Page 183 - Introduction to Paleobiology and The Fossil Record

170 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
with evidence for major climatic changes.
Tropical-type reefs and their rich faunas lived
around the shores of North America and
other landmasses that then lay around the
equator. Southern continents had, however,
drifted over the south pole, and a vast phase
of glaciation began. The ice spread north in
all directions, cooling the southern oceans,
locking water into the ice and lowering sea
levels globally. Polar faunas moved towards
the tropics, and many warm-water fau-
nas died out as the whole tropical belt
disappeared.
The second of the big fi ve mass extinctions
occurred during the Late Devonian, and this
appears to have been a succession of extinc-
tion pulses lasting from about 380 to 360 Ma.
The abundant free-swimming cephalopods
were decimated, as were the extraordinary
armored fishes of the Devonian. Substantial
losses occurred also among corals, brachio-
pods, crinoids, stromatoporoids, ostracodes
and trilobites. Causes could have been a major
cooling phase associated with anoxia (loss of
oxygen) on the seabed, or massive impacts
of extraterrestrial objects. Perhaps this
rather drawn-out series of extinctions is
not a clearcut mass extinction, but rather a
series of smaller extinction events (Bambach
2006).
The end-Triassic event is the fourth of the
big five mass extinctions. A marine mass
extinction event at, or close to, the Triassic- a model of change where normal feedback
Jurassic boundary, 200 Ma, has long been
recognized by the loss of most ammonoids,
many families of brachiopods, bivalves, gas-
tropods and marine reptiles, as well as by the
final demise of the conodonts (see p. 429).
Impact has been implicated as a possible cause
of the end-Triassic mass extinction, but most
evidence points to anoxia and global warming
following massive fl ood basalt eruptions
located in the middle of the supercontinent
Pangea, just at the site where the North Atlan- virtual annihilation of life, with as few as 4–
tic was beginning to unzip. Perhaps the end-
Triassic event is not a clearcut mass extinction
either (Bambach 2006): it may have consisted
of more than one phase, and it seems to be as
much about lowered origination rates as the
sudden extinction of many major groups.
The third and fifth of the “big fi ve” were
the Permo-Triassic (PT) and Cretaceous-
Tertiary (KT) events, and these will now be
presented in more detail.
The Permo-Triassic event
The end-Permian, or Permo-Triassic, mass
extinction was the most devastating of all
time, and yet it was less well understood than
the smaller KT event until after 2000. This
may seem surprising, but the KT event is more
recent and so the rock records are better and
easier to study. The KT event is also more
newsworthy and immediate because it involved
the dinosaurs and meteorite impacts. In the
1990s, paleontologists and geologists were
unsure whether the PT extinctions lasted for
10 myr or happened overnight, whether the
main killing agents were global warming, sea
level change, volcanic eruption or anoxia. The
end-Permian mass extinction occurred just
below the Permo-Triassic boundary, so is gen-
erally termed the PT event.
Since 1995, there have been many addi-
tions to our understanding. First, the peak of
eruptions by the Siberian Traps was dated at
251 Ma, matching precisely the date of the PT
boundary. Further, extensive study of rock
sections that straddle the PT boundary, and
the discovery of new sections, began to show
a common pattern of environmental changes
through the latest Permian and earliest Trias-
sic. Fourth, studies of stable isotopes (oxygen,
carbon) in those rock sections revealed a
common story of environmental turmoil, and
this all seemed to point in a single direction,
processes could not cope, and the atmos-
phere and oceans went into catastrophic
breakdown.
The scale of the PT event was huge. Global
compilations of data show that more than
50% of families of animals in the sea and on
land went extinct. This was estimated by rar-
efaction (see Box 7.1) to indicate something
from 80% to 96% of species loss. Turning
these figures round, the PT event saw the
20% of species surviving. Close study of many
rock sections that span the PT boundary has
shown the nature of the event at a more local
scale (Box 7.2).
The suddenness and the magnitude of the
mass extinction suggest a dramatic cause,
perhaps impact or volcanism. Evidence for a
meteorite impact at the PT boundary has been
presented by several researchers: there have
been reports of shocked quartz, of supposed