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Re: More on azhdarchid height and quad arm posture



Rescued from the truncation demons...

"Increasing the size by just a
quarter gets it large enough to look reasonable on Q. n. So, for artists, if
you have to restore Q. n., it is very ill advised to use the gracile skulls."

In your opinion. We have no idea what the front end of Quetzalcoatlus
northropi looks like, and evidence for robust and gracile necks +/-
skulls in other giant azhdarchids. Anything goes, basically. As the
current taxonomy stands, Q. n is considered more closely related to Q.
sp. than any other azhdarchid, so the smaller Quetz is the most
sensible model. As I said in my last post on this topic, we're really
waiting on the taxonomy of these animals to be examined in detail, and
making claims like these, and others about neck length, beforehand is
pure speculation.

"The Witton pose is very probably wrong."

Pterodactyloid trackways show that their limbs are held in
parasagittal poses, and those of azhdarchids seem to drop almost
vertically from the body, as depicted in my 2008 illustration and
subsequent works. I'm not the only person to illustrate and describe
their postures in this way: Chris Bennett (1997), David Unwin (1997,
2005), Hwang et al. (2002), Mazin et al. (2003) and many others have
done the same. Presumably, the skewed joint at the end of the
metacarpal deflects the distal wing laterally when the arm is folded
up. Thus, I'm happy that my reconstructions are more-or-less
consistent with this idea. The idea that pterodactyloids had sprawled
forelimbs, akin to the Paul (2011) reconstruction of Q. n., is not
supported by any trackway data.

