Page 184 - Introduction to Paleobiology and The Fossil Record

MASS EXTINCTIONS AND BIODIVERSITY LOSS 171

extraterrestrial noble gases trapped in carbon ally not erupted explosively from classic conical
compounds, and the supposed crater has been volcanoes, but emerges more sluggishly from

identifi ed – first in the South Atlantic and, in long fissures in the ground; such fi ssure erup-
2005, off the coast of Australia. These pro- tions are seen today in Iceland. Flood basalts
posals of impact have not gained wide support, typically form many layers, and may build up
mainly because the evidence seems much over thousands of years to considerable thick-
weaker than the evidence for a KT impact (see nesses. Early efforts at dating the Siberian
p. 174). Traps produced a huge array of dates, from
Most attention has focused on the Siberian 280 to160 Ma, with a particular cluster
Traps, some 2 million cubic kilometers of between 260 and 230 Ma. According to these
basalt lava that cover 1.6 million square kilo- ranges, geologists in 1990 could only say that
metres of eastern Russia to a depth of 400– the basalts might be anything from Early
3000 m. It is widely accepted now that these Permian to Late Jurassic in age, but probably

massive eruptions, confined to a time span of spanned the PT boundary. More recent dating,
less than 1 myr in all, were a signifi cant factor using a variety of newer radiometric methods,
in the end-Permian crisis. yielded dates exactly on the boundary, and the
The Siberian Traps are composed of basalt, range from the bottom to the top of the lava
a dark-colored igneous rock. Basalt is gener- pile was about 600,000 years.

Box 7.2 Close-up view of the mass extinction

Paleontologists have studied PT boundary sections in many parts of the world. One of the best
studies so far is by Jin et al. (2000), who looked at the shape of the mass extinction in the Meishan
section in southern China. This section has added importance because it was ratified as the global

stratotype (see p. 33) for the Permo-Triassic boundary in 1995.
Jin et al. (2000) collected thousands of fossils through 90 m of rocks spanning the PT boundary.

They identified 333 species belonging to 14 marine fossil groups – microscopic foraminiferans,
fusulinids, radiolarians, rugose corals, bryozoans, brachiopods, bivalves, cephalopods, gastropods,
trilobites, ostracodes, conodonts, fishes and algae. In all, 161 species became extinct below the

boundary bed (Fig. 7.7a) in the 4 myr before the end of the Permian. Background extinction rates
at most levels amounted to 33% or less. Then, just below the PT boundary, at the contact of beds
24 and 25, most of the remaining species disappeared, a loss of 94% of species at that level. Three

extinction levels were identified, labeled A, B and C on Fig. 7.7a. Jin and colleagues argued that the

six species that apparently died out at level A are probably artificial records, really pertaining to
level B (examples of the Signor–Lipps effect; see p. 166). But level C may be real, and this suggests
that, after the huge catastrophe at level B, some species survived through the 1 myr to level C, but
most disappeared step by step during that interval.
In reconstruction form (Fig. 7.7b, c), the effects of the PT mass extinction are devastating. What
was a rich set of reef ecosystems before the event, with dozens of sessile and mobile bottom-dwellers,

as well as fishes and ammonoids swimming above, became reduced to only two or three species of
paper pectens and the inarticulated brachiopod Lingula (which seems to have survived everything;
see p. 300). The environment had changed too. Sediments show a well-oxygenated seabed before
the event, with masses of coral and shell debris accumulating. After the event, nothing. The sedi-
ments are black mudstones containing few or no fossils or burrows. The black color and associated
pyrite indicate anoxia (see p. 173). This was the death zone.
Read more about the PT mass extinction in Benton (2003) and Erwin (2006). Benton and
Twitchett (2003) is a brief review of current evidence. Web presentations may be read at http://
www.blackwellpublishing.com/paleobiology/.

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