Geoscience Reference
In-Depth Information
In Chapter 9 I listed the two major predictions for the fossil
record made by the Alvarez theory. How well has the dinosaur evi-
dence met them?
PREDICTION
1: Prior to the K-T boundary, the dinosaurs were
not already going extinct for some other reason. Their extinction
was sudden and right at the boundary.
Dinosaur expert Peter Dodson, and the work of Sheehan and col-
leagues, indicate that the first part of this prediction is met: There
was no gradual decline. The dinosaurs did not become extinct well
before the K-T boundary, but lived right up to it.
PREDICTION
2:
Dinosaur fossils are not found above the iridium
horizon.
With a few unconfirmed exceptions, this prediction is also met. The
"Tertiary dinosaurs" from Hell Creek and China may not be that at
all, but instead result from misplacement of the K-T boundary or
from reworking. If it does turn out that a few dinosaurs survived into
the Tertiary, they will not be sufficiently common to falsify this pre-
diction or to figure importantly in earth history.
In addition to these two, there is a question that can be asked,
even if it does not lend itself to a third prediction: Can the Alvarez
theory help to explain the selectivity of the K-T extinction? Since
the days of Baron Cuvier, the French father of taxonomy and pale-
ontology, at the turn of the eighteenth century, the extinction at the
end of the Cretaceous has been known to have been strangely selec-
tive. Marine reptiles, flying reptiles (including the pterodactyl,
named by Cuvier), and the dinosaurs died out, as did many marine
invertebrates, including the ammonites and most of the planktonic
foraminifera. But many terrestrial vertebrates—snakes, crocodiles,
turtles, and mammals—and some plants, survived. The impact-
extinction theory ought to make it easier to explain this peculiar
pattern. On the other hand, if the theory does not help, it may not
be because it is wrong but rather because we lack knowledge and
imagination. After all, no one has yet been able to explain under
any
theory why the crocodiles and turtles survived and the dinosaurs did
not. Were the impact theory also to prove wanting, it would be no
worse off than the theories that geologists have traditionally pre-
ferred—those theories cannot explain the selectivity either. But cer-
tainly, an ability to explain the selectivity of the K-T extinctions
would immeasurably strengthen the Alvarez theory.
