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K-T extinction



Dear list

Tetanurae@aol.com writes:

<< No doubt a really really big rock from the sky would be devistating and
kill  everything, but it cannot be so selective as to kill say
enantiornithines and  not neornithines, or ammonites and not nautiloids.
Baking skies and burning  forests do not selectively kill of basal
ornithopods instead of basal primates, or avisaurids and hesperornithids
instead of anseriformes: they kill  everything. >>

Ammonites mostly lived high in the water-column and had planktonic young.
Nautiloids lived (and live) much lower in the water-column which seems to
have afforded some protection against both the immediate impact effects
(minor on land and sea I contend) and the post-impact ones (major for
everything in my opinion) for many groups.

Also, we will never be able to account for the whole pattern of extinction.
Even under "normal" conditions species go extinct at background rates and
we can rarely discern cast-iron reasons for these in the fossil record.
Under the impact scenario, the populations of many/most species would have
crashed (darkness and temperature change would be tough for almost
everything). Stochastic effects are more likely to be terminal in small
populations than large ones. In other words, the level of extinction *for
no identifiable reason* is likely to have risen dramatically in the
post-impact world, way above normal background levels.

I think the impact scenario accounts for the broad pattern of extinction
among land and sea groups and this is *all we can expect* from any theory
given that simple bad luck must have become an extinction mechanism of
greatly increased importance in such a changed, impoverished and unstable
world.

(I think that much of the pattern that *does* exist, land and sea, revolves
around population size and the degree of reliance on (1) simple
photosynthetically-based food chains (bad news for big land animals and
upper ocean forms) or (2) detritus-based ones (good news for ectothermic
river and lake dwellers, many arthropods, insectivores [prey =
usually/often detritus feeders] and groups which make a living at the
bottom of water-columns). 

Best wishes to all
Chris Lavers