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RE: (extinction)

John Bois wrote:

> Gimme a non-teleological word that does the job, then.  Blind
> selection, perhaps...The bolide blindly selected the extinction of 
> marsupialsbecause they were hyperallergic to dust.

Differential extinction is an observation: some groups went extinct,
others didn't.  Your use of the term 'targeting' (and 'surgical precision'
in early posts) implies that the mechanism was highly selective in which
groups it erased.  Sure, some groups survived, others didn't.  Certain
groups were certainly more vulnerable, just as others had traits that might
have improved their chances of survival (small body size; lack of dependence
on photosynthetic substrates; ability to descend into torpor; ability to
endure protracted periods with little or no food).  But for those groups
that did survive it might have been a close-run thing.  Don't confuse
survival (which is an observation of the fossil record) with invulnerability
(which is an inference).

> Says who?  Are you seriously suggesting dinosaurs were unlucky--that
> theyexperienced extinction and that this had nothing to do with their 
> size, niche components, whatever?  

No, I'm not suggesting that at all (see above).  The extinction of the
non-neornithine dinosaurs likely had a lot to do with body size and its
attendant calorific and thermoregulatory demands.  David and others have
discussed this very articulately.  But your terminology implies (and
occasionally says outright) that the K/T extinction was hit-and-miss: either
you were affected or you weren't; and that some clades crashed to
extinction, while others were immune.  Because lepidosaurs, eusuchians,
champsosaurs, chelonians, mammals and neornithines all survived across the
K/T doesn't mean they were immune, and therefore not 'targeted'.  The entire
'targeting' thing (or 'blind selection') is beside the point.

> The only thing that gives your hypo predominance is that it happened, 
> and so did the bolide, so the bolide must have done it.  But this is 
> circular.

And the iridium spike at the K/T boundary?  And the Chixculub Crater?
Circumstantial, perhaps?  Maybe something collided with the earth about the
time that placentals decided to kick some marsupial ass, and the
neornithines elbowed out the enantiornithines, and a whole bunch of marine
invertebrates suddenly went to meet their Maker.  This, if I understand you
correctly, is your hypothesis.