Geoscience Reference
In-Depth Information
Maastrichtian, for example, is a stage in the Cretaceous system), and
then plotted the percentage of extinction within each stage. Each
data point came from calculating the number of families that became
extinct within a stage as a percent of all the families that lived dur-
ing that stage. (Removing the families that are still alive makes the
denominator of this fraction smaller and the resulting fraction and
peak larger. If half the families in a stage are still alive today, the
denominator is half as large and the peak of percentage is twice as
high as it would have been had they all been extinct. Although
removing extant families affects the height of the peaks, it does not
change their spacing.)
The chart clearly shows that extinction is not continuous (which
geologists have known for a long time), and confirms the location
of the three members of the Big Five that we know fall within
the last 250 million years of earth history: the Permian-Triassic, the
Triassic-Jurassic, and the K-T. Finding the three members of the
Big Five exactly where they were expected to be told Raup and
Sepkoski that nothing was seriously wrong with their methodology.
The truly startling point, however, and the one that sent Raup rush-
ing into Sepkoski's office, is that the peaks show up at regular inter-
vals—every 26 million years. What could it mean?
Before answering that kind of question, when faced with such an
unexpected and unprecedented pattern emerging from a complex
set of data, a scientist has to make certain that the result is not
merely an accident or an artifact of the way the chart was con-
structed. Raup spent the next several months testing these possibili-
ties, trying to "kill the periodicity," as he put it. But no matter what
he tried, the periodicity persisted, and at a confidence level of better
than 99.5 percent.
Like many scientific suggestions, the idea of periodic extinction
was not new. It had been proposed in 1977 by Alfred Fischer, then
at Princeton, and his graduate student Michael Arthur.
6
They assem-
bled data from a variety of such geologic indicators as sealevel, tem-
perature, number of species through time, and isotopic ratios. Their
analysis revealed a cyclical pattern in species diversity with a period
of 32 million years. Fischer and Arthur did not have enough data for
rigorous statistical testing, and partly for that reason their suggestion
was not followed up. Since no one knew why the earth should have
behaved cyclically, the observation itself was discounted.
Raup and Sepkoski, having submitted their huge volume of data
to careful statistical analysis and been unable to falsify their conclu-
sion, were ready to go public. They did so with trepidation. Raup was
