NSF PR 95-77 - October 31, 1995
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Antarctic Fish Fossils Fuel Asteroid Impact Debate
A bed of fossilized fish bones recently discovered
in Antarctica may be the first remains of direct victims
from the catastrophic event 65 million years ago that
wiped out the dinosaurs and 70 percent of the world's
species.
Purdue University paleontologist William Zinsmeister
discovered the fossil "horizon of death" dating from
the boundary of the Cretaceous/Tertiary (K/T) geologic
periods 65 million years ago, while on a National
Science Foundation (NSF)funded expedition early this
year to Seymour Island near the Antarctic Peninsula.
The bone bed, covering more than 12 square kilometers
of the island, rests directly above a layer of iridium,
an element that is rare on Earth but is a common signature
of meteorite impact. Zinsmeister is presenting his
results November 9 at the Geological Society of America
meeting in New Orleans.
A widely believed theory holds that a giant asteroid
hit the Earth and set off the mass extinctions at
the K/T boundary. The impact site most favored by
scientists is a large crater in Mexico's Yucatan.
"The fish bones are an exciting discovery that should
help us better understand environmental changes at
a crucial time in Earth history -- the end of the
Cretaceous," commented Scott Borg, director of NSF's
Antarctic geology and geophysics program. "Seymour
Island is an important site for understanding what
happened on a global scale to the environment at that
time. When the sediments containing the fish bones
were deposited, the island was located in the far
southern latitudes - just like today -- and very far
from the probable impact site in the Yucatan. The
polar region has a very different atmospheric circulation
than the tropics or temperate regions. So, we can
compare the excellent exposures of Seymour Island's
K/T fossils to sites of the same age at other latitudes
to construct a fuller picture."
In the past, Zinsmeister had argued -- based on 20
years of research on late Cretaceous marine fossils
in Antarctica -- that the southernmost continent's
fossil record did not support the asteroid-extinction
theory. Even after finding the bed of fish bones,
he thinks the picture may be more complicated, and
that change was more prolonged than sudden.
"The fossil record in Antarctica suggests that the
final extinction event wasn't immediate, but rather
occurred over a period of time up to 500,000 years,"
he said. "We actually see a decrease in the global
diversity of life starting between 8 and 10 million
years before the impact."
He points out that two important marine animals --
the ammonites, spiralled molluscs related to the modern
nautilus, and the inoceramid bivalves, a clam relative
-- began disappearing from the fossil record about
10 million years before the K/T boundary.
"I think the events at the end of the Cretaceous were
not due to a single catastrophic event, but represent
a conjunction of events -- climatic change, maybe
a period of volcanism, and then, ultimately, a major
impact or catastrophic event," he said.
"The reigning idea is that the Earth had a bad day
from the impact, but I think it had a series of bad
days," Zinsmeister said. "The Earth's biosphere was
already stressed to a critical point, and the impact
could have pushed it over the edge, causing local
catastrophes such as the fish kill in the Antarctic."
Note to reporters: For a photo of William Zinsmeister
holding an Antarctic fossil, call Purdue University
News Office at (317) 494-2096.
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