Evolution__3rd_Edition

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Figure 23.2
The observed extinction rate
for marine animals during
the history of life, from the
Cambrian to the present,
expressed as percentages of
genera going extinct per time
unit (based on almost 29,000
genera). Note the general
decline, and the series of
peaks (for mass extinctions).
Terrestrial life shows a similar
pattern, though there is less
evidence. Redrawn, by
permission of the publisher,
from Sepkoski (1996).
Two of the five are particularly
striking ...
. . . and define the three main
phases of geological history
% Extinction
70
60
50
40
30
20
10
0
Late Cambrian
End-Ordovician
Late Devonian
CHAPTER 23 / Extinction and Radiation 649
End-Permian
Late Triassic
Ca O S D C P Tr J K T
500 400 300 200 100 0
Geological time (×10 6 years)
Permian, the late Triassic, and the end of the Cretaceous. These are sometimes called
the “big five.”
Three of the “big five” are open to doubt. The three less certain mass extinctions are
the ones in the late Ordovician, the late Devonian, and the late Triassic. The big five
may therefore be reduced to a big four, big three, or even a big two, with the two most
important mass extinctions being at the end of the Permian and the end of the
Cretaceous. The end-Permian mass extinction is the biggest in history, with 80–96%
(depending how the estimate is made) of species going extinct. In the Cretaceous–
Tertiary mass extinction, at least half, and perhaps 60–75% of species went extinct.
The observations now explained by mass extinctions have been known about for a
long time. The geologists of the nineteenth century who worked out the main eras of
the Earth’s history did so by looking for characteristic fossil faunas that lasted for a
noticeable time (or rather, depth) in the sediments. Different characteristic faunas were
recognized as different time periods. They recognized three large-scale faunal types and
named them the Paleozoic, Mesozoic, and Cenozoic, with shorter term characteristic
faunas within each of the three. Two major faunal transitions divide the three eras: the
Permo-Triassic boundary, between the Paleozoic and Mesozoic, and the Cretaceous–
Tertiary boundary, between the Mesozoic and Cenozoic. These two major faunal transitions
correspond to the two main mass extinctions. Smaller transitions occurred
between the stages within the main eras, and many of these correspond to smaller, but
still elevated, peaks in Sepkoski’s (1996) graph (Figure 23.2).
The extinction rate, in terms of number of species going extinct per million years,
varies continuously through the history of life. Sometimes it is high, other times it is
low, other times it is in between. No evidence exists for a distinct type of event that
causes mass extinctions. The observed mass extinctions are just the times at the
extreme of a continuum of extinction rates (Section 23.4 below). However, paleobiologists
often study mass extinctions separately from the periods between, and for research
K–T