Re: extinction

[Scott <HORTON@BCRSSU.AGR.CA>]

Janis:
  I don't know how long you've been a member of the list, so maybe you've
already seen this. I posted this note back in January. It is the latest
evidence I know of re: the impact theory of dinosaur extinction:

The Feb issue of _Astronomy_ magazine reports that a new gravimetric survey of
the 65my old Chicxulub crater in Mexico's Yucatan penninsula is even larger than
previously believed. It is 300km in diameter, not 180km. This should put a
stopper in critics who said that the impact was not large enough to have caused
the mass extinction at the end of the Cretaceous.
The smaller 35km Manson crater in Iowa, that was once THE suspected impact, has
been just been re-dated to be 73.8my (too old).
Impacts are still occurring. Astronomers say comet Shoemaker-Levy 9 fragments
will impact on Jupiter this July. Its nucleus is fragmented into about a dozen
pieces. A string of craters just found on the moon Callisto indicates that
similar fragmented comet impacts have occurred before.
I just got Bakker's book _The Dinosaur Heresies_, and haven't got to his
discussion of extinction yet, but I guess I'm a Believer in the impact theory.
Nobody can dispute that the impact occurred, and an impact that large HAD to
cause chaos.
(end of earlier posting)

  In my humble opinion, the evidence for dinosaur decline prior to the KT
boundary layer is due to sampling effects. The other theories, eg. disease,
don't make much sense. Dinosaurs encompassed very diverse groups of animals,
and to think that a disease completely and selectively wiped out all dinosaurs,
pterosaurs, and sea reptiles without affecting the animals is too much to
swallow. A good theory has to explain the extinction of all dinosaurs,
pterosaurs, and sea reptiles at the same time, while allowing small animals
(mammals, snakes, lizards, crocodiles, birds) to survive. It has to explain
not only this extinction event, but other mass exinction events.
  For an excellent discussion of the impact theory vs. other theories, read
"Nemesis: the Death Star" by Rich ...(I've forgotten his last name: Mueller?).
He was a friend of Louis and Walter Alvarez, two of the men who proposed the
impact theory. The author of this book takes the next step, to propose a
mechanism for periodic impacts. It is an excellent demonstration of scientific
method, even if you don't agree with his theories.   :)

Scott Horton
Geophysicist/Computer Programmer