Hopkins Scientist Challenges Theories of Extinction
By Emil Venere

Hopkins paleobiologist Steven M. Stanley has added a new
chapter to the constantly evolving understanding of mass
extinctions, periods when huge percentages of life on Earth
died off.
    He and his former student, Xiangning Yang, a
paleontologist at China's Nanjing University, have discovered
strong evidence for a previously unknown surge of extinctions
that rivaled the infamous die-off that felled the dinosaurs.
    Their research also challenges basic assumptions about
Earth's most devastating mass extinction, 230 million years
ago. Evidence in the fossil record shows that the spectacular
extinction period was actually not as big as scientists have
assumed. Moreover, it was not a single event, but a
combination of two extinction "pulses" 5 million years apart,
according to the Hopkins scientist.
    For more than a decade scientists have believed that up
to 95 percent of all species on Earth ceased to exist at a
point in geologic history marking the end of the Permian
period. Scientists also have accepted that, although those
extinctions might have taken place gradually, they probably
resulted from a single deadly event or chain of events.
    But Dr. Stanley's work casts doubt on both assumptions.
The researchers studied fossil records and developed tests to
analyze the pattern of the extinctions, which possibly were
the result of gigantic lava flows in Siberia that might have
altered the planet's atmosphere.
    Their figures indicate that the number of species killed
during the second of the two extinction pulses was about 80
percent, still the largest mass extinction in Earth's
history, but considerably less dramatic than previous
estimates. 
    "People just keep perpetuating this idea that the
extinctions might have been as high as 95 percent of species,
but it clearly was not," said Dr. Stanley, a professor in the
Department of Earth and Planetary Sciences.
    Perhaps more startling is strong evidence that the first
pulse, a previously undiscovered mass extinction, was a major
die-off in its own right, possibly as large as the cataclysm
that ended the age of dinosaurs 65 million years ago, marking
the boundary between the Cretaceous and Tertiary periods.
    The scientists studied fossils left by several groups of
animals, ranging from ancient clams and snails, to
nautilus-like predators called ammonoids and a group of
shelled, single-celled creatures called fusulinaceans,
relatives of amoebas that looked like grains of wheat or
rice.
    The spindle-shaped fusulinaceans provided the
researchers with some of the most concrete data they needed
to determine that the Permian extinctions were not the result
of one event. Instead, many species were first killed off
during a time referred to by geologists as the Guadalupian
stage of the Permian period. A hiatus of 5 million years
followed before the second, and largest, extinction pulse,
which came during a time referred to as the Tatarian stage,
marking the end of the Permian period.
    The fusulinaceans were abundant on seafloors during the
Permian, and their tiny skeletons are found in limestone
deposits around the world. Their extinction pattern shows
that all the largest genera, or groups of species, of
fusulinaceans, those longer than about 6 millimeters, were
killed during the Guadalupian interval. Included among the
genera that died during that time were all of those
possessing a distinctive, complex wall structure resembling a
honeycomb.
    By lumping extinctions from both the Guadalupian and the
Tatarian intervals together, scientists have overestimated
the Tatarian extinction rate, Dr. Stanley said.
    Only a handful of scientists are aware of the findings,
since the research has not yet been published. One
paleontologist who reviewed the work said Dr. Stanley's
research may reflect a trend in which scientists find better
and better evidence for such extinction pulses, challenging
the more accepted idea that mass extinctions happened in
single catastrophic die-offs.
    "I think this is a very good development," said David
Raup, professor emeritus at the University of Chicago's
Department of Geophysical Sciences. "His data and his
interpretation seem very credible to me.
    "As we do more rigorous analysis, and as we get more
data, I think we are going to see a lot more pulses of
extinction."
    John Sepkoski, another University of Chicago
paleontologist who has reviewed Dr. Stanley's findings, said
he was skeptical about the double-event hypothesis, until he
analyzed the data, which he called "quite convincing."
    One of the problems in gathering data in the past has
been that some of the most vital fossil evidence is located
in obscure and politically touchy places, such as China,
Tibet and Iran, noted Dr. Raup, who was a member of the
Hopkins geology faculty from 1957 to 1965.
    Scientists, however, are now gaining access to places
that had been off-limits, and new information is constantly
coming in. In fact, the Permian extinctions are not the only
die-offs being characterized as having happened in a pulse of
catastrophy.
    Researchers are finding that a similar pattern marked
mass extinctions during the late Devonian period, about 367
million years ago. In that period, many organisms first
became extinct in a similar pulse, followed by a more
devastating event at the boundary between the Devonian and
Carboniferous periods, Dr. Stanley said.