NSF PR 96-75 - November 21, 1996
This material is available primarily for archival purposes. Telephone
numbers or other contact information may be out of date; please see current
contact information at media
contacts.
Impact of Meteorite, Drop in Sea Level Caused Mass
Extinctions 65 Million Years Ago
The scientific dispute over what caused the extinction
of 70 percent of all species worldwide 65 million
years ago is closer to a resolution, with new research
by scientists from UCLA and the University of Washington.
The research was funded by the National Science Foundation
(NSF).
A cover story in the November 22 issue of the journal
Science reports the researchers' evidence
supporting the controversial theory that the extinction
of dinosaurs and many other species was caused by
the impact of a huge meteorite that crashed to Earth
some 65 million years ago.
"The end-Cretaceous extinction is one of the largest
mass extinctions in Earth's history, and its cause
has been among the most contentious, hotly debated
issues in paleontology," says Chris Maples, program
director in NSF's division of earth sciences. "This
work by Marshall and Ward is important because it
may result in more agreement among proponents of different
end-Cretaceous extinction scenarios."
However, the scientists -- paleontologist Charles
Marshall of UCLA and geologist Peter Ward of the University
of Washington in Seattle -- also present evidence
that other factors, including a drop in sea level
prior to the assault by the massive asteroid or comet,
also may have caused some of the extinctions at the
end of what is known as the Cretaceous period. Marshall
and Ward present evidence demonstrating that a combination
of factors caused the mass extinctions.
Marshall applied statistical analysis to a well-preserved
and well-documented fossil record which Ward has collected
-- some 40 species of sea creatures from the Cretaceous
period that include clams and squid-like creatures
with shells called ammonites.
"For the first time, we can estimate the relative
importance of all the major factors that led to the
extinction of the ammonites," says Marshall. The impact
of the asteroid or comet accounts for 50 to 75 percent
of the ammonite extinctions, which is a much higher
figure than some scientists had expected, but lower
than others anticipated, he says. The substantial
drop in sea level, which had peaked at least 10,000
years before the comet struck, accounts for between
zero and 25 percent of the extinctions. In addition,
about 25 percent of the extinctions were due to factors
that would have occurred regardless of the meteorite
or the change in sea level -- what scientists call
"background extinction."
"Some scientists thought the extinctions were due
solely to the impact of the asteroid or comet, others
thought they were due to the sea level change, and
still others thought that background extinction accounted
for most of the extinctions; we're seeing evidence
that all of them were factors," says Marshall.
Marshall and Ward found evidence for a decline in
the abundance of species, and perhaps the extinction
of more species than would have occurred normally,
during the time sea level dropped substantially. In
addition, the scientists found that there was a mass
extinction of species most likely caused by the impact
of the asteroid or comet.
|