Plants

The accumulation of knowledge has meant that scientists today must practice in finer and finer subspecializations: One is not a paleontologist, one is an invertebrate paleontologist specializing in ammonites; or one is a palynologist, an expert not just in fossil plants, but in fossil pollen spores. One of the most beneficial by-products of the Alvarez theory is the way in which it has brought together scientists from an unprecedented variety of disciplines. Pollen specialists, for example, have found themselves for the first time in the same room with dinosaur experts, chemists, physicists, and astronomers, all discussing supernovae, precious metals, impact explosions, and mass extinctions. Advances have been made that would have been impossible had only one group been involved. In this sense, few theories in the history of science have been as fertile as the Alvarez theory.

Two vertebrate paleontologists, William Clemens and David Archibald, along with plant specialist Leo Hickey, were among the first to speak up in opposition to the Alvarez theory, though in much less detail than did Officer and Drake. Only months after the original Alvarez paper appeared in Science," Clemens, Archibald, and Hickey published "Out With a Whimper Not a Bang."14 This 1981 paper and its title have become metaphors for the initial reaction of paleontologists to the proposal that meteorite impact caused the K-T mass extinction. They closed with this paraphrase of T. S. Eliot's lines:

This is the way Cretaceous life ended,

Not abruptly but extended.*-5

Although plants have received far less notice than the more fascinatingly popular dinosaurs, paleontologists have known for a long time that many plant species also failed to survive the K-T boundary. The lack of attention is unfortunate, for fossil plants can tell us a great deal about mass extinctions. First, as the base of most food chains, plants determine much of what happens in the entire realm of biology. Second, because they are so different from animals and are sensitive environmental indicators, fossil plants reveal a lot about the nature of extinction events. Third, pollen and leaf fossils can be present in large numbers, reducing sampling errors and allowing the statistical techniques that add confidence to conclusions about extinction rates and timing.16

"On the whole the pattern of change in land plants and the increasingly cooler affinities of the latest Cretaceous to early Pale-ocene [earliest Tertiary] palynofloras [pollens] are compatible with a gradualist scenario of extinction possibly related to climatic cooling," Clemens, Archibald, and Hickey concluded." That same year, Hickey wrote that the evidence from land flora, together with the difference in the time of extinction of plants and dinosaurs, "contradict hypotheses that a catastrophe caused terrestrial extinctions."18 He based this conclusion on some 1,000 leaf fossils that he and his colleagues had studied, which at the time seemed a large sample indeed.

The first piece of evidence to suggest that the conclusion might be wrong, or at least not universally applicable, came in the 1981 paper by Carl Orth and colleagues in which the first iridium spike was reported in nonmarine rocks from the Raton Basin, proving that the iridium had not been concentrated from seawater.19 They also found that right at the level of the K-T boundary and the iridium spike, the pollen of angiosperms—the flowering plants—nearly disappeared, while that of the ferns rose dramatically. This "fern spike" subsequently turned up at several other K-T localities and in various rock types. Botanists know from studies of modern catastrophes—from the eruptions of El Chichon, Krakatoa, and Mount St. Helens, for example—that ferns are opportunistic plants that move in quickly to colonize a devastated area. Flowering plants later replace them, as happened in the early Tertiary. This scenario suggests that for the flowering plants, the Cretaceous ended not with a whimper but with a bang, quite abruptly.

At the Snowbird II conference in 1988, Hickey and Kirk Johnson reported the results of a new study of nearly 25,000 specimens of mainly leaf fossils from more than 200 localities in the Rocky Mountains and the Great Plains.20 The 25-fold increase in the number of specimens collected over the original Hickey study reflects the impact of the Alvarez theory. Hickey and Johnson found that 79 percent of the Cretaceous plants had gone extinct at the K-T boundary, at the same point at which the fossil pollen changes and the iridium spike appear. This new and statistically more sound evidence caused Hickey, like Peter Ward, to change his mind and conclude that "The terrestrial plant record [is] compatible with the hypothesis of a biotic crisis caused by extraterrestrial impact."21

Speaking to Science reporter Richard Kerr in 1991, Hickey was bluntly honest: "I became a believer. This evidence is incontrovertible; there was a catastrophe. I think maybe [the anticatastrophism] mind set persisted a little too long."22 Like most paleontologists, Kirk Johnson was initially "skeptical of this outlandish theory that attributed the demise of our beloved dinosaurs to some science fiction asteroid."23 His own studies of leaf fossils from the dinosaur beds of Montana made a believer out of him. Archibald also appears to have been converted, at least on the plant evidence, writing in his 1996 book, "Of all the data from the terrestrial realm, the record of plants in the Western Interior seems to me to present the strongest case that extinction was rapid, not gradual, for the species so affected."24

As Johnson has continued to sample the fossil plant record in Montana and North Dakota, he has found more and more new plant species in the uppermost Cretaceous, but next to no new Tertiary ones. This means that more Cretaceous species died out than he had measured earlier. Johnson now estimates the percentage extinction at close to 90 percent.25 Thus the fossil plant evidence thoroughly corroborates predictions ' and 2: As many as 90 percent of Cretaceous plant species disappeared suddenly, right at the K-T boundary; none of them are found above the iridium level.

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