Page 182 - Introduction to Paleobiology and The Fossil Record

MASS EXTINCTIONS AND BIODIVERSITY LOSS 169
Time (Ma)
250 200 150
70
50
26-mya period
30
Percent extinction 20
10
5
2
P Triassic Jurassic
Figure 7.6 Periodic extinctions of marine animal
families over the past 250 myr. The extinction
rate is plotted as percent extinction per million
years. A periodic signal may be detected in a
time series like this either by eye, or preferably
by the use of time series analysis. There are a
variety of mathematical techniques generally
termed spectral analysis for decomposing a time
series into underlying repeated signals. The
techniques are outlined in chapter 7 of Hammer
and Harper (2006), and a practical example that
repeats the classic Raup and Sepkoski (1984)
analysis is given at http://www.
blackwellpublishing.com/paleobiology/. (Based
on the analysis by Raup & Sepkoski 1984.)
The search for a common cause gained cre-
dence with the discovery by Raup and Sep-
koski (1984) of a regular spacing of 26 myr
between extinction peaks through the last
250 myr (Fig. 7.6). They argued that regular
periodicity in mass extinctions implies an
astronomical cause, and three suggestions
were made: (i) the eccentric orbit of a sister
star of the sun, dubbed Nemesis (but not yet
seen); (ii) tilting of the galactic plane; or (iii)
the effects of a mysterious planet X that lies
beyond Pluto on the edges of the solar system.
These hypotheses involve a regularly repeat-
ing cycle that disturbs the Oört comet cloud
and sends showers of comets hurtling through
the solar system every 26 myr.
The debate about periodicity of mass
extinctions raged through the 1980s. Many
geologists and astronomers loved the idea,
and they set about looking for Nemesis or
planet X – but without success. Some impact
enthusiasts found evidence for craters and
impact debris associated with the end-Permian
and end-Triassic mass extinctions, but not for
any of the seven other extinction peaks. And
100 50 0 the evidence for impact is frankly rather weak
except for the KT event.
Most paleontologists rejected the idea
because only three of the 10 supposed mass
extinctions were really mass extinctions (end-
Permian, end-Triassic and KT) – the seven
other high extinction peaks through the Juras-
sic and Cretaceous were explained away as
either too small to signify or as artifi cial (mis-
counting of extinctions, mistiming or a major
Cretaceous Tertiary change of rock facies). Re-study of a revised
dataset by Benton (1995) did not confi rm the
validity of any of the seven queried peaks, and
with only three out of 10 there is no periodic
pattern!
The idea of periodicity of impacts was
reawakened by Rohde and Muller (2005)
who argued for a 62 myr periodicity in mass
extinctions. This cyclicity picks up the end-
Ordovician, late Devonian, end-Permian and
end-Triassic mass extinctions, but it misses
the KT event. It also hints at other intermedi-
ate events in the mid-Carboniferous, mid-
Permian, Late Jurassic, mid-Cretaceous and
Paleogene. Most commentators have been
very unhappy with this study, suggesting it
does not relate closely to the fossil record,
does not replicate the known mass extinc-
tions, and may reflect long-term changes in
sea level. So, the search for periodicity in mass
extinctions and a single astronomical cause
appears to have hit the buffers, but the dis-
covery that perhaps sea level change, or some
other forcing factor might itself be periodic,
is worth further investigation.
THE “BIG FIVE” MASS
EXTINCTION EVENTS
The “big fi ve” or the “big three”?
As noted earlier (see p. 164), there is some
debate about whether there were fi ve or three
mass extinctions in the past 500 myr. We
summarize a few key points about three of the
five events, and then concentrated most atten-
tion on two of the fi ve.
In the end-Ordovician mass extinction,
about 445 Ma, substantial turnovers occurred
among marine faunas. Most reef-building
animals, as well as many families of brachio-
pods, echinoderms, ostracodes and trilobites
died out. These extinctions are associated