What triggers mass extinctions?
(Top Posts - Science - 040509)

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Mystery surrounds the 'reset buttons'
that clear the way for new species

Mass extinctions may follow one-two punch

by Charles Q. Choi
updated 9:12 p.m. CT, Wed., Aug. 8, 2007
  http://www.msnbc.msn.com/id/20180820/
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Complete article:

They are known ominously as the Big Five - the five
greatest mass extinctions over the past 500 million years,
each of which is thought to have annihilated anywhere
from 50 to 95 percent of all species on the planet.

Many unsolved mysteries remain regarding these disas-
ters, perhaps the greatest of which is what caused each
of them.

But research is uncovering how these extinction events
dictated the fate of life on this planet - for instance,
determining which animals first crawled onto land and
which ruled the oceans.

The main suspects behind these catastrophes seem to
come either from above, in the form of deadly asteroids
or comets, or from below, in the form of extraordinarily
massive volcanism. Occasionally, however, unexpected
culprits arise - for instance, otherwise innocuous forests.

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The K-T extinction

The most recent of the Big Five is the most familiar one
 - the cataclysm that ended the Age of Dinosaurs. The
end-Cretaceous or Cretaceous-Tertiary extinction event,
otherwise known as K-T, killed off all dinosaurs save
birds roughly 65 million years ago, as well as roughly
half of all species on the planet, including pterosaurs.

Not only did mammals sweep across the planet after K-T,
but sharks expanded across the seas, explained American
Museum of Natural History vertebrate paleontologist Jack
Conrad.

"Throughout the Age of Dinosaurs, you always had these
large reptile carnivores dominating the water, such as ich-
thyosaurs, mosasaurs and plesiosaurs," Conrad explained.
"Only after they die do you see big sharks becoming really
prevalent. You probably wouldn't have seen orcas or blue
whales either had reptile dominance of the seas not gone
by the wayside."

Although research suggests the planet was on the verge
of environmental upheaval before the K-T extinction
event, the straw that broke the dinosaur's back is widely
thought to have been an impact with an asteroid or comet.

Still, a number of researchers contend evidence commonly
linked with such an impact, such as the metal iridium, which
is rare on the Earth's crust, could also be caused by the mas-
sive volcanic eruptions at the Deccan Flats in India, another
popular contender for the dinosaur-killing catastrophe.

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The Triassic-Jurassic extinction

The end-Triassic, or Triassic-Jurassic extinction event about
200 million years ago is thought by many to possibly have
set dinosaurs on the path to their 135-million-year domination
of much of life on Earth. It also ended life for roughly half of
all species.

Until this disaster, mammal-like creatures known as therapsids
were actually more numerous than the ancestors of the dino-
saurs, known as archosaurs.

"The dinosaurs definitely survived better than the early proto-
mammals did, and the extinction event might have entirely
tipped it in their favor," said Rutgers University paleobiol-
ogist George McGhee.

Of the Big Five, the Triassic-Jurassic extinction has the few-
est number of scientists currently researching it, "although
that's changing right now," said Columbia University paleo-
ecologist Paul Olsen. Its cause remains under great debate,
with the best contender so far being the massive volcanic
eruptions at the "Central Atlantic magmatic province," a
region that encompassed a staggering 4.2 million square
miles, an area larger than Canada.

Another main possibility could be an astronomical impact,
Olsen said, although as with the K-T event, the evidence
for both types of catastrophe can get maddeningly blurry.

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The Permian-Triassic extinction

The largest of the Big Five was the end-Permian or Perm-
ian-Triassic extinction event roughly 250 million years
ago, which eliminated as much as 95 percent of the plan-
et's species.

Before this extinction, marine animals were mostly filter
feeders stuck in place on the seafloor, such as crinoids
or "sea lilies." Afterward, the seas became far more
complex with mobile creatures such as snails, urchins
and crabs.

The most likely final trigger for the end-Permian was again
massive volcanism, this time at the Siberian Traps, which
spewed as much as 2.7 million square miles of lava out,
an area nearly as large as Australia.

Recent evidence suggests, however, that the end-Permian
may have been long in the making.

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The late Devonian extinctions

The late Devonian extinction events were actually two
sharp pulses of death about 360 million years ago, each
just 100,000 to 300,000 years apart.

Each pulse was accompanied by a massive drop in tem-
perature, with the steaming seas of the Devonian - sur-
face temperatures of which were about 93 degrees F -
dropping to about 78 degrees F, "and marine organisms
would not have liked that at all," McGhee said.

As to what caused these cold snaps, the ever-popular
suspects are ash and dust kicked up by either astro-
nomical impacts or massive volcanism.

At that time, plants had made it onto land, as had spiders,
scorpions and similar creatures. Right before the extinc-
tion events, the first proto-amphibians made it onto shore.

However, the invasion of the so-called elpistostegalians
 - distant relatives of the coelacanth - "got wiped out
by these extinction events," McGhee explained.

"It wasn't until at least another 10 million years later that
we got footprints from vertebrates on land again, this time
from the ichthyostegalians, the proto-amphibians we're all
descended from. Who knows how the world might have
been different."

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The Ordovician-Silurian extinctions

The earliest of the Big Five, the end-Ordovician or Ordovi-
cian-Silurian extinction events some 444 million years ago,
are reckoned by many to be the second largest.

These also consisted of a pair of die-offs, apparently involv-
ing massive glaciation and a resulting fall in sea levels. The
cause of this glaciation remains a mystery, but one idea was
that land plants actually caused it, pulling so much carbon
dioxide out of the atmosphere that global cooling resulted,
McGhee explained.

Curiously, even though the end-Ordovician led to a huge
loss of life, in a way it actually had very little impact on the
persistence of lineages. Although the four other Big Five
extinction events led to huge changes in which animals
rose to prominence, the same animals that dominated be-
fore the end-Ordovician dominated afterward.

Otherwise, "one neat thing about mass extinction events
is that they're often reset buttons, where you change what
dominates the globe," Conrad said. "You open the door
to things like us to live."

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Further references:

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Mass Extinction
  http://en.wikipedia.org/wiki/Mass_extinction
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Major Extinctions
  http://prohuman.net/science/major_extinctions.htm
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