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Re: Nesting with Dinosaurs.




On Fri, 21 Apr 2000, Matt Wedel wrote:

> Seriously, though, if egg-eating (ovivory?) by mammals was a significant
> factor in the extinction of dinosaurs, shouldn't the dinos have died out
> at, like, the end of the Triassic?  If not, why not?

This question assumes as fact something which is clearly not: that mammals
and birds changed little from the Triassic.

Birds.  If neornithine domination resulted from superior competitiveness
over enantiornithines--and this hypothesis has just as
much evidenciary support as the K/T "smart bullet" or sole
survivor hypotheses--the same traits would probably affect dinosaurs as
well.  For example, improved flight characteristics, more flexible
behavioral responses could pose a greater threat, and increase
predation load particularly on hatchlings.  After all,
extant birds are a major predator on the juveniles of ratites.  

Irony usually functions to ridicule ridiculous ideas.  The above idea
seems rather grounded.  If not, why not?

Mammals. Towards the end of the Cretaceous, the placenta evolved.  This is
a big deal.  And while placentas don't crack eggs, the structures,
behaviors they facilitate may well have.  Relaxation on size selection
seems to have occurred, at least the record doesn't contradict that
statement.  (See response to Kenneth Carpenter).  Also, selection against
_small_ size seems to have been operating on dinosaurs towards the end of
the Cretaceous.  A very conservative hypothesis would seem to be that
mammals preyed on (eggs and juveniles of) small dinosaurs and were thus
relieved of size selection themselves.  If this is not a conservative
hypothesis, why not?  I'm not saying this _did_ happen.

Finally, I take it as a given that once mammals crossed some size
threshold they could be as tyrannical towards dinosaurian reproductive
mode as they are today.  This size threshold is probably very low
(judging by size relationships of extant prey offspring and their
predators!).