event to account for the demise of the dinosaurs."3 Writing in 1982, Archibald and Clemens used different words to make the same point: "At present, the admittedly limited, but growing, store of data indicates that the biotic changes that occurred before, at, and following the Cretaceous-Tertiary transition were cumulative and not the result of a single catastrophic event."4

In October 1982, only two years after the original paper appeared in Science and before most paleontologists had even begun to take it seriously, Luis Alvarez gave a long, detailed, and unusually personal talk at the National Academy of Sciences, the most prestigious invitational scientific society in the world, of which he was a member. -When his remarks appeared in print, they offended paleontologists, geologists, and others who preferred to see a certain level of polite discourse maintained in science. He had begun his talk with a preemptive declaration of victory: "That the asteroid hit, and that the impact triggered the extinction of much of the life in the sea—are no longer debatable points."6

Writing about a field trip to Hell Creek, Montana, source of Tyrannosaurus rex and the bedrock of dinosaur studies, Alvarez noted that "[The husband of one of his co-workers] tripped over a previously undiscovered Triceratops skull on one occasion. So we have not been a group of people each working in his own little compartment, but rather we have all thought deeply about all phases of the subject."7 To suspicious geologists, this casual statement implied that he thought that their field was so easy that complete novices could not only stumble across new discoveries, they could solve persistent problems with thought that, however deep, could not, and need not, have gone on for long.

"A physicist can react instantaneously when you give him some evidence that destroys a theory that he had previously believed in. But that is not true in all branches of science, as I am finding out," Luis claimed.8 Not every physicist reacted as he described, however. Less than a year later, astrophysicist Robert Jastrow, a professor at Dartmouth College, wrote: "So there we are. The asteroid theory was very attractive because it explained so much in a simple way, and many people will regret its passing. However, the evidence against it is very strong."' Thus as early as 1983, two physicists, with the utmost confidence, came to exactly the opposite conclusion about a matter of geology.

Where do physicists gain the self-assurance to make pronouncements in a field in which they have little or no training and experience? Walter Alvarez has observed that science is a hierarchy from the sophisticated and mathematical to the complex and nonmathe-matical.10 The order runs, roughly: mathematics, physics, chemistry, astronomy, geology, paleontology, biology, psychology, and sociology. Fields high in the hierarchy use mathematics to explain the laws that they have derived. Though the discovery of these laws may require great feats of intellect, the laws themselves can often be simply stated (£ = mc'). Fields lower down deal with history, life, the human brain and behavior, which cannot be described mathematically or simply (with a few exceptions, such as plate tectonics). Scientists in these fields must handle (literally) dirty rocks and messy, squishy things like whole organisms. Physicists expect that someday they will be able to roll everything into one grand unified theory; scientists from geology "on down" would scarcely dream of such an aspiration. All this translates into a false sense of superiority on the part of those at or near the top of the hierarchy and gives them, seemingly without a moment of doubt, the nerve to make pronouncements about the fields below them. Ironically, in his article Jastrow acknowledged this hierarchy, but he failed to consider that it might apply not only to Luis Alvarez, but to himself.

Of course, as with other forms of prejudice, such attitudes are wrong, even dangerous. Mathematics is not "better" than psychology; it is merely different. Not every physicist could be a successful biologist. Just as we are finding that different kinds of intelligence exist, so each field probably attracts those most amenable to its special set of problems, techniques, and ways of thinking.

At the time of Luis's talk at the National Academy, plant paleontologist Leo Hickey was arguing that the plant extinction had been moderate and had occurred at a different time than the dinosaur extinction. After describing Hickey as a "very good friend" and a "close personal friend" of Walter Alvarez, Luis said that "Hickey has behaved quite differently with respect to the [Raton basin fern spike] ... he ignored it."11

Alvarez directed most of his disdain at William Clemens, the vertebrate paleontologist and faculty colleague of Walter Alvarez, whom Luis also described in his talk as a friend. Their difference centered on the interval between the K-T boundary at Hell Creek, Montana, and the highest recovered dinosaur bone, which occurred at some distance below the boundary, producing a barren interval that became known as the "ghastly 3-m gap." Luis Alvarez describes in great detail how he used a variety of techniques to convince

