if it could notice the change, why would that change affect its ability to live?") We have a good geological record of ocean temperatures, from the measured oxygen isotope ratios, and again the dinosaurs had weathered many such sudden changes in temperature,25 with no apparent trouble. We have seen that a sudden temperature change should be expected as a result of a bolide impact, and Ken Hsu et al.25 have observed it at the K-T horizon. Toward the end of their paper, they say, "Our data suggest to us that the catastrophic environmental changes at the beginning of the Tertiary were the consequences of mass mortality in the oceans, not the causes." And as a final argument against sea level changes, and temperature excursions as the cause of the extinction of the dinosaurs, there is no way to associate the iridium layer (which must be explained) with such environmental changes, except through a bolide impact, which would certainly cause such changes. (And we should remember that there is good evidence that the earth lias been hit by 10 kilometer diameter bolides, many times during its 4.5 billion year history.)

I think it is now time for those who felt comfortable in ignoring the impact theory, seven years ago, to recognize that it is the only existing theory that agrees with all the known observations. So I feel the shoe is now on the other foot, and those who are pushing for the acceptance of non-bolide theories of the K-T extinction should now tell how they can overcome the many objections I've outlined in this talk, to keep their theories alive. If they can't do that, and I don't see much chance that they can, they have only two choices: they can either accept some form of bolide impact theory, or come up with a new, non-bolide theory that agrees with all the known observations. I think the chances of finding such a theory are rather remote, but I can't rule them out. But before any scientists embark on such a search, they should remember that bolide impacts in the 10 km diameter class are part of uniformitarianism, in the same sense that magnetic reversals are, (infrequent, but certainly not unexpected) so if the K-T extinction was not caused by a bolide impact, where is the evidence for such an expected impact, with its now well understood signature?

Almost all of the genera that have ever existed on the earth have undergone extinction. So the five major extinctions shown in figure 6 stand out above a huge background of ordinary extinctions. Until recently, it was thought that mass extinctions and background extinctions differed only in their "intensity," as measured, for example, by the number of families lost per million years. I believe our work led people to reexamine that postulate, and it has now been found, by Dave Jablonski, to be incorrect.26 He points out that genera can protect themselves from ordinary extinctions, in two ways (l) by diversifying, meaning evolving larger numbers of different species—they all have to be killed before the genus can be said to be extinct—and (2) by spreading themselves over wide geographical areas. But Jablonski finds that those protective measures don't help a genus escape a mass extinction. I think it is correct to say that a background extinction takes place when a genus loses the Darwinian battle for the survival of the fittest—one species against another. But in the K-T mass extinction which Jablonski studied, there was no way that a genus could have evolved to protect itself from an occurrence that happens suddenly once every one hundred million years or so. So the fittest still do survive, but Darwin's slow-acting natural selection can have played no part in making the survivors more fit. And I think we can add that as a postdiction of our theory, to bring the total up to 15 separate tests, all passed with high marks.

Some of you may have read the talk I gave in Washington several years ago, in which I said that all but one of our predictions had been fulfilled and fortunately that partial failure had no negative effect on our theory that the K-T extinction was caused by a bolide impact. We couldn't help noticing that there were five major extinctions in the last 500 million years (Figure 5), and that a bolide of the kind that caused the K-T extinction should hit the earth on the average of once every 100 million years (Figure 4). We wouldn't have been responsible scientists if we hadn't suggested that all of the five "majors" could have had similar origins, in which case, we should find iridium layers associated with all five. So we and other groups have looked for iridium enhancements at the four other major extinctions, with not as much success as we had hoped, but we certainly didn't "strike out." The most severe mass extinction was about 250 million years ago, at the Permian-Triassic boundary. We analyzed clay from a Chinese P-T layer, and showed that its iridium content went sharply downward—one of the signatures of a volcanic deposit. A Chinese group reported a large iridium enhancement in that same layer, in the same quarry. It has been our experience that when two groups analyze the same sample, the one that finds the smaller amount of iridium is usually right; it is easy to contaminate a sample, but quite impossible to accidentally "decontaminate" one. Of course, we hoped that the Chinese measurements were correct, but they were later revised downward, to be in substantial agreement with ours. There are reasons to believe that both of our groups looked below the real P-T boundary, so we plan to look higher up. We will join forces with our Chinese friends next year, and will collect P-T samples over a much wider span of time.

Carl Orth and his group27 have found an iridium enhancement near the Late Devonian mass extinction, another "major", but he cites evidence to show that it came from the ocean, through a biological concentration process. So at the moment, the K-T extinction appears to be the only one among the majors that was certainly caused by the impact of an iridium-ricli bolide.

For mass extinctions of the second rank, there are at least two that have associated iridium enhancements. We and Ganapathy independently found the one at the Eocene-Oligocene boundary, about 38 million years ago. We have since shown that it consists of two or perhaps three closely spaced iridium "spikes." And very recently, using our new detector, we have found a sharp iridium spike (Figure 15), laid down eleven million years ago. This figure shows a set of points with larger error bars, where the detector was used in the "search mode." Then, after the 150 parts per trillion spike showed up, we remeasured ail the neighboring points for longer times, and thereby reduced their statistical errors. We plan to make a continuous search of the last 250 million years of the geological record in this way, over the next few years.

Three years ago, Dave Raup and Jack Sepkoski28 showed evidence that mass extinctions occurred periodically, with a time interval of 26 million years. (The three known Ir enhancements are all consistent with the Raup and Sepkoski timetable—11, 38 and 65 million years ago.) Figure 16 is a replot of their data, by my colleague, Richard Muller. The arrows are spaced 26 million years apart. You may have heard that Rich and his colleagues have suggested that the regularity shown in figure 16 derives from a postulated solar companion star, called Nemesis, with a period of 26 million years.29 According to their theory, it perturbs the Oort Cloud near perihelion, and sends a shower of a billion comets into the inner Solar System, where several hit the earth by chance, in a time interval of a few million years. (The observed "fine structure" of the E-O iridium enhancement is best explained by a comet shower, but that shower could have come from a single "passing star," with no requirement for periodicity.) I am sorry that I don't have time to tell you more about this interesting and still developing story.

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