Re: Massive answer to the mess of pterosaur mass

[MICHAEL HABIB <habib@jhmi.edu>]

Hey everyone,

Some comments on the pterosaur mass issue (specifically the Quetz. mass issue). 
 I think Mark's mass estimate technique has a lot of merit, and I have run some 
launch calculations with his 250 kg Qn that came out reasonably well.  I have 
tended to prefer a slightly lower mass (closer to 200 kg) myself, but that 
margin is actually a lot smaller than it seems.  By the same token, my mass 
estimate and Jim's are also not actually so far off as they might seem, given 
how much estimate error there can be.

I do think that Jim's estimate is a bit on the low side; he and I have actually 
discussed this a bit already, and our disagreement in this regard is minor in 
the big scheme of things.  One reason that I find a greater mass (around 200 
kg) is because my planform has a somewhat lower aspect ratio and slightly 
greater area. Thus, my version is more optimal at a greater mass.  I also note 
that extrapolating a mass of Quetzalcoatlus from the span:mass regression on 
Greenewalt's bird data (with most passerines removed for several reasons) 
yields a mass estimate of just over 200 kg.  Calculating the mass of Qn such 
that the span loading is the same for a wandering albatross also gives me a 
mass of 200kg, using the albatross data I have on hand.  I actually might 
expect that to be an underestimate because the span loading is probably a bit 
higher for large pterosaurs than large birds, based on both build and 
allometric effects.  The caveat there is that body weight in long-distance 
travelers like albatrosses varies a great deal just within individuals.  Thus, 
while I get about 200 kg using the average, the estimate can swing a great deal 
using a heavily fueled versus skinny set of animals.

I do think that the torso may be a bit larger than Jim gives credit for, though 
perhaps I'm just reading into his analogy of a large football player too much.  
While the torso was quite short, my estimates suggest that it should be quite 
deep, as well, and the overall dimensions of the pectoral girdle would be 
impressive, to say the least.  The humerus is truly massive; anyone who has 
seen it on exhibit (or up close) knows what I mean.  That one element would 
almost fill my thorax and abdomen (though I grant I'm a small person).  The 
scapulocoracoid would need to be quite deep, and the flanges quite expanded.  
The precise allometry is difficult to calculate, but my rough estimate is that 
the football player would have to be a very good sized linebacker at least :) 
Add in the huge limbs and giant neck and head, and you have a pretty darn big 
critter.

I've also started toying with some bone cross-section techniques for estimating 
mass, but this is harder than it sounds because we have little a priori reason 
to assume a specific safety factor for pterosaurs.  It was probably near that 
of birds, but we cannot say for certain.  Interestingly, my first swing at that 
gave me a (admittedly rough) mass of about 220 kg.  The interesting thing there 
is that the technique would be expected to give the maximum (ie. heavy fuel 
load) weight.  As such, I wonder if perhaps Mark's method does the same, and 
that our estimates are more congruent than they seem (with 250 or thereabouts 
being the maximum fueled mass for a large animal).

Regardless, the real punchline, I think, is that these large pterosaurs were 
not the "ultralights" that is often expected.  Clearly, they were reasonably 
heavy animals with plenty of power generation.  This holds whether Qn was a 150 
kg animal or a 250 kg one.

Cheers,

--Mike H.