Clemens that such a gap was only to be expected when rare creatures had suddenly gone extinct, yet Clemens stubbornly refused to accept the obvious. Luis's description of his attempt to persuade his "friend" goes on for four dense pages; its detail suggests that there must have been something more than friendship and science behind it—Alvarez seems to be trying to show Clemens up not only as wrong but as unreasonable, even unscientific: "I really cannot conceal my amazement that some paleontologists prefer to think that the dinosaurs, which had survived all sorts of severe environmental changes and flourished for 140 million years, would suddenly, and for no specified reason, disappear from the face of the earth ... in a period measured in tens of thousands of years. I think that if I had spent most of my life studying these admirable and hardy creatures, I would have more respect for their tenacity and would argue that they could survive almost any trauma except the worst one that has ever been recorded on earth—the impact of the K-T asteroid."12

Shortly after he gave his talk, the review article by Archibald and Clemens in American Scientist" appeared, just in time for Alvarez to incorporate a critique of it as an afterword to his written remarks. According to Alvarez, after they had ignored the iridium evidence and pooh-poohed impact, Archibald and Clemens offered only two alternatives as the cause of the K-T extinction: supernova explosion and the spillover of Arctic seawater (which would have lowered global temperatures). Alvarez said that he was "quite puzzled to see that in 1982, two knowledgeable paleontologists would show such a lack of appreciation for the scientific method as to offer as their only two alternative theories to that of the asteroid, a couple of outmoded theories. . . . Today, both of them are as dead as the phlogiston theory of chemistry."14 To accuse a scientist not only of being wrong, but of being ignorant of the proper use of the scientific method, is the deadliest of scientific insults—tantamount to saying that the person in question is not truly a scientist. Such a remark would certainly strain any friendship.

Polls do not decide scientific matters, but since our interest is not only in what caused the K-T extinction, but in how scientists reacted to the Alvarez theory, it is useful to consider the results (normalized to equal 100 percent) of a poll taken in the summer of 1984 of over 600 paleontologists, geophysicists, and other geologists from six countries:

• 24 percent agreed that an extraterrestrial impact at the K-T boundary caused the mass extinction.

• 38 percent thought that a K-T impact had occurred but that other factors caused the mass extinction.

• 26 percent thought that no K-T impact had occurred.

• '2 percent believed that there had been no K-T mass extinction. '

As David Raup points out, over 60 percent of the polled scientists believed that an extraterrestrial impact ended the Cretaceous. Not bad considering the nearly 200-year influence of uniformitarianism.'6

When the Society of Vertebrate Paleontologists held its annual meeting in Rapid City, South Dakota, in '985, the controversy was in full bloom. According to reporter Malcolm Browne, writing in the New York Times, the assembled paleontologists claimed that the argument over impact had "so polarized scientific thought that publication of research reports has sometimes been blocked by personal bias." 7 (The discussion in this and the following three paragraphs is taken from Browne's article.) One said that "Scientific careers are at stake."'8 Some linked the cosmic winter that might result from meteorite impact with the nuclear winter that might result from World War III. Those who denied that cosmic winter could have occurred might also deny nuclear winter, thus branding themselves as pro-nuclear militarists.

Here among their own, not far from the best dinosaur collecting fields in the world, the vertebrate paleontologists let loose. The general thrust of the comments, though not the polite tone, was expressed by Robert Sloan of the University of Minnesota: "My own analysis of the fossil record suggests that the Cretaceous extinctions were gradual and that the catastrophic theory is wrong."'9

William Clemens announced that he had discovered dinosaur fossils near Prudhoe Bay in Alaska, which, as it is today, was in the Arctic during the late Cretaceous and therefore subject to long periods of darkness. If dinosaurs could survive six months of Arctic winter and darkness, how could a few months of alleged cosmic winter kill them off? Said Clemens, "But survive they did, as we see in the fossil record."20

