Geoscience Reference
In-Depth Information
C
HAPTER
12
ARE EXTINCTION
AND CRATERING PERIODIC?
They're regularly spaced in time.
David Raup
During the 1970s, when the Alvarezes were developing their the-
ory, a young paleontologist named John Sepkoski was at work at the
University of Rochester, compiling the ranges of geologic ages dur-
ing which each family of fossil organisms lived. (Recall that biolo-
gists subdivide organisms into kingdom, phylum, class, order, family,
genus, and species. We belong, in the same order, to the animals,
chordates, mammals, primates, hominids, genus
Homo,
and species
Homo sapiens).
Sepkoski was not going to all this trouble in order to
study mass extinction, but rather to learn more about how biologic
diversity has changed over geologic time. Meteorite impact was the
furthest thing from his mind.
Sepkoski scoured the world literature of paleontology, searching
out even the most obscure journals in the most unfamiliar lan-
guages, slowly adding information to his database. The data he en-
tered for each family were simple: name, geologic age of the oldest
and youngest recorded occurrences of species belonging to the fam-
ily, and the literature references. Sepkoski was fortunate to have had
the encouragement of his senior colleague at Rochester, David
Raup, who happened to be predisposed toward the statistical ap-
proaches to which a large database lends itself. By 1978, both scien-
tists had moved to the University of Chicago, further strengthening
a department of paleontological powerhouses. There, Sepkoski con-
tinued to upgrade and polish his compendium, until it contained
3,500 families and 30,000 genera.
2
One day a senior colleague, the
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