consider the possibility that entire groups will be lost
for purely random reasons. (3) The calculation of Raup and Jack Sepkoski
(column of August 1982) that major extinctions stand higher and more distinctly
above the background level than previously recognized.
Some refinements and major surprises were added to this growing theme
in papers presented at Indianapolis. Jack Sepkoski, a former student of
mine now flourishing mightily at the University of Chicago, has spent
years compiling the most consistent and complete data set on extinctions
ever developed--a listing at the family level that includes everything
from protozoans to mammals. With these data, we finally have a basis for
the fine-scaled consideration of quantitative patterns in extinction that
this second strategy demands. (Good science may require genius and imagination,
as these columns so often emphasize, but never forget that new conclusions
are the fruit of hard empirical work as well; without these labors, highfalutin
thought is so much waffling.)
Using the Sepkoski data, Raup and Sepkoski have now identified a striking
cyclicity in mass extinctions for 225 million years since the great Permian
dying. Every 26 million years, with eight hits and just two apparent misses
(a pattern too regular and striking to be accidental on statistical grounds),
we find a peak of mass extinction; all previously identified major extirpations
lie right on the highs of this 26-million-year cycle. What cause could
yield a periodicity so regular, yet so widely spaced? If we understand
geology aright, no purely internal process of climate, volcanism, or plate
tectonics cycles so regularly with such a long period. Raup and Sepkoski
therefore speculate that some astronomical cycle must be involved--a
solar or galactic properly, although for the moment, we have no idea what.
If the cycles are so frequent and caused by events so utterly beyond an
organism's control or anticipation (how can populations track a 26-million-year
cycle?), and if the mass extinctions shape life's pattern so fundamentally,
then mass dying is not ordinary death extrapolated.
David Jablonski, a paleobiologist from the University of Arizona at
Tucson, then added two cogent points to emphasize the abruptness and the
different character of mass extinctions. For abruptness, Jablonski noted
that the raw data of mass extinctions often include a long period of apparently
slow and steady decline among groups that crash more profoundly at the
peak itself. These slow declines have long been taken as a sign of continuity
between normal and mass extinction. But are they real or an artifact of
our imperfect geological record?
For more than one hundred years, geologists have sought terrestrial
agents to associate with mass extinction. The litany is long, yet all
but one have failed--mountain building, volcanism, fluctuations in
temperature, to name just a few old and unsuccessful favorites. Falling
sea level represents the one good correlation (and the 26-million-year-cycle
theorists had better take it into account). Nearly all mass extinctions
are preceded by a marked regression of sea level.
Falling sea level may well be a causal factor in extinctions (our Fossil
record is strongly biased toward shallow-water marine invertebrates),
but it also imposes an obvious artifact upon the data. As sea level falls,
fewer sedimentary rocks are deposited to hold the fossils of these regressive
epochs. Perhaps the slow decline that precedes most mass extinctions only
records the decreasing volume of available rock for finding fossils, not
a true and gradual decrease presaging the later peak.
Jablonski used a clever method to measure the potential artifact. Some
forms disappear from the record as sea level falls, only to be found again
when seas return to deposit more rocks after the mass extinction itself.
These temporary losses must be an artificial effect of falling seas and
decreasing amounts or fossiliferous rock. Jablonski refers to these reappearing
groups as "Lazarus taxa."
By counting the number of Lazarus taxa that disappear before, but reappear
after, a mass extinction, Jablonski can estimate how much of a counted
slow decline before a mass extinction might be the artificial result of
less available rock for finding fossils, and how much must record a real
and gradual event tying peaks of mass extinction with normal times before.
In some cases, subtraction of the Lazarus taxa still leaves a residue
of slow disappearance, and the pattern must be real (decline of ammonites
before the Cretaceous extinction, for example). But for many groups at
the Cretaceous, measured slow decline can be explained entirely by the
artifact of decreasing available rock. Thus, the Cretaceous extinction,
and others as well, may be more abrupt than we have previously realized.
The asteroid's candidacy is strengthened, and its dominant role affirmed.
Mass extinction is something quick and special.
Jablonski then examined the behavior of groups during normal times and
during episodes of mass extinction to see if he could detect consistent
differences that might accentuate the special character of mass extinctions.
He found some intrigu-
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