PRINCETON, N.J. — As a paleontologist, Gerta Keller has studied
many aspects of the history of life on Earth. But the question
capturing her attention lately is one so basic it has passed the
lips of generations of 6-year-olds: What killed the dinosaurs?

The answers she has been uncovering for the last decade have
stirred an adult-sized debate that puts Keller at odds with many
scientists who study the question. Keller, a professor in
Princeton’s Department of Geosciences, is among a minority of
scientists who believe that the story of the dinosaurs’ demise is
much more complicated than the familiar and dominant theory that a
single asteroid hit Earth 65 million years ago and caused the mass
extinction known as the Cretacious-Tertiary, or K/T, boundary.

Keller and a growing number of colleagues around the world are
turning up evidence that, rather than a single event, an intensive
period of volcanic eruptions as well as a series of asteroid
impacts are likely to have stressed the world ecosystem to the
breaking point. Although an asteroid or comet probably struck Earth
at the time of the dinosaur extinction, it most likely was, as
Keller says, "the straw that broke the camel’s back" and not the
sole cause.

Perhaps more controversially, Keller and colleagues contend that
the "straw" — that final impact — is probably not what most
scientists believe it is. For more than a decade, the prevailing
theory has centered on a massive impact crater in Mexico. In 1990,
scientists proposed that the Chicxulub crater, as it became known,
was the remnant of the fateful dinosaur-killing event and that
theory has since become dogma.

Keller has accumulated evidence, including results released this
year, suggesting that the Chicxulub crater probably did not
coincide with the K/T boundary. Instead, the impact that caused the
Chicxulub crater was likely smaller than originally believed and
probably occurred 300,000 years before the mass extinction. The
final dinosaur-killer probably struck Earth somewhere else and
remains undiscovered, said Keller.

These views have not made Keller a popular figure at meteorite
impact meetings. "For a long time she’s been in a very
uncomfortable minority," said Vincent Courtillot, a geological
physicist at UniversitÈ Paris 7. The view that there was anything
more than a single impact at work in the mass extinction of 65
million years ago "has been battered meeting after meeting by a
majority of very renowned scientists," said Courtillot.

The implications of Keller’s ideas extend beyond the downfall of
ankylosaurus and company. Reviving an emphasis on volcanism, which
was the leading hypothesis before the asteroid theory, could
influence the way scientists think about the Earth’s many episodes
of greenhouse warming, which mostly have been caused by periods of
volcanic eruptions. In addition, if the majority of scientists
eventually reduce their estimates of the damage done by a single
asteroid, that shift in thinking could influence the current-day
debate on how much attention should be given to tracking and
diverting Earth-bound asteroids and comets in the future.

Keller does not work with big fossils such as dinosaur bones
commonly associated with paleontology. Instead, her expertise is in
one-celled organisms, called foraminifera, which pervade the oceans
and evolved rapidly through geologic periods. Some species exist
for only a couple hundred thousand years before others replace
them, so the fossil remains of short-lived species constitute a
timeline by which surrounding geologic features can be dated.

In a series of field trips to Mexico and other parts of the world,
Keller has accumulated several lines of evidence to support her
view of the K/T extinction. She has found, for example, populations
of pre-K/T foraminifera that lived on top of the impact fallout
from Chicxulub. (The fallout is visible as a layer of glassy beads
of molten rock that rained down after the impact.) These fossils
indicate that this impact came about 300,000 years before the mass

The latest evidence came last year from an expedition by an
international team of scientists who drilled 1,511 meters into the
Chicxulub crater looking for definitive evidence of its size and
age. Although interpretations of the drilling samples vary, Keller
contends that the results contradict nearly every established
assumption about Chicxulub and confirm that the Cretaceous period
persisted for 300,000 years after the impact. In addition, the
Chicxulub crater appears to be much smaller than originally thought
— less than 120 kilometers in diameter compared with the original
estimates of 180 to 300 kilometers.

Keller and colleagues are now studying the effects of powerful
volcanic eruptions that began more than 500,000 years before the
K/T boundary and caused a period of global warming. At sites in the
Indian Ocean, Madagascar, Israel and Egypt, they are finding
evidence that volcanism caused biotic stress almost as severe as
the K/T mass extinction itself. These results suggest that asteroid
impacts and volcanism may be hard to distinguish based on their
effects on plant and animal life and that the K/T mass extinction
could be the result of both, said Keller.

Note: A longer version of this news release appeared in the
Princeton Weekly Bulletin: