[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

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



"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...."

This stuff concerning total height is fraught with difficulties because
the head height of an animal really depends on its stance and posture. A
walking azhdarchid may have it's neck  held forward (like a walking
giraffe, I suppose) to move it's centre of gravity forward, but a
standing one may find more stability with the neck brought back
somewhat. When you're swinging a 3 m long neck around, that can lead to
some pretty substantial differences in height. Also, how long do you
imagine the back legs to be? Seeing as the azhdarchid neck _pretty much_
has to shoot straight from the torso, the errectness of the torso
(defined by the ratio between hind- and forelimb) is going to effect the
angle of the neck and any subsequent height calculation. Your shorter
torso, of course, will lead to an even more errect posture. Attributes
like shoulder heights are more comparable - I figure Q. northropi to
stand with a shoulder height of something like 2.5 m.

As an aside, I have my doubts that azhdarchids could stick their beaks
into the air. The only azhdarchid skull preserving the entire cranium,
Zhejiangopterus, suggests that the occipital condyle faces ventrally.
With their relatively stiff necks, the skull probably always faced
somewhat downwards. 

"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."

We've made some models of full-size monster azhdarchids, too: they
aren't very accurate but get the sense of scale across nicely. There's a
whole bunch of picutres of them being made at
http://www.flickr.com/photos/markwitton/153546729/in/set-72057594082071497/,
battling with getting the thing airborne
(http://www.flickr.com/photos/markwitton/165854970/in/set-72057594082071497/)
and, finally, hanging above the heads of visitors
(http://www.flickr.com/photos/markwitton/165854973/in/set-72057594082071497/).
This really isn't a posture to scrutinise, though: we had half-a-day to
get this and 3 other pterosaur models errected, so we couldn't worry too
much about making sure every joint was perfect. It was more a case of
'that looks all right - on to the next!'

"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 :-)"

Now THAT's an image that someone needs to draw. Just imagine what a
flock of azhdarchids could do to the paint job on your car.

"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."

Yeah, and this is, to be blunt, an issue for anyone wanting to work on
giant azhdarchids: it's something that is incredibly appealing to anyone
who wants to work on pterosaur functional morphology and biomechanics
but is extremely difficult with what is available in the literature.
Prior to making my calculations I did make attempts to see the specimens
myself, but, well, it just didn't happen. Hence, I've had to work with
what is available in print, and this may be the cause of our
disagreements over proportions.

"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."

Don't you have to make a decision at some point, though? I mean, you
can use a range of densities or tissue quantities or whatever but,
ultimately, don't you have to say "this is my preferred mass
estimate/range of estimates"? How do you decide where to draw those
lines?

"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."

Good point. 

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

Sorry, I'm not quite clear what you mean here: are your 'bits'
different tissue densities (say, muscle, fat, air space) or your body
segments? In the case of the former, I don't have a problem determining
tissue density (although, of course, density is variable across a single
tissue type - more mines there, then), but rather the confidence in
which you can model their distribution in the body of an extinct animal.
Apologies if it seems I'm beating the same drum over and over here, but
I've tried to get my head around modelling soft tissues in pterosaurs
and just can't find a way to do it that doesn't involve lots of
arbitrary decisions. I might be missing something, though. 

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

No argument there.

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

Somehow, I have reservations about that :-)"

I know it sounds a bit crazy, but I stick by it. If dry skeletal mass
is consistently related to total body mass (which it appears to be in
both mammals and birds) and we assume that fossil tetrapods bore the
same relationship, then modelling dry skeletal mass might provide a more
reliable route to estimating mass. Note that I don't doubt the _method_
of serial tissue modelling, just the confidence we can have in the
results. 

"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
:-("

A 5 m Anhaguera comes out as 11.9 kg for me (based on Wellnhofer's 1991
AMNH specimen), the same mass as a big condor. I find most
ornithocheiroids to be very light for their wingspans, a consequence, no
doubt, of their tiny bodies and hindlimbs. This is another aspect we
need to consider in scaling pterosaurs: there are a lot of differences
in body proportions that make certain groups a lot lighter than others.
It would be silly, for instance, to estimate the mass of an azhdarchoid
based on the masses of ornithocheiroids. Once we boil mass estimation
down to a reliable art, there's going to be all sorts of interesting
comparisons to do between different forms.

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