Re: Wilkinson's pterosaur wings (long)

[MICHAEL HABIB <habib@jhmi.edu>]

>  I just happened upon Matt Wilkinson's latest paper in the Journal of 
> Evolutionary Biology (details below). 
>  Have not had time to read it in detail (pedagogical duties), but it 
> seems to be an excellent piece of work
>  and the clearest exposition of the basic mechanics of pterosaur wings 
> published so far. 

I had a chance to read this paper quicky last week, and am going over it again. 
 Overall, I think the paper has a lot of good things going for it, and it is 
quite well written, in my opinion.  Matt is generally very articulate (and a 
good guy, to boot), and that comes through in the paper.

That said, there are a few things I would question in the paper (though, 
admittedly, most are not that important to the major thrust of the work):

1) I am still skeptical that the pteroid could actually sustain the bending 
loads it would experience in the anteriorly-facing, fully extended position 
that Wilkinson (2007) prefers.  I would like to see a quantitative test of 
pteroid strength in conjunction with the proposed model.  This is actually my 
most pressing concern with the pteroid and propatagium configurations proposed 
in the paper.

2) While the pteroid and propatagium do seem to be important (Wilkinson makes a 
good case for that, and I would have expected it to be to begin with), I do 
have a feeling that the poor observed performance of the experimental wings 
lacking propatagia stems largely from using poor approximations of pterosaur 
wings.  The importance of the pteroid could easily be exaggerated if the rest 
of the wing is poorly modeled, structurally.  Wilkinson certainly addresses the 
problems with the latex rubber wing model, but I was not entirely clear on what 
the specifics of the structure in the wing model used by Wilkinson (2007).  

3) The same sort of issue as in #2 applies to the model with the horizontal 
pteroid position.  The poor lift/drag ratios measured for that configuration 
might be a result of modeling the propatagium, and especially the wing spar 
posterior to it, as too thin a structure (and thus generating inboard 
separation at low angles of attack).  Again, just a guess, but one worth 
looking into in more detail, I think.  Still, I think the argument is strong 
that the propatagium was somewhat deflected during flight (though I suspect it 
was narrow, rather than the wide propatagial configuration favored in the 
manuscript.  To Wilkinson's distinct credit, he does address both 
possibilities).

4) I was skeptical of a few points made on page 1669, but largely out of 
differences in personal hypothesis:

-For example, Wilkinson (2007) states that "all pterosaurs must have become 
transiently bipedal to enable deployment of the wings prior to take-off".  This 
need not follow, in fact.  It could be that this is how pterosaurs launched, 
but I seriously doubt it.  There is substantial evidence to suggest that 
pterosaurs utilized a leaping, quadrapedal launch dominated by the forelimbs 
(which as been discussed here previously, especially by Jim Cunningham).  The 
evidence isn't conclusive, of course, but it does counter the claim that there 
is no viable alternative to a transient bipedal launch with early wing 
deployment.  Furthermore, Wilkinson's own comments regarding the problems of a 
bipedal, running launch cycle follow immediately after the above statement, 
which (in my opinion) simply furthers the likelihood that a running launch was 
not the rule for large pterosaurs.

-On the same page, Wilkinson (2007) suggests that pelagic pterosaurs would be 
able to utilize gravity assisted launches from cliffsides.  While I agree that 
this might have been available some of the time, it seems extremely unlikely as 
a general rule, given terrestrial pterosaur trackways and the fact that only 
two modern groups of birds are really obligate cliff-launchers (frigatebirds 
and tropicbirds) and neither is very large by pterosaur standards (they both 
also have reduced launching power in the hindlimbs, and hindlimb power is 
largely launch-limiting in birds, though probably not launch-limiting in 
pterosaurs).  The upshot is actually that Wilkinsons' comments on 
Quetzalcoatlus should really extend to all large pterosaurs (which isn't really 
a counter to his points at all, but rather means that his comments on 
level-ground launching should extent more broadly).

-Obviously, as most of you here know, I'm not a fan of the broad-chord wing 
hypothesis for large-bodied pterosaurs.  That's a whole other discussion at 
some point, however (and will come up in Munich, and possibly SVP as well).  
Incidentally, Matt told me at SVP that he wasn't particularly attached to the 
broad-chord model, but merely that he used it because it seemed more accepted 
when he was running his model, which seems fair enough to me.

-Lastly, and it's a very minor point, Wilkinson references the 
"ultra-lightweight skeletons" of giant pterosaurs as a means of reducing wing 
loading.  To an extent this is accurate; large pterosaurs do obviously have 
thin-walled bones with heavy pneumatization.  However, like birds, pterosaurs 
seem to have just as much bone mass per total body mass as non-volant species.  
Just as in birds, the lightweight construction of the bones does not so much 
reduce total weight as much as allow for very strong bones, with a high 
skeletal volume, without *adding* a lot of weight.  It is more an adaptation 
for not being super heavy than for becoming super light.  The main reason I 
point this out here is that there are other comments in the paper, surrounding 
the discussion of launch and landing, that suggest an overall tendency in 
pterosaurs to reduce loadings and stall speeds (and that the deflected 
propatagium would further help).  In actuality, I think Wilkinson may be 
selling himself
 short a bit: the trend seems to be towards *high* wing loadings in most 
pterosaurs, as best I can tell (which is expected for the pelagic species, 
especially).  This would make the generation of high lift coeficients during 
landings more important, and actually extends the utility of the 
pteroid/propatagial model that Wilkinson describes.


Overall, these are mostly minor points.  The only truly pressing issues are the 
structural abilities of the pteroid, the composition of the wing, and the shape 
of the anterior wing posterior to the propatagium.

Cheers,


--Mike H.