Re: Pterosaur size

[Michael Habib <mhabib5@jhmi.edu>]

On Wednesday, December 13, 2006, at 12:13 PM, don ohmes wrote:

> Why does relatedness make a difference? Basic dragonfly design. Big 
> then, small now. Why? And you're just guessing anyway, about kinship. 
> Not that kinship is relevant.

Kinship is very relevant.  If you want to do a trend analysis, you have 
to consider the phylogenetic non-independence of the units you are 
comparing.  Most of the trends you refer to are, as best I can tell, 
actually the result of clade-specific mechanical differences.  If you 
wish to demonstrate that the trends are, in fact, not a product of 
clade membership then you must correct for kinship.  Essentially, you'd 
have to either show the trends within lineages with low functional 
variance, or utilize independent contrasts.  In all likelihood, you'd 
have to do both.  --MH

> Kinship is irrelevant, although frequently there, especially at any 
> given point in time. _Functional equivalents_. They are probably 
> related, but so what?

See above --MH


> Great category. But again, relatedness doesn't count, although 
> probably helpful in determining functional equivalence. What is the 
> largest ectothermic herbivore now? Then? How do they trend through 
> time?  Those are the questions that are relevant.

It is relevant, but the full question is "how does the size of 
ectothermic species trend over time accounting for phylogenetic 
relatedness and functional differences?"  It is very interesting; it is 
also very difficult to approach and much more complicated than plotting 
simple raw values against time.

> Unlike Jim and Mike, I don't think today's environment could evolve a 
> Quetz-sized volant. Or tomorrow's environment either.

I'm still not clear on why you think this is the case.  Is it 
constraint from an evolutionary standpoint, or mechanistic?  You seem 
to imply in several cases that you feel it is mechanistic.  Can you 
explain what mechanical factors suggest to you that Quetz would not be 
viable today?  Because I cannot find anything to suggest this.  
Landing, launch, and steady state are all feasible for Quetz in a 
modern atmosphere (which, as was previously mentioned, isn't really 
that different from the Cretaceous atmosphere from the perspective of a 
large-bodied flyer at a relatively high Re).  --MH

> In fact, I think the current size limit for flapping flight is below 
> Argentavis, and somewhat above the largest birds living...

Which I find odd, because the most recent mega-volants died in the Late 
Pliocene.  From the perspective of the atmospheric effects you cited 
before, that is modern.  Argentavis might not be viable from a food 
perspective standpoint, but that's because we don't have a full 
megafauna at the moment.  --MH

> Plot the largest aquatic air breathers through time. Do the same for 
> terrestrials. Very different charts. Opposite slope, for one thing. 
> Different "lifespans" and evolutionary rates, too. Or so it appears to 
> me. Why?

Good question, but you first need to correct for kinship and make sure 
the trends are real.  --MH


Cheers,

--Mike H.