Page 177 - Introduction to Paleobiology and The Fossil Record

164 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
died out during normal times than during the
more spectacular mass extinctions.
Nonetheless, mass extinctions fascinate
paleontologists and the public because these
were times of concentrated misery, and repre-
sent perhaps unusually intense environmental
catastrophes. But how is a mass extinction to
be defined? All mass extinctions share certain
features in common, but differ in others. The
common features are:
1 Many species became extinct, perhaps
more than 30% of plants and animals of
the time.
2 The extinct organisms spanned a broad
range of ecologies, and typically include
marine and non-marine forms, plants and
animals, microscopic and large forms.
3 The extinctions were worldwide, covering
most continents and ocean basins.
4 The extinctions all happened within a
relatively short time, and hence relate to
a single cause, or cluster of interlinked
causes.
5 The level of extinction stands out as con-
siderably higher than the background
extinction level.
It is hard to define these terms more precisely,
first because each mass extinction seems to
have been unique, and second because it is
sometimes hard to pin down exactly the
timing and scale of events.
Paleontologists commonly talk about the
“big fi ve” mass extinctions of the last 540 myr,
the Phanerozoic, and the current extinction
crisis is sometimes called the “sixth extinc-
tion”. The five mass extinctions (Fig. 7.2) are
the end-Ordovician, Late Devonian, end-
Permian, end-Triassic, and Cretaceous-
Tertiary (KT) events. Study of the Neoproterozoic
reveals a further one or two possible mass
extinctions, before and after the Ediacaran
(see p. 242) so perhaps we should refer to the
“big six” or the “big seven” such events.
The notion of five somewhat similar mass
extinctions throughout the Phanerozoic has
been questioned, however. In a careful statisti- loss of 80–96% of species. The assumption
cal survey, Bambach (2006) has shown that
there were perhaps only three real mass extinc- lies are wiped out is based on the observation
tions, the end-Ordovician, the end-Permian
and the KT events. The Late Devonian and
end-Triassic events do not stand out so clearly
above background extinction rates at those
MASS EXTINCTIONS
Minor
0 Major Intermediate extinctions
Cenozoic Tertiary
100 Cretaceous
Mesozoic
Jurassic
200
Triassic
Permian
Time (Ma) 300
Carbon-
iferous
Devonian
400
Paleozoic Silurian
Ordovician
500
Cambrian
600 Prec. Vendian
Figure 7.2 Mass extinctions through the past
600 myr include the enormous end-Permian
event 251 Ma, which killed two or three times
as many families, genera and species (50% of
families and up to 96% of species) as the
“intermediate” events. These were global in
extent, and involved losses of 20% of families
and 75–85% of species. Some of the minor mass
extinctions were perhaps global in extent,
causing losses of 10% of families and up to 50%
of species, but many may have been regional in
extent, or limited taxonomically or ecologically.
times; each lasted perhaps over 5 myr, and
each was caused as much by depressed origi-
nation rates as by elevated extinction rates.
In trying to defi ne and scale mass extinc-
tions, the end-Permian event is in a class of
its own, because 50% of families disappeared
at that time, and this scales to an estimated
that a higher proportion of species than fami-
that families contain many species, all of
which must die for the family to be deemed
extinct. Hence, the loss of a family implies
the loss of all constituent species, but many