From J. David Archibald, State University California
In Gail Vines’s otherwise balanced article on mass extinctions
(Inside Science 126, 11 December 1999),
some comments regarding the extinctions at the
end of the Cretaceous warrant a response.
There have been recent claims that the impact in the Yucatan peninsula sent
shock waves well into North America. There is, however, no geological evidence
for a tsunami reaching South Dakota at this time, as the author claims, because
there are no known end-Cretaceous marine deposits from this state.
Throughout the western interior of North America only terrestrial deposits
are known at the end of the Cretaceous. This is exactly why the western interior
preserves the best terrestrial record of biotic events during the
Cretaceous/Tertiary transition. From this record we know that only 50 per cent
of vertebrate species became extinct. This percentage is based on actual
fossils, unlike the conjectural 75 to 80 per cent extinction cited in the
article.
As to the likely role of volcanism in these extinctions, both fossils and
radiometric dates show that the eruption of the Deccan Traps on the Indian
subcontinent began well before the end of the Cretaceous and would have had
long-term effects on the extinctions.
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Coupled with the greatest increase in land mass known for at least 250
million years and the accompanying loss of coastal environments, this was
already a time of increased environmental stress. The asteroid impact may well
have been the coup de grâce, but had the environment been intact its
effects would have been far less severe and may have left the dinosaurs as the
largest terrestrial vertebrates.
San Diego
