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Dino extinction: gradual or overnight?



Dear Paul,

>I have seen this claim on other occassions and have to ask; how
>good is the data on the great dinosaur fade? It would seem to me
>that even under the very best of conditions, large terrestial
>animals would still leave a very spotty and pathetic fossil
>record, not the kind of data that one would want to base
>population trends on...

I agree with you that population trends are probably not reliably
represented in the fossil record.  But from everything I've read,
what many paleontologists believe is reflected in the fossil
record in the last "days" of the Cretaceous is not a decline in
dinosaur populations, but a decline in dinosaur DIVERSITY.  To
extract the data given in _The Dinosaur Heresies_, the Judith
River Formation contains six major (herbivorous) dinosaur genera:
Corythosaurus, Lambeosaurus, Prosaurolophus, Centrosaurus,
Styracosaurus, and Chasmosaurus.  But this diversity deteriorated
until the Edmonton-Hell Creek epoch, where 70-80% of the
herbivorous dinosaur specimens are Triceratops, and most of the
rest are Edmontosaurus.  IMHO, this apparent "Triceratops
monopoly" made dinosaurs as a group vulnerable to changes in the
environment too fast for evolution to make them adapt.  My humble
view is that the dinosaurs died out as a result of a "double-
whammy": first something happened to deteriorate the dinosaurs
diversity, then a second "something" happened to strike down the
remaining species.  My view is that it was this second "something"
(maybe an asteroid/comet impact) that forced the dinosaurs into a
rapid final extinction, and that without it the dinos would have
recovered and re-diversified in the Tertiary (the way they
apparently did after a similar diversity decline at the end of the
Jurassic).  So in this sense, the dinosaurs' FINAL extinction was
quick, but it was the gradual decline of diversity in dinosaur
species (what caused THAT is another question) that set the stage.


                    -- Dave