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Re: Pterosaur size

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


--Mike H.