Page 191 - Introduction to Paleobiology and The Fossil Record

178 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
EXTINCTION THEN AND NOW
Extinction events
Somewhere between background extinction
and mass extinction have been many times
when rather large numbers of species have
died out, but perhaps only in one part of the
world, or perhaps affecting only one or two
ecological groups. These medium-sized extinc-
tions are often classed together as extinction
events, but clearly each one is different. Many
extinction events have been identifi ed (see Fig.
7.2), and some of the better-known ones are
noted briefl y here.
The fi rst is the Ediacaran event, about
542 Ma, which is ill defined in terms of timing,
but it marks the end of the Ediacaran animals
(see pp. 242–7). Some Ediacaran beasts may
have survived into the Cambrian, but the
majority of those strange quilted jellyfi sh-like,
frond-like and worm-like creatures disap-
peared, and the way was cleared for the dra-
matic radiation of shelly animals at the
beginning of the Cambrian. Because of the
antiquity of this proposed mass extinction, it
is hard to be sure that all species became
extinct at the same time, and some would
argue that this was not a mass extinction at
all. Causes are equally debated, with some
evidence for a nutrient crisis or a major tem- 48) was beginning to break up, with the
perature change. An older putative mass
extinction, at the start of the Ediacaran, some
650 Ma, might have been triggered by global
cooling, the “snowball Earth” model (see p.
112), but this is equally debated.
An extinction at the end of the Early Cam-
brian marked the disappearance of previously
widespread archaeocyathan reefs (see p. 268).
A series of extinction events occurred
during the Late Cambrian, perhaps as many
as five, in the interval from 513 to 488 Ma.
There were major changes in the marine
faunas in North America and other parts of
the world, with repeated extinctions of trilo- Cenomanian-Turonian extinction event some
bites. Following these, animals in the sea
became much more diverse, and groups such
as articulated brachiopods, corals, fi shes, gas- fishes and ichthyosaurs that fed on them, is
tropods and cephalopods diversifi ed dramati-
cally during the great Ordovician radiation
(see p. 253).
There were many further extinction events
or turnover events in the Paleozoic, between
the Late Devonian and PT mass extinctions,
including a substantial extinction phase bet-
ween the Middle and Late Permian, some
10 myr before the PT event. This Middle–Late
Permian extinction, the end-Guadalupian
event, may turn out to be a mass extinction
in its own right. Numerous marine and non-
marine groups were hard-hit at that time, and
it has been hard to identify until recently
because its effects were sometimes confused
with the end-Permian event, because of lack
of clarity about dating.
There were further such events at the end
of the Early Triassic and in the Late Triassic.
The Late Triassic extinction event, more com-
monly called the Carnian-Norian event (after
the stratigraphic stages) occurred some 15–
20 myr before the end-Triassic mass extinc-
tion. The Carnian-Norian event was marked
by turnovers among reef faunas, ammonoids
and echinoderms, but it was particularly
important on land. There were large-scale
changeovers in floras, and many amphibian
and reptile groups disappeared, to be followed
by the dramatic rise of the dinosaurs and
pterosaurs. At this time, many modern groups
arrived on the scene, such as turtles, crocodil-
ians, lizard ancestors and mammals. The
cause of these events may have been climatic
changes associated with continental drift. At
that time, the supercontinent Pangaea (see p.
unzipping of the Central Atlantic between
North America and Africa.
Extinctions during the Jurassic and Creta-
ceous periods were minor. The Early Jurassic
and end-Jurassic events involved losses of
bivalves, gastropods, brachiopods and ammo-
nites as a result of major phases of anoxia.
Free-swimming animals were unaffected, and
the events are undetectable on land – they
may be partly artificial results of incomplete
data recording. Events have been postulated
also in the Mid Jurassic and in the Early Cre-
taceous, but they are hard to determine. The
94 Ma, associated with extinctions of some
planktonic organisms, as well as the bony
probably associated with sea-level change.
Extinctions since the KT event have been
more modest in scope. The Eocene-Oligocene
events 34 Ma were marked by extinctions
among plankton and open-water bony fi shes
in the sea, and by a major turnover among