Ammonites

Ammonites were mollusks, like the squids and octopuses, that ranged from about a centimeter to a meter in size. Their intricately coiled and chambered shells provide some of our most beautiful fossils. The ammonites first appeared in the mid-Paleozoic, survived even the deadly end-Permian extinction, when 96 percent of all species died, and expired at the end of the Cretaceous, 330 million years after their arrival. Geologists, including Darwin (whose contributions to that field earn him the title geologist) for over a century had noted that the ammonite extinction marked the K-T boundary. Yet when modern geologists took a closer look, they were not so sure. Peter Ward of the University of Washington, a leading expert on ammonites, tells of being invited to Berkeley in 1981, not long after the Alvarez theory had appeared, to give a talk to assembled Alvarezes and Berkeley paleontologists.4 He told them of a theoretical study he had just completed, which showed that the ammonites should have gone extinct suddenly. This was music to the ears of the Alvarezes, who lost no time in inviting Ward to dinner. According to Ward, William Clemens, a Berkeley paleontologist whose office was down the hall from Walter's and who was to become a bitter opponent of Luis's, left immediately after the talk without comment.5 A year later, after having done extensive ammonite collecting, Ward returned to Berkeley to give another talk, this time announcing that his new findings showed that the ammonites had apparently gone extinct well below the boundary, thus contradicting prediction 1 and undercutting the Alvarez theory. This time Luis Alvarez stayed away, while Clemens invited Ward to dine.

One of the best locations for finding ammonites near the K-T boundary is in the Bay of Biscay off Spain, at its border with France, where the relevant geologic section is well developed and well exposed in huge, wave-cut sea cliffs. Ward's favorite outcrops were near the town of Zumaya, where a strong iridium anomaly was found in the same kind of thin clay layer as at Gubbio. In '983, not long after the appearance of the Alvarez theory, Ward reviewed the life and death of these fascinating creatures in an article in Scientific American." Because the highest (youngest) ammonite he could find occurred some '0 m below the K-T boundary—equivalent to tens of thousands of years before the impact—Ward concluded that the Alvarezes were wrong: "The fossil record suggests . . . that the extinction of the ammonites was a consequence not of this catastrophe but of sweeping changes in the late Cretaceous marine ecosystem . . . studies ... at Zumaya suggest they became extinct long before the proposed impact of the meteoritic body."7 At the end of his article, however, Ward added a crucial caveat: "This evidence is negative and could be overturned by the finding of a single new ammonite specimen."8

Ward's reference to negative evidence was meant to emphasize that the absence of evidence is not evidence of absence. Failure to find a fossil species at a given horizon near its upper limit does not prove that the organism had already gone extinct at that level, as Ward understood—maybe a more diligent search would turn it up. The only way to make progress against the Signor-Lipps effect is to return and search again. Ward returned to Zumaya to do exactly that. "Finally, on a rainy day," he writes, "I found a fragment of an ammonite within inches of the clay layer marking the boundary."9

Thus encouraged (and perhaps influenced by the appearance on the Zumaya beach first of armed Spanish soldiers and later of disgruntled Basques, each asking what he was doing to their rocks), Ward began to enlarge his collecting areas to include other sites along the Bay of Biscay where the K-T boundary is exposed. Without the impetus of the Alvarez theory, Ward would not have gone to this extra trouble. What would have been the incentive to spend more of his life on these same cliff faces? His redoubled effort paid off. Within the first hour of collecting near the French town of Hen-daye, just around the corner from Spain, he found abundant ammonites in the last meter of Cretaceous rock. In papers given at the second Snowbird conference in '988, Ward reported that "collecting... east and west of the Zumaya section ultimately showed that ammonites are relatively common in the last meter of the Cretaceous sediment.He thought that their scarcity at Zumaya had stemmed "from some aspect of ammonite ecology, rather than collection failure or preservation effects."" For now vanished reasons, the ammonites had migrated away from Zumaya, but had not gone far. Only a short distance away, at Hendaye, they showed up in abundance. In 1994, Ward summed up: "My decade-long study ... in Spain and France ultimately showed that the ammonites had remained abundant and diverse right up until the end of the era; the last ammonites . . . were recovered just beneath K-T boundary clay layers."12

Ward's latest approach to the problem of ammonite extinction is to apply statistics. Considering the difficulties inherent in trying to locate the true extinction horizon of a species, using statistics not only makes sense, it is essential. At best, for a given horizon in time, scientists have access to only a tiny fraction of the geologic record. Combine that with the Signor-Lipps effect, and you can see that the chances are vanishingly small that the last surviving individual of any fossil species—say, the last ammonite to have lived and be fossilized—will ever be found. The time of the true, last survivor will always lie above the horizon of the highest specimen recovered. One way around this difficulty is to collect large numbers of samples and then to apply statistical techniques, as Ward and Charles Marshall of UCLA have done for the ammonites at Zumaya.13 This allowed them to define a range over which a given ammonite species most probably became extinct. They found that a few ammonite species disappear all the time in a kind of background extinction, and that, prior to the K-T boundary, a drop in sea level apparently had killed off a few others, though this happened gradually. They concluded that 50 percent of the Zumaya ammonite species had undergone a sudden extinction right at the boundary.

What lessons can we learn from the story of the Biscay ammonites? Most important, and contrary to the initial impression of Ward and others, half the ammonite species lived right up to the K-T boundary, when they suffered an extinction that was both sudden and catastrophic, corroborating prediction 1. Ammonites are not found above the iridium, corroborating prediction 2. Although their numbers waxed and waned throughout the Cretaceous, as they had done for the preceding 330 million years, as nearly as we can tell given the vicissitudes of the fossil record, the final demise of the ammonites is fully consistent with the Alvarez theory.

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