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Re: If the asteroid had missed...



>  
>  Certainly.  However, the animals that seem to do most of the 
> predation on extant large nesters (e.g., armadillos, birds, lizards) 
> are not extravagantly different from pre-boundary animals.  

True, but "pre-boundary" in this case extends back well into the Mesozoic.  Add 
in the additional non-avian dinosaur nest predators, and we have a rather long 
history of probable hatchling attrition long before the K/T boundary.

> Most 
> explanations for the absence of large dinosaurs invoke preresidential 
> advantage of current niche holders.  Yet, terror birds staged a large 
> carnivorous come back in SA.  Why was this the only one (?).  And why 
> did it ultimately fail?  I agree that the invaders were quite 
> different from boundary types...still.  

Good questions.  The preresidential advantage card is still playable, however, 
because the terror birds evolved in a world where there were many large, 
terrestrial mammals.  Looking at it from a probablistic point of view, it seems 
like preresidential advantage kept such large carnivorous comebacks to a 
minimum (though just being "playable" doesn't mean it is the main effect).  
Then there is anatomy/historical constraint.  For example, crown group birds 
are not a "good" group for generating large-bodied folivores.  It has happened 
a few times now (and they aren't really obligate folivores), but there is a lot 
of historical baggage to overcome.  After all, there is only one living species 
that is a volant truly folivorous bird.

>  We have the bolide data.  But we still have questions that may argue 
> for a role in species' interactions: why enanti's out before the 
> boundary? why pterosaurs draining before?  I fully understand that the 
> bolide was responsible for a great many things.  But I believe the 
> spectacular nature of that event obscures the pervasive, consistent, 
> and ineluctable quality of species' interactions--and the speciation 
> they drive. 

That's reasonable; obviously species interactions are going to matter, and you 
pose some good questions.  Such additional trends in diversity may be the 
result of species interactions or they may be the result of additional abiotic 
dyanamics.

For what it's worth, I'm not entirely convinced that pterosaurs were actually 
"draining" beforehand.  The observed patterns are actually just as explainable 
by a combination of luck and morphologic/behavioral trends.  In terms of luck, 
most of the pterosaur record is really smatterings of individual specimens 
punctuated by a few spectacular sites (like Niobrara Chalk and Santana 
Formation).  The fact that we haven't happened to find such a site nearer to 
the K/T boundary is not really suggestive of a decline.  In addition, the 
latest Cretaceous diversity seems to be dominated largely by azhdarchoids 
(especially azhdarchids), which have the rather annoying habit of living inland 
and refusing to die in large colonies.  So the abundance could be quite high; 
we really don't know.  The argument for loss of phylogenetic diversity (ie. 
loss of higher clades) is a bit stronger, though that could still be a result 
of disproportionately good luck in the lower Cretaceous.  

The last few years of late Cretaceous pterosaur discoveries seem to 
increasingly demonstrate wide geographic extend and abundance for Late 
Cretaceous pterosaurs.  However, they also increasingly support a loss of 
higher phylogenetic diversity.  So, I suppose you could argue either way.  

Cheers,

--Mike H.