The most vicious attack came from Robert Bakker, formerly of the University of Colorado Museum, the originator of the theory that the dinosaurs had been warm-blooded and fast moving: "The arrogance of those people is simply unbelievable," he said of the pro-impactors. "They know next to nothing about how real animals evolve, live, and become extinct. But despite their ignorance, the geochemists feel that all you have to do is crank up some fancy machine [presumably the iridium analyzer] and you've revolutionized science. The real reasons for the dinosaur extinctions have to do with temperature and sea level changes, the spread of diseases by migration and other complex events. But the catastrophe people don't seem to think such things matter. In effect, they're saying this: 'We high-tech people have all the answers, and you paleontologists are just primitive rock hounds.'"21

Luis Alvarez liked a fight and gave as good as he got. In a second article in the New York Times, Browne quoted from Luis Alvarez's just published autobiography: "I don't like to say bad things about paleontologists, but they're really not very good scientists. They're more like stamp collectors."22 (Showing that they did agree on something, Jastrow had a couple of decades earlier compared geology to the collecting of butterflies and beetles.)

Alvarez was echoing the great British nuclear physicist and Nobelist, Ernest Rutherford, who divided science into physics and stamp collecting.23 Rutherford's offensive statement may have stemmed from a burst of professional pride and can be excused as such. Alvarez's remark, on the other hand, like Bakker's, seemed much more personal and demeaning of an entire field of scholarship.

It got worse. Jastrow told Browne, "It is now clear that catastrophe of extraterrestrial origin had no discernible impact on the history of life as measured over a period of millions of years."24 Alvarez retorted: "There isn't any debate. There's not a single member of the National Academy of Sciences who shares Jastrow's point of view. Jastrow, of course, has gotten into the defense of Star Wars, which for me personally indicates he's not a very good scientist. In my opinion, Star Wars doesn't stand a chance."25

Jastrow rejoined by pointing out that Alvarez had flown on the companion plane to the Enola Gay in the raid that destroyed Hiroshima and had been one of only five physicists willing to appear before the Atomic Energy Commission to denounce as a security risk Dr. J. Robert Oppenheimer, who had headed the Manhattan Project, on which he had been Alvarez's superior. In his autobiography, however, Luis Alvarez wrote that he had told the Oppenheimer inquiry panel that he had no doubt of Oppenheimer's loyalty to the country. 2 6,2 7 The Alvarez-Clemens debate continued in Browne's article, with Alvarez saying that "he considers Clemens inept at interpreting sedimentary rock strata and that his criticisms can be dismissed on grounds of general incompetence."28

Tragically, as this debate sank to ever lower depths, Luis Alvarez discovered that he had terminal cancer of the esophagus. He told an interviewer, "I can say these things about some of our opponents because this is my last hurrah, and I have to tell the truth. I don't want to hold these guys up to too much scorn. But they deserve some scorn, because they're publishing scientific nonsense."29

Luis Alvarez died on September 1, 1988, ending one of the most versatile, successful, and combative careers in modern science. At Snowbird II, held just six weeks later, one of the participants proposed two minutes of silence in his honor. Walter rose to say, "My father would have been mortified. He'd much rather have a good fight in his memory."30


If impact did not kill the dinosaurs, what did? Michael Benton, writing nearly three decades after Jepsen, found a total of about 65 seriously proposed ideas.31 (He omits such recent suggestions as AIDS and terminal constipation.) Taking a very coarse cut, Benton's list can be aggregated as follows:

• Medical problems ranging from slipped discs to disease.

• "Evolutionary drift into senescent overspecialization." (Some of us may feel that we too are suffering from this malady—a sort of reptilian chronic-fatigue syndrome.)

• Competition with other animals, especially mammals.

• Floral changes: New plant species were unsuitable for dinosaurs or poisoned them.

• Climate change: too hot, too cold, too wet, too dry.

• Atmospheric change: high levels of oxygen, low levels of carbon dioxide.

• Oceanic and topographic change: Seas retreated (we know that during the late Cretaceous they did); or large volumes of fresh, cold Arctic Ocean water spilled into the Atlantic, lowering temperatures and causing drought.

• Volcanism, whose resulting soot and ash could have had the same lethal effects as predicted for impact.

• Extraterrestrial events such as supernovae explosion and meteorite impact.

