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Massive answer to the mess of pterosaur mass (say that three times fast)



Hi folks (nods to Jim),

"Mark, I've taken a quick look at your Qn-giraffe drawing, and assuming
the human height at 1.75 M, have made a few rough measurements on the
drawing. I was only able to measure two of the three dimensions due to
foreshortening into the plane of the drawing, so your dimensions are
actually a bit larger than I recite here."

Ah, I'm afraid to admit that I'm not the tidiest artist. The overall
height and gross proportions are roughly correct, but I'm afraid my
drawing does, as you've found, contain a bit of slop. At one point in
its life it was very nicely measured and neat, but repositioning limbs,
erasing dodgy bits of shading and my impatience took their toll on
proportions overtime over time. The piece was never intended to be a
diagram: it's was just meant to show how big a 10 m span azhdarchid
really is in a more appreciable way than the typical 'crucifiction'
diagrams we see (you know, the ones with silhouettes of wingspread
pterosaurs positioned next to a man showing how big his last fishing
catch almost was). In my view, the sense of scale is much greater when
the pterosaur towers over people and looks like it could, if it wanted
to, eat the unlucky guy conscripted into standing next to it in the name
of science.

"PhIV-1 and outer wing are short, but I didn't measure them in the
drawing."

This is based on skeletal reconstructions I've done of standing
azhdarchoids and azhdarchids. So far as I can make out, the flexed wing
fingers only poke marginally above the shoulder in tapejarids and don't
poke above the height of the shoulders at all in azhdarchids. 

"For talking purposes only, lets just take a quick average of only the
humerus and r/u volumes 0.5*(1.73+1.52)=1.625 , ignore the larger torso
discrepency, and assume the whole thing is large by that 1.625 
"average" amount, which would imply a mass of 250Kg/1.625=154Kg. 
Probably by coincidence, this is fairly close to the guestimate of 150Kg
that I usually use for Qn, based on the wing area and optimal lift
coefficient for the aspect ratio that I use.  Needless to say,
mathematical rigor is sorely lacking in the numbers I used above, so
what I've just said is strictly for talking purposes -- but, in short; I
think your 250Kg estimate may be a bit heavy."

Two things here: firstly, some of my measurements are based on
Zhejiangopterus rather than Quetzalcoatlus sp. - sadly, Zhejiangopterus
is better described that Quetzalcoatlus (anyone familiar with the
Zhejiangopterus literaure will appreciate how desperate this case is),
and I needed a complete list of osteological measurements for
calculating the mass. Hence, I had to resort to Zhejiangopterus to fill
the many gaps not covered in the Quetzalcoatlus literature. Where
possible, though, my measurements for the Quetzalcoatlus estimate came
from the real McCoy. I guess my 'Q. northropi' might be better labelled
as 'generic azhdarchid of Q. northropi proportions', then. 

Secondly: Changing the proportions of the torso may not change the
results as much as it would using 'traditional' methods of determining
mass. My method of calculating mass involved geometrically modelling
simplified pterosaur skeletons - breaking them down into simple shapes
like cylinders, cones, frustrums and prisms and the like - a typical
pterosaur would have something like 70 or 80 consituent parts. Bone
histology and the like would then be factored in to account for
pneumaticity and medullary cavities, total skelelton mass would be
calculated, and then a regression analysis performed based on the neat
little relationship between skeletal mass and body mass reported by
Prange et al. (1979). As you might expect, my pterosaur torsos feature
seperate calculations for each compenent - scapulocoroacoirds, sternums,
ribs and what have you. Hence, shortening the torso would only really
involve reducing the size of the dorsal vertebrae: this doesn't affect
the weight very much because, relatively speaking, they add very little
mass to the overall skeletal mass. To really knock the mass of the torso
down, you need to attack the size of the big elements in there:
sternums, shoulder girdles and all those other chunky bits.

"As an aside, when I do pterosaur mass estimates, I take a series of
cross sections, actually distribute each of those sections into
proportions of bone, fat, muscle/ligaments/tendons, air sacs, marrow,
voids, lung tissue, skin, etc. and apply seperate densities to each."

We discussed doing something similar here but decided against it on
grounds that we really don't know what's going on with pterosaur soft
tissues. Sure, we know that they're pneumatised and should expect all
your normal vertebrate tissues, but in what quantities? We could
extrapolate some data from birds or bats, I suppose, but how confident
can we be about that? No extant birds or bats are anywhere near as big
as even modestly proportioned pterodactyloids, so whose to say that they
didn't do things differently? Equally, what bird density do you use? As
bird body density is related to flight style, you can't chose a bird
density for your pterosaur without setting a bias in any subsequent
flight calculations. I suppose you can calculate an average, but then
you might risk overlooking an important adaptation towards a particular
flight- and lifestyle. This soft tissue stuff is a minefield, hence my
attempt to model mass without looking at any at all.

"When I do so, I find that the all-up weight of a pterosaur tends to be
approximately similar to that of an albatross with the same wing span
(which isn't really all that much of a coincidence, aerodynamics and
atmospheric physics being what it 
is....)."

Hey, funnily enough, I find similar things with my smaller pterosaurs:
their masses tie pretty well to equivalently sized birds. It's these
smaller ones that we really should be talking more about, actually:
working on giants like Q. northropi is all very fun and impresses the
girls, but any conclusion we may draw them is based on extrapolation and
theory - it's much safer to work with animals that we have complete
datasets for. With that in mind, I'll point out that a big Pteranodon (6
m wingspan) in my mass estimation comes out at 36 kg: still a lot higher
than most published estimates but a figure that I'm much, much more
confident about than any currently available for our giraffe-sized
friends. This figure, incidentally, refers to a Bennett 2001-esque
Pterandon as opposed to all the Eaton 1910-based forms normally used in
mass calculations. Any thoughts?

Mark

--

Mark Witton

Palaeobiology Research Group
School of Earth and Environmental Sciences
University of Portsmouth
Burnaby Building
Burnaby Road
Portsmouth
PO1 3QL

Tel: (44)2392 842418
E-mail: Mark.Witton@port.ac.uk