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Re: Pelagornis chilensis




--- Mike Habib <habib@jhmi.edu> schrieb am Fr, 17.9.2010:

> Von: Mike Habib <habib@jhmi.edu>
> Betreff: Re: Pelagornis chilensis
> An: "dinosaur@usc.edu List" <dinosaur@usc.edu>
> Datum: Freitag, 17. September, 2010 22:37 Uhr
> I have now had a chance to read
> through the new Pelagornis paper from JVP, and drool over
> the exceptional specimen. It's a very nice paper with
> some wonderful description and analysis. I do,
> however, have some skepticism regarding the body weight and
> flight comments. I am interested to know what other
> list members think of the same:
> 
> >From the paper:
> 
> "There exists a correlation between the mass of a bird and
> the least circumferences of the femur and tibiotarsus, with
> log M = 2.411Âlog CFâ0.065 and logM= 2.424Âlog CT +
> 0.076, where M is the body mass in gram, CF the least femur
> circumference, and CT the least tibiotarsus circumference
> (Campbell andMarcus, 1992). With least shaft circumferences
> of 58.4 and 64.1 mm for the femur and tibiotarsus of the
> Chilean pelagornithid, this results in mass estimates of
> 15.6 and 28.6 kg, respectively. Even the larger of these
> values is much less than the estimated mass of 71.9 kg for
> A. magnificens (Campbell and Marcus, 1992), and not
> significantly above the mass of the heaviest extant volant
> bird, the Mute Swan, Cygnus olor, whose males can reach
> â20 kg. These low values are nevertheless plausible,
> because the bones of pelagornithids were exceedingly
> thin-walled, and the hind limbs, which had to bear the
> weight of the bird, are very small. In combination with the
> very narrow wings, these low weight estimates testify highly
> efficient soaring abilities of pelagornithids, which appear
> to have been among the most proficient avian long-distance
> soarers."

Not having handled any material personally, I can only say that Pelagornithidae 
bones are indeed *extremely* thin-walled judging from the usual state of 
published material (long bones are usually crushed to the point of being barely 
recognizable - indeed, some have been assigned to Pelagornithidae more due to 
their size and bad preservation than due to any recognizable feature, because 
the latter were all but obliterated). 

To arrive at a more certain conclusion, one would need to consider, perhaps, 
Diomedeidae (which are closest in gross morphology) and Anhimidae (which 
*might* be closely related to Pelagornithidae, and have a very low volume/mass 
ratio). With the material now at hand, there should be some rather 
uninformative specimens available for thin-sectioning, which should provide 
satisfying answers to the question of how heavy/strong their bones were 
(http://onlinelibrary.wiley.com/doi/10.1002/jmor.10029/abstract and 
doi:10.1006/cres.1997.0102 give some ideas on how to tackle this).

As regards the proximal humerus' "locking mechanism", Mayr & Smith discuss this 
in their 2009 paper on the Belgian specimens 
(http://onlinelibrary.wiley.com/doi/10.1111/j.1475-4983.2009.00930.x/abstract). 

I would not discount the "automatic takeoff" technique (also note that this 
only applies to the gigantic spp.), as long as we don't know where these guys 
nested; for all we know they might only ever have alighted on high/volcanic 
islands to breed 
(http://si-pddr.si.edu/dspace/bitstream/10088/2006/2/SCtP-0090-Lo_res.pdf p.260 
seems to imply that at least one species nested in the Appalachians or on 
low-lying islands that today are hills in the Carolinan Piedmont). Albatrosses 
can get away with their extremely clumsy takeoff/landing techniques because 
their way of life minimizes the total time spent in either, and of course by 
choosing predator-free islands to breed; they regularly seem to spend 1-2 years 
without ever landing. (In that regard, is there any takeoff footage of 
_Phoebastria albatrus_ from Torishima?)

Pelagornithidae were such a unique, extreme and specialized lineage - and 
highly successful at the same time - that they do not compare well with e.g. 
large pterosaurs (which were also extreme and specialized in their own regard, 
though not as unique), except in the loosest terms of crude physics; i.e., both 
must have some sort of vaguely similar mechanism to overcome gravity. But even 
so they had to approach the problem from different angles. Ditto _Argentavis_ - 
its soaring technique must have differed from that of Pelagornithidae, if it 
did not travel all the way to the ocean to forage there (and there is really no 
good reason to think it did).

Think of extremely high skyscrapers - these too face the problem of gravity, 
and are "anatomically" constrained by it. Yet every single one of them is 
unique in *how exactly* it solves the problem, how it is constructed etc. And 
physics stays physics, no matter whether a human mind or the blind 
trial-and-error of evolution goes mano a mano with it - you can trick your way 
around it, but you cannot overcome it, meaning that there are only so many ways 
to trick your way around it, and they are also constrained by the initial 
material at your disposal. That the results are as outwardly similar and 
"anatomically" different as they are comes as no big surprise.

Thus, a comparison between Pelagornithidae and _Argentavis_, let alone 
pterosaurs, is probably highly insightful in the framework of comparative 
anatomy and histology, but the data and conclusions of one taxon do not seem to 
me to have specific bearing on the ecology of the others, except perhaps in a 
falsificationist sense (by pointing out theoretically possible adaptations 
found in *neither*). 

That the extinction of pterosaurs left a niche for huge soaring predators wide 
open is not very hard to figure out. Correct me if I'm wrong, but IIRC huge 
pterosaurs included both pelagic predators and terrestrial scavengers. That 
would make it *rather* interesting that huge soarers reappeared early in the 
Paleogene in (or rather: over) pelagic habitats, but that the niche stayed 
vacant until the early Neogene in terrestrial habitats.

Any pterosaur researchers and flight-interested biophysicists who consider the 
question relevant can at this point do much worse than to contact Mayr, and 
keep track of his research, to ultimately cooperate in a study outlining the 
specific adaptations to the problem found in either group, and in which 
respects they differed and in which ones not. (One would also need to consider 
teratorns, because Pelagornithidae alone are too singular to make a good 
comparison with anything)


Regards,

Eike