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Re: Massive answer to the mess of pterosaur mass (say that threetimes fast)



Comments inserted.
JimC

----- Original Message ----- From: "Mark Witton" <Mark.Witton@port.ac.uk>
To: <jrccea@bellsouth.net>; <dinosaur@usc.edu>
Sent: Thursday, January 17, 2008 2:41 PM
Subject: Massive answer to the mess of pterosaur mass (say that threetimes fast)



Ah, I'm afraid to admit that I'm not the tidiest artist.

Welcome to the club. I'm the founding member.

The overall height and gross proportions are roughly correct,

I didn't measure your height. A Qn would probably normally stand about 14 feet at the crest. A giraffe is what, about 17 feet? (I did a Qn/giraffe comparison sketch back about 1999, but my giraffe was bigger than yours). A Qn could most likely reach the tip of his snout about 23 or 24 feet into the air if he chose to, but I doubt if he'd usually bother.... Your torso is way out of scale with the wings -- way too large relative to the wings., which are themselves too large relative to the man. The torso size is pretty much defined by the scapulocoracoid, and the scapulocoracoid size is pretty much defined by the relationship between the glenoid, the proximal humerus, the dp crest, and the coracoid flange (which is defined by the dp crest). The proximal humerus and dp crest of Qn are preserved, so there is a very good idea of how deep the chest is. Consequently, the distance between the Qn notarium socket and the acetabulum would be expected to be roughly about 64 to 66 cm. Yours appears to be on the rough order of 88 or 89 cm. That's about 36% too long.


but I'm afraid my drawing does, as you've found, contain a bit of slop.

As I mentioned, I've been there myself. Particularly when working in the wee hours.


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).

I feel the same way about the TMM skeletal sculpture. It is very good for what it is -- but, it isn't intended to be a research tool. I know where you're coming from and identify.


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.

I'd be more worried that it might poop on me. I'm of the impression that the little suckers processed their food pretty quickly :-)

"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.

The length of the outer wing should be on the order of 8.9 feet (270 cm), which is roughly about a foot higher than the shoulder. I do consider that to be a marginal distance above the shoulder. I wasn't concerned about it because it doesn't constitute a truly significant fraction of the volume of the animal.


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.

I'll buy that, except that the linear proportions seem to range from about 1/6 to 1/3 larger than Qn would be expected to be. I'm lucky in that I do have some of the allometric ratios between Qsp and Qn, and a large fraction of Qsp is preserved. That said, I'm not at liberty to release the dimensions or photographs, so can only speak generically.


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.

Unfortunately, you seem to have assumed those elements were about 4/3 the linear size they oughta be, which might make them about 2.2 to to 2.3 times heavier than expected.


"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.

I don't have a problem with the way you approached it. Only with your dimensions, which would affect your weight.


Sure, we know that they're pneumatised and should expect all
your normal vertebrate tissues, but in what quantities?

Exactly. For that reason, I made all those tiddly bits I mentioned specifiable as percentages of the gross area in each cross section so I could easily get a feel for how much weight difference would result from different assumptions re percentages and tiddly bit densities. As we both know, in some cases it can be significant. In others, it doesn't make all that much difference.


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?

I find them to be about equally interesting for the things they did differently and the things that they did similarly. Both are informative.


Equally, what bird density do you use?

A multiplicity, for each type of tissue. Again, making the densities easily variable gives a feel for how much impact the changes have on gross weight. However, we need to keep in mind that an actual individual may vary his weight as much as about 50% during the course of a single long flight.


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.

You'd never choose "A" density anyway. You'd choose a number of densities, depending upon what bits you're working on.


I suppose you can calculate an average,

That would seem like a waste of time to me. Probably to you as well.

but then
you might risk overlooking an important adaptation towards a particular
flight- and lifestyle.

I have the impression that you feel that averaged densities are an oversimplification. So do I.


This soft tissue stuff is a minefield,

Agreed.

hence my attempt to model mass without looking at any at all.

Somehow, I have reservations about that :-)

(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.

Strongly agreed. That's why I much prefer working on Qsp and tend to carry Qn numbers along as a sort of stepchild (because Qn is what most everyone else seems to be interested in). No offense to real stepchildren and step-parents. If you raise a kid, he or she is yours -- no matter what anyone else may say. Did I mention that I strongly agree with the paragraph above? It bears repeating.


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?

Yeah. Have you done an estimate of the 5 meter span Brazilian juvenile Anhanguera piscator? I don't remember the specimen # off the top of my head. I've done a fairly detailed weight breakdown of that specimen using my method (and, like you -- filling in missing bits with guestimates from other animals), and I'd love to compare notes. Can send you my raw data if it would be of any benefit to you, and if I can still find it. I consider the weakest part of my mass estimate for piscator to be the head. Ran out of time to do the head mass estimate before I ran out of work to do on it :-(


JimC