Geoscience Reference
In-Depth Information
percent at the K-T. About 67 percent of reptile and amphibian fami-
lies disappeared, opening the way for the rise of the dinosaurs; 33 per-
cent of all insects, which usually survive almost anything, disap-
peared. Prior to the extinction, most marine organisms made their liv-
ing anchored to the seafloor; those that came after tended to crawl on
or to float above the bottom. This led Richard Monastersky of
Science
News
to joke that we owe our modern seafood menu of "lobster
bisque, fried calamari, seared tuna, and even sea urchin sushi"
5
to the
evolutionary path laid open for the ancestors of these delectable crea-
tures (well, most of them anyway) by the Permian-Triassic mass
extinction.
If high iridium levels, shocked quartz, or spherules were to be
found in Permian-Triassic boundary layers, the case for impact
would be greatly strengthened. So far, each has been reported, but in
no instance have the reports been confirmed to the satisfaction of
even the pro-impactors. Iridium at the Permian-Triassic boundary,
for example, appears to be a factor of 10 lower than at the K-T.
At the 1996 meeting of the Geological Society of America, Greg
Retallack showed photomicrographs of quartz from the Permian-
Triassic in Antarctica that he claimed exhibited planar deformation
features.
6
Specialists such as Glenn Izett and Bruce Bohor, however,
were unconvinced.
7
Finally, there is no good candidate crater, as
Araguinha (see Table 4) appears to be too small.
Douglas Erwin of the National Museum of Natural History of
the Smithsonian, in his definitive account of this boundary,
The
Great Paleozoic Crisis,
8
lists theories that have been proposed to ex-
plain the Permian-Triassic mass extinction. His list is not as long as
Jepsen's (page ix), but numbers 14 and includes such familiar K-T
suspects as global cooling and flood basalt eruptions. The latter idea
received a recent boost in a paper by Paul Renne of the Berkeley
Geochronology Center and his colleagues,
9
who used the argon-
argon method to date volcanic rocks of Permian-Triassic age from
southern China at 250.0 ± 0.2 million years, exactly the same age
his group obtained for the Siberian basalts. They propose that vol-
canic sulfur emitted during the Siberian eruptions caused a strong
pulse of acid rain, as Retallack has argued happened at the K-T.
The acid rain, together with increased concentrations of various vol-
canogenic poisons, caused the great Permian-Triassic mass extinc-
tion. But some geochronologists doubt that Renne's age results are
quite as precise as he claims. As we saw in the case of the Deccan
intertrappeans, the exact timing of events, though crucial to the
argument, is exceedingly hard to pin down. Erwin has written that
