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In the mid-1980s, the confidence in this picture began to wane.
It was shown then that the mammals occurred not in rocks of true
Cretaceous age, but in Tertiary channel deposits cut down into the
Cretaceous rocks (see p. 134], thus evaporating the evidence for
early replacement of dinosaurs by mammals. Standing the argument
on its head, others have reported finding dinosaur remains in Ter-
tiary rocks in the Hell Creek region and claim that the dinosaurs did
not become extinct at the K-T after all! 4 4
Further work at Hell Creek eroded the attempts at precise cor-
relation of the rock strata there. According to Jan Smit, previous
workers had placed the K-T boundary at Hell Creek between 2 m
and 12 m too high (the same point Smit made for the sediments at
Mimbral). 4 5 When a boundary is set too high, species immediately
below it appear to have gone extinct earlier than they actually did.
By placing the K-T boundary too high, according to Smit, the false
impression was created that the dinosaurs had gradually disappeared
and had been replaced by mammals well before the end of Creta-
ceous time. The Z coal beds, thought to mark the bottom of the Ter-
tiary, were shown not to be of the same age at different locations,
eliminating their usefulness as a time marker. 46 ' 4 7 Finally, the Hell
Creek strata were found to be riddled with gaps. At the 1995 meet-
ing of the Geological Society of America, J. K. Rigby, Jr., of Notre
Dame, reported paleomagnetic studies which showed that 300,000
to 500,000 years of the rock record were missing there. 4 8 The mod-
ern view is that the rocks of Hell Creek cannot be matched from
one spot to another with sufficient resolution to make the precise
chronology of dinosaur extinction clear.
As sometimes happens, the more a phenomenon is studied, the
more questions are raised and the less confident the answers become.
The Hell Creek strata now appear so complicated and full of gaps
that it is hopeless to attempt to use them as the litmus test of dino-
saur extinction. Some scientists realized that they needed to take an
entirely different approach from the traditional, one that provided
large enough samples so that statistics could be employed, and which
did not depend on the precise location of the K-T boundary at Hell
Creek. This view gave rise to two important studies.
SURVIVAL ACROSS THE K-T BOUNDARY
AT HELL CREEK
The Museum of Paleontology at Berkeley contains more than
150,000 curated specimens of nonmarine vertebrate fossils—mam-
mals, dinosaurs, turtles, snakes, and so on—from the Hell Creek and
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