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Re: extinction



>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.

But see Mayerhoff et al (1994) for a differing opinion of the Chicxulub
structure (volcanic).

>  In my humble opinion, the evidence for dinosaur decline prior to the KT
>boundary layer is due to sampling effects.

See Clemens (1992), Archibald (1992a), (1992b); who claim that the evidence
for sudden dinosaur extinction is due to *exactly the same thing*

>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.

The sea-reptiles suffered from the sea-level fall which drained their
habitat, the shallow epicontinental seaways.

A good theory has to explain the extinction of all dinosaurs,
>pterosaurs,

How about:

Dependent on large, bulk food intake which was affected by climate change,
not adapted to rapid temperature fluctuations caused by climate change.

>and sea reptiles at the same time,

Sea-level fall destroyed habitat - affected climate

while allowing small animals
>(mammals, snakes, lizards, crocodiles, birds) to survive.

Mammals, birds - "warm blooded", relatively low bulk food reqiurements,
adaptable to temp fluctuations.
snakes, lizards - low food intake requirements, unspecialized to
particulare niches
crocodiles - riverine (equable temperature retained), low food requirement,
not affected directly by changing flora

It has to explain
>not only this extinction event, but other mass exinction events.

The major extinctions show no evidence of impact associations eg Permian,
Devonian

Refs

Archibald, J.D. (1992a) Letter to the editor. Science, 256: 160

Archibald, J.D. (1992b) Dinosaur extinction: how much and how fast?.
Journal of Vertebrate Paleontology, 12(2) 263-264

Clemens, W.A. (1992) Letter to the editor. Science, 256, 159-160

Mayerhoff, A.A. et al (1994) Chicxulub structure; a volcanic sequence of
Late Cretaceous age. Geology, 22(1): 3-4

Chris Nedin                                 cnedin@geology.adelaide.edu.au
Dept. of Geology and Geophysics
University of Adelaide
South Australia
5005