Evolution__3rd_Edition

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Box 23.2
Changes in Extinction Rates and Changes in
the Sedimentary Record
Mass extinctions do not correspond to prolonged hiatuses in
the fossil record a absolute dating of the rocks either side of the
extinction events rule that possibility out. However, the quality
of the sedimentary record could influence the observed extinction
rates in other ways. The amount of sedimentary rock per geological
time interval changes through time, due to changes in the amount
of sedimentary rock originally laid down and the amount of it
preserved up to now. Figure B23.2 illustrates how these changes
(a) Real pattern
Species 1
Species 2
Species 3
(b) Quality of sedimentary record improves over time
Quality of
sedimentary
record
Time
interval 1
Time
Time
interval 2
Species 1
Species 2
Species 3
Low extinction
rate observed
in time interval 1
(c) Quality of sedimentary record deteriorates over time
Quality of
sedimentary
record
Time
Time
interval 1
Time
Time
interval 2
Species 1
Species 2
Species 3
High extinction
rate observed
in time interval 1
Figure B23.2
Changes over time in the quality of the sedimentary record
can influence the observed extinction rate. (a) Assume, for
simplicity, that some species were continuously present
through two successive geological time intervals. (b) If the
first stage had a poor sedimentary record and the second
stage a good sedimentary record, few species will have their
last recorded representation in stage 1 and stage 1 will have
an artifactually low extinction rate. (c) If the first stage has
a good sedimentary record and the second stage a poor
sedimentary record, many species will have their last
representation in stage 1, and stage 1 will have an
artifactually high extinction rate.
CHAPTER 23 / Extinction and Radiation 659
can influence the observed extinction rate. When a geological
time interval with a good sedimentary record follows an interval
with a poor sedimentary record, few species are likely to have their
last representation in the earlier interval because many fossils are
preserved in the later interval. The earlier interval then has an
artifactually low observed extinction rate. The opposite result is
observed when an interval with a poor record follows an interval
with a good record. In this case, the earlier interval has an
artifactually high observed extinction rate.
Peters & Foote (2002) used published data on the amount of
exposed marine sedimentary rocks in the USA for 77 conventional
time units from the Cambrian to the present (thus each unit
averaged about 7 million years a the Cambrian began about
540 million years ago, Figure 18.1, p. 526). They also used
Sepkoski’s database for the time distributions of marine fossil
genera. They constructed a model of the effect illustrated in
Figure B23.2, and used it to predict the observed extinction rates
given changes through time in the amount of sedimentary rock.
The model had two versions, one in which the underlying real
extinction rate was constant and another in which the real
extinction rate steadily decreased from the Cambrian to
the present.
Figure B23.3 illustrates the results for the two models. We can
notice two things. One is the outstandingly good fit between the
model and observations a outstandingly good, given the noise in
the data from taxonomic and other sources of error (for instance,
the sedimentary rock data are for the USA, but Sepkoski’s database
is global). Much of the variation in extinction rates is accounted for
by variation in the amount of sedimentary rock. Secondly, some
peaks in the observed extinction rate are explained by one version
of the model but not the other. For instance, the Permian, Triassic,
and Cretaceous extinctions at 240, 200, and 65 million years ago,
respectively, are not explained by the model with decreasing
extinction rates (Figure B23.3a) but are explained, or are
better explained, by the model with constant extinction rates
(Figure B23.3b). The reason is uncertain and requires further
research. Meanwhile, we can fence-sittingly conclude that some
mass extinctions may stand out from the extinction rates that would
be predicted from the amounts of rock alone a or they may not.
Peters and Foote suggest two interpretations of their findings.
One is more radical. Almost all changes in extinction rates, including
the classic mass extinctions, may be artifactual a reflecting changes
in the sedimentary record, not changes in the biological extinction
rate. The search for causes of mass extinctions in such factors as