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02:05 PM ET 03/26/97
Asteroid didn't kill dinosaurs, scientists say
LONDON (Reuter) - A team of London scientists Wednesday
challenged the popular theory that an asteroid wiped out the
dinosaurs 65 million years ago.
The mass extinction had already been under way for several
million years when the asteroid smashed into the Earth, giving
what amounted to the ``coup de grace,'' they wrote in the
Journal of the Geological Society.
According to the 22 experts, led by Norman MacLeod of the
Natural History Museum, a period of violent volcanic eruptions
combined with a big drop in global sea level to weaken the
dinosaurs' dominance of the planet.
There is plenty of evidence that an asteroid did smack into
the Earth near what is now southern Mexico, sending out huge
tidal waves and throwing up a cloud of dust that brought 5,000
years of winter.
``The end-Cretaceous impact event may have contributed to
the ...mass extinction in the manner of a coup-de-grace blow to
the Earth's environment,'' they wrote.
``However, the fossil record indicates that most loss in
biotic diversity associated with the...mass extinction had
already taken place by the time the asteroid arrived.''
McLeod's group's theory is not new and has been argued for
some time. But this is an especially large and concerted effort
by scientists not only from the museum, but from University
College London and Birkbeck College.
They noted a drop in sea levels by about 330 feet and a huge
flow of lava onto what is now the Indian subcontinent -- both of
which would have been highly disruptive to climate.
Fossil evidence showed the giant dinosaurs had been thinning
out, they added.
It was the extinction of the dinosaurs that allowed mammals,
and eventually humans, to evolve. Tiny shrewlike mammals
survived the huge changes 65 million years ago, eventually
evolving into the diverse forms that now dominate.