Evolution__3rd_Edition

660 PART 5 / Macroevolution
(a)
Extinction rate per lineage (Myr)
0.16
0.12
0.08
0.04
0
Observed
Model
400 300 200 100 0
Geological time (Myr)
Figure B23.3
Changes in the quality of the sedimentary record alone can
account for most of the observed changes in the extinction
rate. The model used the basic idea of Figure B23.2 to predict
the observed extinction rate, from the real changes in the
sedimentary record between successive geological time
intervals. (a) The observed and predicted (“model”)
extinction rates using absolute geological time in millions
of years, and assuming that the extinction rate decreases
asteroid impacts could well be mistaken. The power laws of
Section 23.4 would also need to be reinterpreted. Peters and Foote’s
work does not affect whether or not extinction rates fit a power
law. However, the work suggests that any power law may arise not
because of factors influencing biological extinction, but because
of factors influencing how much sedimentary rock is deposited and
then preserved and brought to the surface in successive geological
time intervals. This radical interpretation returns to the skepticism
of Lyell and Darwin, though with a new and exact model of how
the quality of the fossil record influences observed extinction rates.
Their second interpretation is that some common factor may
cause changes both in the sedimentary record and in the extinction
rate. For instance, changes in the extinction rate may be explained
by changes in the sea level. Extinction rates tend to go up when the
(b)
1.20
Extinction rate per stage
0.80
0.40
0
400 300 200 100 0
Geological time (Myr)
Observed
Model
steadily over time. (b) The observed and predicted
(“model”) extinction rates assuming all geological time
intervals have the same length and that the extinction
rate is constant. The “observed” graph in (a) is much
the same as in Figure 23.2 although this one uses a
slightly updated version of Sepkoski’s dataset. The
observed rates differ in (a) and (b) because of the
different treatment of the geological time stages. From
Peters & Foote (2002).
sea level falls (see Figure 23.5), because (as noted above) the
habitat available for many marine animals is reduced. But when
the sea level goes down, the amount of sedimentary rock will also
go down. Thus, the same factor could cause both an increase in real
biological extinctions, and a change in the quality of the
sedimentary record. Moreover, other extinction factors, such as
climate, plate tectonics, and perhaps even vulcanism and asteroid
impacts, may be associated with changes in sea level. Thus, the
results of Peters and Foote do not rule out a role for the traditional
causes of mass extinctions. However, their research raises the
standard of evidence needed to demonstrate a real increase in
extinction rates. A convincing demonstration of a mass extinction
must take account of artifacts caused by the fluctuating
sedimentary record.
Different mollusks grow up in different ways. In gastropod snails, planktonic
and direct development are two of the main types of development. With planktonic
development, the egg is released into the surface waters of the ocean and develops
into a larval form which disperses among, and feeds on, the microscopic organisms
(called “plankton”) that float near the ocean surface. After a while, the larva settles and
..