Let us put these theories up against the criteria deduced by David Raup from his career-long study of extinction: "For geographically widespread species, extinction is likely only if the killing stress is one so rare as to be beyond the experience of the species, and thus outside the reach of natural selection."32 But how widespread is widespread? Raup answered this question at Snowbird I, concluding that: "Modern biogeography is too robust for mass extinction to result from annihilation of life in a single region. ... A global or near global crisis or environmental deterioration is required."33 Let me rephrase Raup's two prerequisites:

I . For a species that lives over a wide area to be driven to extinction, the cause of death not only has to be powerful, it must also be outside the experience of the species—not of individuals but species. This means that the cause must be so rare as to appear no more often than once every few hundred thousand or few million years.

2. The extinction of over 50 percent of all living species—a mass extinction—requires killing on a global scale; mass death in a region or two will not do the job.

Which of the theories summarized by Benton are rapid enough in their action to be beyond the reach of natural selection (or migration) and are also global in their reach? Medical problems, competition, and floral changes are the stuff of natural selection; they also tend to be regional. Climatic, atmospheric, and oceanic changes are widespread and therefore appealing, but they tend to occur on geologic time scales. Rather than causing mass extinctions, these gradual changes would give organisms opportunity to evolve or to migrate in response.

Changes in sea level are worth a special look, for they are the most-cited cause of dinosaur extinction. Consider the most recent dramatic change in sea level, the rise that occurred when the last glacial ice melted. The earth has a fixed amount of water during any one period of geologic time; the more that is locked up in ice, the less that is available to fill the oceans. Thus as glaciers form, sea level drops; when glaciers melt, sea level rises. During the Pleistocene Ice Ages, which ended (or at least paused) 15,000 years ago, sea level first fell and then, when much of the ice melted, rose by hundreds of meters. Although many large mammals became extinct, no mass extinction resulted. Indeed, sea level has risen and fallen throughout the history of the earth as glaciers have waxed and waned, sea floors have spread, continents have collided, and oceans have opened and closed. The record is shown by the so-called Vail curve of sea level, developed by researchers at Exxon (Figure 22).

Some of the major shifts in sea level occurred near in time to major geologic boundaries and extinctions, but many did not. The much-touted change in sea level during the Cretaceous actually

Jurassic Sea Level
figure 2 2 The rise and fall of sea level over the last 600 million years. Many abrupt changes fail to coincide with major extinctions and geologic boundaries. The levels of the Big Five extinctions are indicated by arrows. [The Vail curve, after Raup.34]

began near the middle of the period. During the long run of the dinosaurs, from Eoraptor in the Triassic 230 million years ago to the last T rex at the end of the Cretaceous, many changes in sea level, some up, some down, many larger and quicker than the K-T change, were all nevertheless survived. In any event, let us remember that the dinosaurs lived on land. A drop in sea level, which some vertebrate paleontologists propose to explain their demise, would by definition open up more land surface on which the dinosaurs could live. The claim that such a drop caused the extinction of creatures that had lived for 160 million years appears to be contrary to logic.

David Fastovsky, co-author of an excellent book, The Evolution and Extinction of the Dinosaurs,35 and Peter Sheehan, a paleontologist with the Milwaukee Public Museum, put it this way at the third Snowbird conference: "It is counterintuitive to posit that an increase in land surface area (as occurs by definition as the result of a drop in sea level) will be accompanied by habitat fragmentation [claimed by

Archibald to be a result of a drop in sea level36]; why should the terrestrial realm be 'fragmented' by increasing the habitable area? An increase in land should provide opportunities and space not previously available to land-dwelling organisms."37 Peter Ward has said, "We just do not know how a regression [drop in sea level] could kill anything."38

Of the theories on Benton's list, only massive volcanic activity and extraterrestrial events meet Raup's criteria of being global, infrequent, and lethal. Dinosaur specialist Dale Russell, reviewing the evidence in 1979, thought volcanism was unlikely to provide the answer because it tends to be gradual and episodic, rather than sudden like the K-T extinction.39 In Chapter 6 we saw how the details of the Deccan eruptions fail to corroborate the volcanic alternative. Now it is time to turn from theory to the dinosaur fossil record.

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