Geoscience Reference
In-Depth Information
tists might be able to tell whether the dinosaurs were already de-
clining well before the boundary as the anti-impactors claimed, or
whether they were found right up to the boundary. At least they
might be able to shrink the ghastly gap.
But who would do all the work of collecting the required large
number of new dinosaur specimens? To the museum professionals,
the answer came at once—the volunteers] Sheehan spent three
summers in North Dakota and Montana collecting in the Hell Creek
formation, accompanied each time by 16 to 25 carefully trained and
closely supervised volunteers from the "Dig a Dinosaur" program of
the Milwaukee Public Museum, who paid $800 for the privilege.
They spread out in "search parties," scouring the Hell Creek terrain
for any sign of a dinosaur fossil. When a volunteer found a specimen,
a paleontologist went over to make the identification, which was
then logged into the computer. Almost all specimens were left in
place rather than being collected and removed. To reduce the effect
of different sedimentary environments, the collectors restricted their
efforts to one of three sedimentary facies (distinct rock types that
sedimentologists can identify). The volunteer workers logged the
amazing total of 15,000 hours of careful fieldwork and found 2,500
dinosaur fossils. A key point of the study, one that differs from ear-
lier work, was that they recorded not only whether a given species
persisted at a certain level, but how many times it occurred. In other
words, they measured not only taxonomic diversity (how many spe-
cies are present no matter how rare), but what they called ecologi-
cal diversity (how many individuals are present). This is a distinction
with a difference: A species that was almost but not quite eliminated
would leave taxonomic (naming) diversity unchanged—only a single
individual would retain the taxon's name on the list. On the other
hand, ecological diversity (number of individuals) would have plum-
meted, showing that it is obviously the more informative measure
for tracing patterns of extinction.
The Milwaukee crew divided the Hell Creek formation into
three units of approximately equal thickness, with the top one
reaching up to the K-T boundary, and measured the number of
dinosaur families in each third. Their most diligent search found
dinosaur fossils within 60 cm of the K-T boundary, thus shrinking
the ghastly gap over which Luis Alvarez and Clemens had tangled
far into the Berkeley night. Their focus on ecological diversity
allowed them to conclude: "Because there is no significant change
between the lower, middle, and upper thirds of the formation, we
reject the hypothesis that the dinosaurian part of the ecosystem was
