Evolution__3rd_Edition

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Position (m)
300
340
330
KT
0 100 200
Ir (ppt)
An iridium anomaly is associated
with the KT boundary
Peak at
3,000 ppt
CHAPTER 23 / Extinction and Radiation 651
Figure 23.4
The iridium (Ir) concentration increases suddenly by 2–3 orders
of magnitude at the Cretaceous–Tertiary (KT) boundary rocks
at Gubbio, Italy. (100 ft ≈ 39.5 m.) Redrawn, by permission of
the publisher, from Alvarez et al. (1990). © 1990 American
Association for the Advancement of Science.
purposes it is convenient to distinguish between “mass extinctions” and “background
extinctions.” Background extinctions are the extinctions that are going on all the time,
even when the extinction rate is not exceptionally high.
23.3.2 The best studied mass extinction occurred at the
Cretaceous–Tertiary boundary
The mass extinction at the end of the Cretaceous has been found in all regions of the
globe, and affected more or less every group of plants and animals (Figure 23.3). The
fossil record of small, abundant, microfossil groups such as Foraminifera provides
the best evidence for the fine-scale pattern of the extinction, but the demise of larger
groups provides its drama. Some groups, such as the dinosaurs and ammonites, were
driven finally extinct; most large groups were drastically reduced in diversity, though
some odd groups, such as crocodiles, were not noticeably affected at all. The obvious
question is: why did it happen?
In 1980, Alvarez et al. published an influential observation. They sampled the rocks
of the Cretaceous–Tertiary boundary from Gubbio in Italy, and found exceptionally
large concentrations of rare earth elements, particularly iridium (Figure 23.4). These
elements also have high concentrations in extraterrestrial objects. Alvarez and his colleagues
explained the biological mass extinction, and the geochemical iridium anomaly,
by the collision of a large asteroid with the Earth. Since then, similar iridium anomalies
have been found in Cretaceous–Tertiary boundary rocks at several other sites. Some
geologists have argued that the iridium anomaly could have had a terrestrial cause, by
volcanic eruptions; but asteroids are the most widely accepted explanation.