By Will Dunham
 WASHINGTON
(Reuters) - The dinosaurs were wiped out by an asteroid that smacked the Earth
65 million years ago, but they survived another cataclysmic event -- perhaps another
asteroid impact -- that snuffed out 80 percent of all species about 200 million
years ago, scientists said on Thursday.
By studying the fate of a type
of marine plankton, single-celled organisms called Radiolaria, researchers found
that the mass extinction was a sudden event, not the prolonged die-off that experts
previously had thought. The extinction occurred at the boundary between the Triassic
and Jurassic periods during the Mesozoic era.
The event provided the death
knell for most species and helped crown the dinosaurs, which arose earlier in
the Triassic, as the rulers of the Earth, said Peter Ward, a University of Washington
paleontologist who led the study.
Ward said this calamity had tremendous
similarities to two of the other five mass extinctions that have ravaged Earth
over the past 500 million years. Like those, Ward said it appears this mass extinction
was caused by a giant rock from space.
``We know now that asteroid impact
can cause rapid extinction,'' Ward said in an interview. ``It may not be an asteroid.
But if it isn't an asteroid, it acts like an asteroid, put it that way.''
Most
scientists believe an asteroid strike caused the mass extinction at the end of
the Cretaceous period that killed the dinosaurs and ushered in the age of mammals.
In February, scientists presented evidence that an asteroid or comet impact also
caused the even bigger extinction at the boundary between the Permian and Triassic
periods 250 million years ago.
EVIDENCE OFF THE COAST OF BRITISH COLUMBIA
Ward's
team gathered evidence about the extinction 199.6 million years ago at two remote
sites in the Queen Charlotte Islands off Canada's British Columbia coast, examining
fossil samples indicating a collapse of the plankton population.
The researchers
found an abrupt drop in the rate at which inorganic carbon was turned into organic
carbon by life forms through processes such as photosynthesis.
The organic
carbon decline coincided with the disappearance of more than 50 species of radiolarians,
which served as a food source for numerous marine species and whose disappearance
was an indicator of a major biological crisis.
The study was published
in the journal Science.
Ward said the research indicated it took less than
10,000 years for the mass extinction to unfold. It could have taken place even
more quickly -- perhaps in an instant, he added.
``This thing was real
fast,'' Ward said.
At the time, most dinosaurs were relatively small, and
they were locked in a survival-of-the-fittest battle with other well-adapted animals,
including the mammal-like reptiles -- the biggest of which were among the major
herbivores of their day.
``These suckers are huge, they're hulking,'' Ward
said.
But the mammal-like reptiles -- whose earlier forms gave rise to
the first true mammals -- perished in the calamity.
``One of the great
mysteries has been ... why would these creatures, which are seemingly better adapted
for eating a variety of plant sources, die out and the dinosaurs not? And the
answer is: Mass extinction doesn't give a hoot about your adaptations for everyday
life. There's a lottery involved, for whatever reason,'' Ward said.
Also
nearly wiped off the planet were the ammonoids -- marine predators that resembled
a giant squid in coiled cone shell.
DEATH FROM THE SKY?
Ward said
there are ongoing studies to try to confirm an asteroid as the cause. Ward said
he has found evidence of little carbon molecules called buckminsterfullerenes
-- or buckyballs -- that hint at a space rock as the culprit.
He said a
massive crater in Quebec called the Manicouagan structure, which measures 60 miles
wide, could be the impact site. The crater has been dated to 214 million years
ago, but Ward said the date may be too old.
Ward said alternative theories
include an explosion of a nearby star that could have blown off the Earth atmosphere's
ozone layer and sent temperatures soaring, or massive volcanic activity, possibly
related to the breakup of the archaic supercontinent known as Pangea.
Scientists
know very little about the mass extinctions that took place 350 million and 420
million years ago, Ward said. |
