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Re: Cretaceous Mass Extinction

I'm not a palentologist, but I wrote a book on mass extinctions a few years
back (Vanishing Life: The Mystery of Mass Extinctions, Scribner's Young
Adult, 1993), and have been following the subject for years. Your questions
and other's comments prompt a few observations.

We know an asteroid hit at the time the dinosaurs and other species died
out. We don't know what killed them. The coincidence is very strong
circumstantial evidence that the impact was the root cause, but I've seen
many different theories how the impact caused the extinctions. AMong the
more provactive is the notion that the critical factors were the target
rock (anhydrates, which would have released lots of CO2 and sulfur oxides),
and/or the angle of impact.

Recent data suggests that diversification of placental mammals and birds
started in the late Cretaceous, not just after the impact. This makes
sense, since it's unlikely that merely one species of each group survived
the extinction.

Virtually everyone agrees that everything larger than some certain size was
wiped out, but it's never been clear what that size was. Size may be a
factor partly because in general there are larger numbers of small animals
than of large ones. Kill 99% of all rhinos alive today, and they probably
will go extinct. Kill 99% of all mice, and they will survive, because tehre
are so many more mice. While there is evidence that overall biomass
decreased sharply (from carbon isotopes), we don't know what the kill
ratios were of the species that survived.

If you look at all the big extinctions, each seems to have its own distinct
patterns. The KT is the only one with clear signs of an impact. Conversely,
there are other times when there are clear signs of (smaller) impacts that
no one has been able to link with mass extinctions.

Jeff Hecht, science writer