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Tiny Teeth Shed Light On Ancient Comets


U.S. Department of the Interior
U.S. Geological Survey
Central Region Outreach Office
P.O. Box 25046, MS 150
Denver, CO 80225-0046

Contact: Heidi Koehler
Phone: (303) 236-5900 ext. 302   Fax: (303) 236-5882

News Release: Embargoed until 3/20/98

Tiny Teeth Shed Light on Ancient Comets

Minuscule fossil animal teeth, known as conodonts, indicate that a
370-million-year-old comet that slammed into Nevada could be as much
as five times larger than scientists initially suspected.

"From conodonts discovered in debris from the impact, I have re-calculated
the crater depth and size," said Charles A. Sandberg, geologist emeritus
with the U.S. Geological Survey in Denver. "The crater left by the ancient
Nevada comet is now interpreted to have been at least one mile deep and
45 miles wide. It is comparable in size to one that struck the Chesapeake
Bay area about 35 million years ago. On the basis of this comparison, the
Nevada comet could have been as large as three miles in diameter, much
larger than our initial calculation of one kilometer. It may have been part of
the first of a series of comet showers that led to a mass extinction of many
forms of life three million years later."

Startling new evidence provided by these conodonts on the depth of the
crater produced by the Alamo Impact, 130 miles north of Las Vegas, will
be presented at the North-Central Section meeting of the Geological
Society of America in Columbus, Ohio on Friday, March 20, by Sandberg
and his co-author Jared R. Morrow, USGS volunteer and instructor at the
University of Colorado.

Conodonts are the microscopic teeth of primitive, boneless, eel-like
animals. The shape of these animals is similar to that of modern hagfish,
best known from the fjords of Norway. Conodont animals lived in many of
the world's oceans from the Cambrian through Triassic Periods of geologic
time (550 to 210 million years ago). The largest known conodont teeth,
found near the town of Alamo in southern Nevada, measure nearly a
half-inch in length, but most are not much larger than the head of a pin.

Evidence presented by Sandberg in October 1997 at the annual GSA
meeting in Salt Lake City showed that impact-related phenomena, such
as grains of shocked quartz and higher-than-usual levels of the element
iridium, occupied a circular area at least 120 miles in diameter, but the
size of the crater could not be determined then. Now, new conodont
evidence for its dimensions has been found in small blocks of fallout
debris from the impact crater. Iridium is an element found in asteroids and
comets but not common on Earth. Shocked quartz grains are produced by
the force of an impact on sandstone rocks.

This impact occurred during the Devonian Period of geologic time, 370
million years ago, when an ancestral Pacific Ocean covered most of
Nevada. The impact occurred offshore from a carbonate platform, very
much like the modern Australian Barrier Reef or the Bahamas Bank. Shock
waves from the impact and ensuing tsunami waves crashed against the
carbonate platform and coastline in a semicircular area 100 miles from
north to south. As the carbonate platform collapsed, blocks of rock
hundreds to thousands of feet across were torn from the seabed, twisted,
and transported seaward. As tsunamis of decreasing intensity
reverberated back and forth across the ocean basin, broken pieces of rock
and other ejecta from the impact were deposited over the carbonate
platform and high-water deposits were stranded in a semicircular band
along the coastline to the east.

The small blocks of impact-fallout debris recently found within the breccia
of large jagged blocks contain carbonate spherules formed from limestone
fragments that recrystallized within a superheated cloud, shocked quartz
grains, and bits and fragments of rocks blasted from the crater. Most
importantly, they also contain conodonts ejected from rocks that lay a mile
below the Devonian sea floor at the time of impact.

The timing of the Alamo Impact coincides with the demise of some Late
Devonian reefs in Belgium and Germany and with unusual breccias in
Germany and Austria. Earlier work by Sandberg and his co-authors has
already shown the possibility of two other times of impacts between that
of the Alamo Impact and the time of the Late Devonian mass extinction.

This presentation at the Geological Society of America section meeting is
part of a Pander (Conodont) Society Symposium, exploring the effects of
extraterrestrial impacts on major extinctions of ancient life. The symposium
will include talks by four USGS geologists and scientists from Austria,
Canada, England, Germany, and Sweden. The society is named after a
German-speaking paleontologist, Christian H. Pander, who first described
conodont microfossils in 1856. Since the middle of the 20th Century,
conodonts have become the most useful microfossil for dating marine
rocks, mainly because of their widespread distribution, fast rate of
evolution, and rapid recoveries from near-extinctions.

The keynote address by Hans-Peter Schonlaub, Director of the Austrian
Geological Survey, will stress the importance of conodonts in dating
impacts and extinctions during much of the time that life existed on Earth
and the scarcity of impact craters that are currently recognized and well
dated.

As the Nation's largest water, earth and biological science and civilian
mapping agency, the USGS works in cooperation with more than 2000
organizations across the country to provide reliable, impartial, scientific
information to resource managers, planners, and other customers. This
information is gathered in every state by USGS scientists to minimize the
loss of life and property from natural disasters, contribute to the sound
conservation, economic and physical development of the Nation's natural
resources, and enhance the quality of life by monitoring water, biological,
energy and mineral resources.

For additional information prior to March 17 contact:

Charles A. Sandberg, Geologist
sandberg@usgs.gov
Ph: 303.236.5763
Fax: 303.236.0459

Jared R. Morrow
morrow@ucsu.colorado.edu
Ph: 303.545.9983

For the October 1997 press release on the Alamo Impact, go to:
http://www.usgs.gov/public/press/public_affairs/press_releases/pr372m.html