Evolution__3rd_Edition

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The fossil evidence can be tested
Asteroid impacts are one factor,
among many, associated with mass
extinctions
CHAPTER 23 / Extinction and Radiation 653
It can also be difficult to correlate events at different geographic localities, because
absolute dates are often unavailable. The incompleteness of the fossil record also introduces
uncertainty: a species may appear to go extinct suddenly at what is really a gap in
the sedimentary record (look at Figure 23.3a and c for what, controversially, may be
examples). For all these reasons, evidence from the fossil record is controversial when
used to show either sudden or gradual, or synchronous or asynchronous, extinction
patterns.
Despite these problems, the evidence can still be used (Figure 23.3). An increasing
amount of evidence suggests that the mass extinction was sudden and synchronous. Let
us look at one such study, by Ward (1990). He first collected ammonites from around
the time of the Cretaceous–Tertiary boundary at a site in Spain, in 1986; further collections
were made later and a larger study was possible in 1989. If the real extinctions
were synchronous, the 1989 evidence should show more synchronous extinctions than
the 1986 evidence; and vice versa if the real pattern was non-synchronous. The former
was observed (Figure 23.3d). Ward’s result tends to support the idea of an exactly
synchronous extinction at the Cretaceous–Tertiary boundary; but it is not enough to
convince a skeptic. It concerns only one taxon in one region, and the extinctions there
could easily have been synchronous without the same being true of the rest of the
world. Thus the supporters of Alvarez’s theory accept evidence such as Figures 23.3a
and c as showing synchroneity and attribute evidence like Figure 23.3b to the imperfections
of the fossil record; critics argue the other way round.
In summary, there is good evidence for both suddenness and synchroneity of
extinctions at the end of the Cretaceous, and the evidence appears to improve in more
thorough fossil samples. However, the evidence is not complete enough to have persuaded
everyone.
23.3.3 Several factors can contribute to mass extinctions
The Cretaceous–Tertiary mass extinction is only one of several mass extinctions, and
asteroid impacts are only one of several factors hypothesized to cause mass extinctions.
Figure 23.5 summarizes evidence for the main factors that have been hypothesized to
cause mass extinctions. For asteroid impacts, the figure only shows evidence for impact
craters. We see that the Cretaceous–Tertiary mass extinction is the only mass extinction
to be associated with a large impact crater. Major craters exist for the Jurassic that
are not associated with mass extinctions. Asteroid impacts therefore seem to be neither
necessary nor sufficient to explain mass extinctions. Evidence from iridium anomalies
tells the same story. Measurements of iridium have been made for other mass extinctions,
some of which have small increases, but most do not. Small increases, of about
one order of magnitude, may be better explained by terrestrial processes that concentrate
iridium, rather than by an asteroid collision. The iridium spike at Gubbio is much
larger, by three to four orders of magnitude (Figure 23.4). None of the five major mass
extinctions except the Cretaceous–Tertiary one are generally accepted to have been
caused by an asteroid impact.
Figure 23.5 also summarizes evidence for other factors that have been hypothesized
to cause mass extinctions. These factors include: changes in sea level (and climate),