Evolution__3rd_Edition

652 PART 5 / Macroevolution
The asteroid impact theory for the
Cretaceous–Tertiary mass
extinctions ...
. . . is supported by four lines of
evidence ...
. . . including synchronous
extinctions
The exact means by which such an impact could have precipitated the mass extinction
has been considered in detail by Alvarez, and other authors. Alvarez et al. originally
suggested that the impact would have thrown up a global dust cloud, which would
have blocked out sunlight for several years until it settled again. When Krakatoa
erupted in 1883, it ejected an estimated 18 km 3 of matter into the atmosphere, and
this took 2.5 years to fall down again. The asteroid that hit the Earth at the end of
the Cretaceous is estimated to have been 7.5–9 miles (12–15 km) in diameter. Such
an asteroid, whose kinetic energy they described as “approximately equivalent to that
of 108 megatons of TNT,” would have produced an explosion about 1,000 times
as large as the eruption of Krakatoa. The loss of sunlight alone would have been
enough to cause the extinctions, but the impact could have had other destructive side
effects too. Global warming, acid rain, extreme vulcanism, and perhaps an associated
global fire, are some of the possibilities. An impact on the scale suggested by
Alvarez et al. would have been capable of causing the mass extinction at the end of
the Cretaceous.
Since Alvarez et al.’s original publication, geologists have found an increasing quantity
of evidence that supports his idea. The evidence is of four main kinds. The geochemical
evidence, of which the iridium anomaly was the first example, has broadened in
space, as similar anomalies have been found in Cretaceous–Tertiary boundary rocks at
other sites, and in kind, as other chemical signatures of an asteroid impact have been
detected. Secondly, we now have evidence of the impact crater itself. A geological structure
(called the Chicxulub crater), buried beneath sediments off the Yucatan coast of
Mexico, was the site of the impact. The structure is large enough, with a diameter of
probably about 112 miles (180 km), and it dates to the Cretaceous–Tertiary boundary.
The third kind of evidence is of physical structures that would have been generated by
the impact. Rocks, such as shocked tektites and quartzes, which are suggestive of a high
velocity collision, have been found from several Cretaceous–Tertiary sites, including
Chicxulub. As the evidence has fallen into place, many geologists have come to accept
that the mass extinction was caused by an asteroid. (But not all geologists, as we shall
see in Section 23.5 later.)
A fourth kind of evidence comes from the pattern of extinctions in the fossil record.
If Alvarez’s theory is correct, the extinctions at the Cretaceous–Tertiary boundary
should have been sudden, concentrated in a short interval of time, and not preceded by
any decline through the Cretaceous; they should be synchronous in different taxa and
geographic localities; and they should coincide with the iridium anomaly. This is a
highly testable and stimulating set of predictions.
There are some problems in the evidence. You might think you could simply
peer into the fossil record and observe whether extinctions were sudden or gradual,
synchronous or spread out in time. In reality it is not so easy. How, for instance, do we
observe the exact time of an extinction? The last appearance of a species in the fossil
record will usually precede its final, true extinction (and a species certainly cannot
appear after its true extinction). The species’ population may decline before it finally
disappears, which would reduce the chance of leaving fossils. Moreover, even if the
population is constant, its chance of fossilization will still be much less than 100%.
Species therefore appear to go extinct in the fossil record before they actually did. This
“push backwards” is greater for forms that are less likely to leave fossils.
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