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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.
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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 dis-
appeared, while that of the ferns rose dramatically. This "fern spike"
subsequently turned up at several other K-T localities and in vari-
ous rock types. Botanists know from studies of modern catastro-
phes—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 sug-
gests 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 John-
son 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.
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The 25-fold increase in the num-
ber 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 evi-
dence caused Hickey, like Peter Ward, to change his mind and con-
clude that "The terrestrial plant record [is] compatible with the
hypothesis of a biotic crisis caused by extraterrestrial impact."
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Speaking to
Science
reporter Richard Kerr in 1991, Hickey was
bluntly honest: "I became a believer. This evidence is incontrovert-
ible; there was a catastrophe. I think maybe [the anticatastrophism]
mind set persisted a little too long."
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Like most paleontologists,
Kirk Johnson was initially "skeptical of this outlandish theory that
attributed the demise of our beloved dinosaurs to some science fic-
tion asteroid."
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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 topic, "Of all the data from the terrestrial realm, the record
of plants in the Western Interior seems to me to present the strong-
est case that extinction was rapid, not gradual, for the species so
affected."
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