Re: The extinction of small dinosaurs (long again)

[Michael Bruce Habib <mbh3q@virginia.edu>]

> So what is it that selected against enantiornithines and small dinos at
> the K/T, or against small pterosaurs during the Cretaceous?  And what
> disturbance effects were there _during_ the K/T such that ithe balance of
> power so dramatically shifted from pterosaurs to birds?

Wish I knew, it would make one heck of a publication.  
There are probably a host of important traits at work 
there that we just haven't pulled out yet.

Just the same, there are distinct possibilities. First off, 
at the K/T, there was a significant difference in average 
body size between pterosaurs and birds, and body size has 
been shown in several cases to correlate with extinction 
risk.

As for enantiornithines and small dinos, hard to say.  Keep 
in mind that 'small' is a relative term.  After all, there 
are numerous small cats having severe problems with human 
disturbances.  They may be small as cats go, but they're 
still decent sized predators, and good sized mammals 
(remember that the average mammal is mouse-sized), and thus 
tend to have small population sizes.

> My understanding is that pterosaurs did not disappear in an event--rather
> a gradual loss of diversity--first small then large species becoming
> extinct.  Is this not true?

I am not sure one way or the other.  Perhaps a literature 
search is in order.  I was under the impression that it was 
more polarized: a loss of small forms, a long lag, and then 
the loss of large forms.

> Yes.  I just don't agree with the view that species residing in a
> niche maintain dominion over it (because they can always adapt to a
> threat from the surrounding competing species) unless turfed out by
> some environmental change.  Again, I feel the environmental change is
> the threat posed by the surrounding species.  I'm not sure you would
> argue with this in "normal" times--I suppose I'm saying the ptero/bird 
> shift happened in normal times.  But I suppose this may not be true.

Of course, whether truly "normal" times exist is one hell 
of a question.  Obviously a difference in opinion here, 
probably nothing that can be done about that. I do tend to 
see the exclusion principle as very strong. However, I 
would not argue this because they can 'adapt to anything', 
but rather because I...

a) do not think competition acts the same way at higher 
levels of scale and diversity (ie. family level and above)

b) do not see why there should be strong selection to 
derive traits close to those of another species (and thus 
increase competitive interaction), at least not across 
entire clades (see "a").

Keep in mind that this sort of niche 'inertia' has occurred 
in some cases.  Dinosaurs radiated into large terrestrial 
vertebrate positions opportunistically as best we can tell, 
and mammals did the same.  Both groups obtained larger size 
and morphological diversity only after the established 
forms got whacked.  These examples indicate that there is 
some advantage to being established.

> Agreed.  But this is a _process_.  Species may compete for non-critical
> resources, and gradually encroach on each other's territory.  Arms races
> may ensue and one species may be better positioned (by virtue of its
> pre-existing body plan) to usurp the other.

Fair enough.  Again, would the process act the same at all 
scales?  At greater scales, would all systems (ie. species 
interactions) work in the same direction?

> Agreed.  But I'm trying to argue that because pterosaurs became extinct
> over the Cretaceous, that the biology of the organisms was the major
> factor.  I think this is the prevalent view (right???).  

Not sure, actually.  It believe it was a little while back, 
but I think it's more unsure these days.  Again, I think a 
shot at the recent literature might be worthwhile; I'd 
certainly be interested either way.  


Here's a note from Michael Benton (In Vertebrate 
Palaeontology, 1997):

"The idea that simple competition can have major long-term 
effects in evolution is probably an over-simplification of 
a complex set of processes.  Competition between families 
or orders of animals is very different from the ecological 
observation of competition within or between species.  In 
palaeontological examples like this, competition has often 
been assumed to have been the mechanism, but the evidence 
has generally turned out to be weak."

Mike