Geoscience Reference
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which were lithified (turned into rock) by the resulting heat and
pressure; erosion later exhumed and exposed them for us to see
today. By studying the composition and features of sedimentary
rocks and their fossils, sedimentologists and paleontologists can re-
construct the history of ancient landscapes in great detail. They
know that the oldest formation in the Hell Creek area, the Bearpaw
Shale, formed from muds deposited on the floor of the ancient sea.
Above the Bearpaw lies a sandstone that formed from the beaches
left behind as the seaway retreated. It in turn is overlain by two for-
mations deposited by streams that meandered back and forth across
marshy, nearshore floodplains: the Cretaceous Hell Creek forma-
tion and higher up, the Tertiary Tullock formation. Near the base of
the Tullock lie several thin beds of coal that formed in the reducing
conditions in the coastal swamps. Geologists had come to accept,
as the K-T boundary in Montana, the lowermost of these coals,
the "Z coal," in part because no dinosaur remains occur above it.
(Admittedly, a certain amount of circular reasoning is going on
here.) The uppermost dinosaur fossils were thought, especially by
Clemens, to lie about 3 m below the Z coal and no closer, suggest-
ing that the dinosaurs had gone extinct scores of thousands of years
before the boundary.
From decades of study, vertebrate paleontologists had come to
conclusions about the end of the dinosaurs that the Alvarez theory
was initially unable to shake. For example, although 36 dinosaur
genera occur in rocks dating some 10 million to 11 million years
before the K-T boundary, those immediately below the boundary
contained only about half that many genera. To most paleontolo-
gists, this was a clear indication that the dinosaurs were on the way
out well before the end of the Cretaceous and that, if anything,
impact had delivered only a coup de grace. Dale Russell pointed
out, however, that the higher number of genera from the older rocks
was a total obtained by adding together all those found at 25 loca-
tions around the world, whereas the smaller, later number had come
from only the three North American sites. This suggested that the
difference might be only a sampling effect. Nevertheless, by the
mid-1980s, most paleontologists who had studied the Hell Creek
fossils had firmly concluded that the dinosaurs had already died out
some 20,000 to 80,000 years prior to the K-T boundary, before the
putative arrival of any meteorite. Some paleontologists presented evi-
dence that the dinosaurs had already started to be replaced by mam-
mals well down in the Cretaceous, and claimed that this showed the
dinosaurs had started to disappear well before the K-T boundary.
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