Geoscience Reference
In-Depth Information
genera that lived during the late Cretaceous alone, which is by far
the most studied period.
4
1
Although dinosaur specimens are few, they occur in late Creta-
ceous rocks on every continent and at dozens of sites around the
world. To study their extinction, all we need are dinosaur-bearing
sections that extend from the late Cretaceous up to the K-T and at
least a short distance beyond it into the Tertiary. At how many
places in the world can such sections be found? The answer, shock-
ingly, is
three:
Alberta, Wyoming, and Montana. The Hell Creek for-
mation near Glendive, Montana, is by far the best studied. Dinosaur
research is continuing in other countries today, in Argentina and in
China, for example, and in time more sites will meet the criteria.
But up to now, to provide the litmus test for dinosaur extinction
theories, paleontologists have had no alternative but to rely on fos-
sils from the upper Great Plains, and from the Hell Creek formation
in particular.
What, then, do we know about the dinosaurs from the Great
Plains? At Snowbird I, Tom Schopf pointed out that our knowledge
of the Maastrichtian (latest Cretaceous] dinosaurs derives from only
16 known species, which have in turn been identified from only 200
individual specimens.
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2
No dinosaur has captured our imagination
better than the horrific
Tyrannosaurus rex,
yet only a handful of
complete skulls has ever been found.
When this paucity of hard evidence is added to the problems of
gaps, migration, Signor-Lipps effect, dissolution, location of bound-
aries, channel cutting, reworking, and so on, it is clear that any con-
fidently definitive statement about the demise of the dinosaurs
based on scarce fossil evidence is apt to be wrong. A bang can easily
be mistaken for a whimper, and vice versa.
To H
ELL
C
REEK AND
B
ACK
The Hell Creek formation achieved notoriety in 1902 when famed
dinosaur hunter Barnum Brown discovered there the first, magnifi-
cent
Tyrannosaurus rex.
Over the more than nine decades since, fos-
sil hunters and serious paleontologists have returned again and again
to northeast Montana and Hell Creek to collect and to decipher.
How did the Hell Creek rocks come to be? In late Cretaceous
times, a shallow sea stretched across western North America from
Canada to Mexico. As it shrank and retreated, the sea left behind a
sequence of sediments formed in different marine and nearshore
environments. Subsequent deposits entombed these older sediments,
