Encyclopedia of Evolution.pdf - Online Reading Center

an out of oxygen, and the landscape eroded severely. What
could have caused such a severe event? Several causes have
been suggested:
Loss of continental shelf area. One early suggestion,
from the 1950s, was that smaller continents collided
together to form the world supercontinent of Pangaea (see
continental drift). Since many species of organisms
known from the fossil record lived in shallow water, the
collision of several continents into one would have drastically
reduced the amount of coastline environment in which
they lived. A massive continent, furthermore, would have
had more severe terrestrial climates. Oceans ameliorate temperature
extremes, which is why the summers are hotter
and winters are colder in Minneapolis than in Seattle. The
interior of Pangaea, then, would have had extreme climatic
conditions that would have caused at least some extinctions.
Finally, species that had evolved separately might have been
thrown together into a cosmopolitan mix, in which a few
“winners” would outcompete “losers” that had formerly
been dominant in their own separate lands. While it is
beyond dispute that Pangaea formed during Permian times,
many scientists have concluded that the resulting changes in
terrestrial climate and marine habitat would not have been
great enough to cause the Permian extinction. It certainly
would not account for the worldwide loss of oxygen and
massive death of vegetation.
Asteroid impact. Could the extinction have been caused
by an asteroid, as was the case with the Cretaceous Extinction?
Scientists are much less likely to discover evidence
of such an impact in deposits 250 million years old than in
deposits 65 million years old. However unlikely it might seem,
evidence of a large asteroid impact was recently found: the
Bedout Crater off the coast of Australia. The date of this crater
corresponds closely to the time of the Permian extinction.
Volcanic eruptions. A massive set of volcanic eruptions
known as the Siberian Traps occurred at the Permo-Triassic
boundary. (“Traps” comes from the Swedish for staircase or
steps, referring to the successive layers of lava flow.) These
eruptions continued for a few million years. They cover an
area of Siberia larger than the entire European Community.
The gases ejected from these eruptions could have caused
worldwide devastation. Sulfur dioxide (SO 2) reacts with
water to produce sulfuric acid, a component today of acid
rain. Severe acid rain may have killed much of the vegetation
on land and photosynthetic organisms in the oceans, causing
the collapse of food chains in both. Other large volcanic
eruptions during Earth history were not associated with mass
extinctions, but the Siberian Trap eruptions may have had
a worldwide impact because they ejected more sulfur than
other volcanic eruptions. The lava from these eruptions is rich
in sulfur minerals, suggesting that these eruptions would have
ejected even more sulfur dioxide than most volcanic eruptions
observed today. Volcanoes also eject large amounts of carbon
dioxide (CO 2), a greenhouse gas that leads to global warming
(see greenhouse effect). Isotope ratios suggest a nine
degree F (16°C) increase in global temperature. The resulting
death of plants could have caused the worldwide plunge in
oxygen levels.
Permian extinction
Release of methane. Paleontologist Michael Benton
suggests, though without direct evidence, that the moderate
global warming caused by the volcanoes caused yet another
set of catastrophes to occur: gigantic global burps. Deep
underneath continental shelf sediments, especially in polar
regions, there are today (and may have been during the Permian)
very large deposits of methane hydrate, which is an icelike
combination of water and natural gas. Pressure and cold
temperature keep the methane in a solid state, but if the ocean
waters became warm, the methane might evaporate explosively,
bubbling quickly through the ocean and into the atmosphere.
There is evidence that such “burps” have occurred in
the geologically recent past: an eruption 55 million years ago
apparently caused a global warming of about 3–4°F (5–7°C)
over a 10,000 year period. Methane, though short-lived in
the atmosphere, is a strong greenhouse gas. By enhancing
the global warming that was already going on, these methane
eruptions could have started a positive feedback loop in
which more global warming caused the release of even more
methane. Methane reacts with oxygen, and this would have
worsened the already dire problem of anoxia. When methane
reacts with oxygen, it produces carbon dioxide, which is also
a greenhouse gas.
The Permian extinction had some permanent effects on
the history of life. During the Permian, brachiopod shells
were relatively common, and mollusk shells were relatively
rare; while a few million years after the extinction event,
bivalve mollusk shells evolved into an explosive radiation of
new species, while the brachiopods remained, and remain,
rare. This occurred not because, under normal seafloor conditions,
mollusks were superior to brachiopods, but because
mollusks either got lucky, or else most of them were better
adapted to the post-disaster world of the earliest Triassic.
The implications of the Permian extinction for the
future of the Earth are both good and bad. Many biologists
believe that human activity is causing a sixth mass extinction.
Humans are causing species to become extinct at
a rate as great as what occurred in previous mass extinctions
(see biodiversity). The bad news is that continued
destruction of vegetation can, in fact, lead to massive soil
erosion and, if this destruction continues yet further, can
even contribute to a depletion of oxygen from the atmosphere.
At the end of the Permian, fossil evidence of plants
(e.g., pollen) nearly vanished, and so did oxygen. There is
no need to speculate that life on Earth is strongly dependent
on the work of plants: The “experiment” has in fact been
done. The good news is that the Earth can recover from an
almost total extinction event. No matter what humans do to
the Earth, and probably no matter what happens to it from
other causes, except for the final explosion of the sun several
billion years from now, the Earth will recover. But this
good news is also bad. It would take, from the viewpoint
of human history, forever for the Earth to recover. The
human economy is dependent upon the continued stability
and smooth operation of natural systems. For humans, even
a slight change in global temperature would spell agricultural
and economic disaster. Because civilization depends