Mark

On 26 June 2013 23:00,  <GSP1954@aol.com> wrote:
> Back in
> http://pterosaur-net.blogspot.co.uk/2010/02/tmm-42489-2-hypersonic-uberbass-slide.html
>  Witton did what we were all doing, assuming that since
> both the giant Q. n. holotype and half sized Javelina specimens were
> Quetzalcoatlus, that the giant had the same gracile skull form. This is very
> improbable, there are differences in the wing elements in the two Javelina
> creatures that imply they are different genera, and the gigantic 
> Hatzegopteryx that
> may be the same taxon as Q. n. apparently has a stout skull.
>
> Because the stout Javelina skull is not much longer than that of the medium
> sized gracile skulls it seems to be not much larger, but that is an
> illusion because the beak before the combined nasal antorbital opening is 
> shorter
> than in other azhdarchids. And the rostrum is far deeper. When I restored the
> skull using Zhejiangopterus as a guide for the back end, it was large for
> the medium sized Javelina skeletons even by pterosaur standards, so the stout
> skull probably comes from a larger taxon. Increasing the size by just a
> quarter gets it large enough to look reasonable on Q. n. So, for artists, if
> you have to restore Q. n., it is very ill advised to use the gracile skulls.
> Best to go with the robust snout until better info comes along who knows when
> (odd that no more material I’ve heard of has been produced from the
> Javelina of late). Because the cervicals that are said to go along with the 
> stout
> skull are not out and about there is no good way to scale it up to the Q. n.
> holotype. May have been changes in proportions in any case.
>
> There is considerable variation in relative neck length in azhdarchids.
> There is no particular reason to presume that Q. n. had as long a neck 
> relative
> to the body as in some other azhdarchids, the strong possibility of a stout
> skull suggests it has a neck/trunk length ratio at the lower end of the
> azhdarchid range.
>
> As for the height of the shoulder, and the posture of the humerus during
> quadrupedal locomotion, the recent tendency of restoring an erect arm posture
> has been irking me, so I got out my trusty pigeon skeleton and had a gander.
>
> When birds and presumably pterosaurs fly, the wing elements are oriented to
> minimize drag, presenting their slenderest aspect to the airflow. So the
> elbow joint is oriented with the radius-ulna condyles pointing ventrally,
> keeping the frontal profile of the elbow joint minimal because the distal end 
> of
> the humerus is broader transversely than it is antero-posteriorly. Because
> the radius condyle is lateral to that for the ulna, when the humerus and
> lower arm is in flying posture the radius is in front of the ulna, so the 
> plane
> of the radius and ulna that cannot rotate much along their long axis is
> correspondingly close to horizontal, and the same follows for the wrist and
> hand elements. That keeps the overall plane of the arms as thin as possible to
> the airflow of course. Because the arm is directed laterally this is a
> sprawling posture. When birds flap their wings downward, the humerus does not
> drop vertically into an erect posture. That’s because the dorso-laterally
> oriented shoulder glenoid prevents the humerus from dropping much below
> horizontal. So the wing flaps downward mainly at the elbow joint, which is 
> easy
> because the large radius-ulna condyles are directed ventrally. During this 
> action
> the planes of the radius-ulna and hand remain edge on to the airflow.
>
> It is emphasized that having the radius-condyles directed ventrally both
> keep the frontal profile of the wing bones as streamlined as possible, while
> allowing the wing to be flapped downwards at the elbow.
>
> No one as far as I know is has done a detailed articulation diagram of a
> pterosaur arm in quad posture, one with those nifty overlaying joint
> articulations. Or articulated a well preserved 3-D set of casts in this pose. 
> Which
> is a problem. (There are casts of the medium sized Javelina azhdarchid arms.
> But during the QN project they could not be articulated into a logical wing
> posture, suggesting they have been distorted, are from differing specimens,
> or something along those lines.)
>
> Witton and company are restoring the arms of ground moving azhdarchids as
> follows (see fig 8 in Witton and Naish 08 PLoS ONE and assorted life
> restorations). The humerus posture is like that of ungulates, erect, nearly
> parasagittal, sloping down and back, with the radial-ulnare condyles 
> apparently
> directed ventro-anteriorly. The plane of the articulated radius and ulna is 
> also
> close to fore and aft, roughly paralleling that of the midline plane of the
> body. Same for the main wing metacarpal. That in turn allows the wing
> finger to fold directly aft of the radius-ulna and metacarpus, along the same
> plane, rather like in birds when the hand is folded along the radius-ulna.
> That’s a good thing, because you wouldn’t want the wing finger to be directed,
> say, medially, because the outer wing would then be jabbing into or banging
> against the body and that would be bad all round. All the more so because the
> winger fingers does not seem to have been able to totally collapse against
> the metacarpus. So if the wing finger were directed medially during ground
> locomotion that would be, as they say, awkwwward. Right?
>
> The Witton pose is very probably wrong.
>
> For one thing, is it really possible to position an azhdarchid humerus in
> an erect, ungulate like posture? Certainly is not with birds. Now, I agree
> with the research that I peer reviewed that joints of living animals often
> have somewhat more mobility than dry bone manipulation may seem to indicate.
> But when I try to put a flying bird humerus in the ungulate like pterosaur
> posture of Witton there is no way. The latero-dorsally facing shoulder glenoid
> won’t come close to allowing it. (So when birds fold their wings, the
> humerus does a peculiar roll along its long axis so the normally ventrally 
> facing
> surface rotates to face laterally, don’t know if anyone has worked out how
> pterosaur wings may have folded in detail.) So the arms of flight capable
> birds are always sprawling, they cannot go erect. Pterosaur shoulder joints
> are not identical to birds, but I am very skeptical that the humerus could act
> like that of an ungulate, someone needs to do proper diagramming of the
> posture with 3-D preserved bones to show it is practical, assuming no one has
> published such results.
>
> But, let’s say for the sake of discussion that it is possible to get the
> humerus to act like that of an antelope. Now the Witton posture is feasible,
> right?
>
> Now it’s the elbow that is the problem.
>
> The condyle for the radius is of course lateral to that for the ulna. In
> birds, the radius condyle is somewhat proximal to that for the ulna, ns part
> to facilitate wing folding via the radius pushing on the medial carpals which
> is not relevant here. From what I can see on a photo I have of the Q. n.
> humerus, the placement of the radius condyle relative to that of the ulna is
> not as proximal as it is in birds, presumably because pterosaurs did not fold
> their wings the way birds do.
>
> So, if the pterosaur humerus is in a ungulate like posture when
> quadrupedal, with the radius condyle lateral to that of the ulna, then the 
> radius in
> markedly more lateral to the ulna, and the plane of the radius-ulna plus 
> metaca
> rpus is then not longer parasagittal, but is much more transverse, so the
> wing finger folds medially, which means that the outer wing is banging into
> the body which looks like a bad thing. This is how the articulations work in
> birds if you go ahead and put the humerus into an ungulate pose.
>
> If the plane of the radius-ulna-metacarpus-wing finger is to be kept
> parasagittal when the humerus is erect, then the proximal radius must be 
> strongly
> anterior/proximal to the proximal ulna at the elbow joint. This does not
> seem possible (requires completely disarticulating the elbow joint of birds 
> and
> very probably pterosaurs) and I have not seen detailed illustrations
> showing it would work. That is logical because if this articulation were true,
> then either the distal humerus condyles would have to face forward during
> flight to keep the radius ulna flat on to the airstream, which would maximize 
> the
> drag of the elbow joint and prevent the wing from being flapped downwards
> at the elbow, or the distal humeral condyles would face ventrally to avoid
> those two problems, but that would put the radius-ulna, metacarpus and wing
> finger flat on to the airflow which is aerodynamic madness. Or the elbow joint
> would have to allow the rigid radius-ulna to roll along their combined long
> axis up to 90 degrees at the elbow joint which also is not possible in
> birds and presumably pterosaurs. Or, the radius and ulna would have to be
> significantly crossed along their long axes permanently, which is not likely
> either, have not seen diagrams showing that that happened, and that would have
> meant maximal drag at the elbow during flight. Or through active long axis
> twisting
>
> So, as far as I can tell, the Witton way is wrong one way or another. If
> the humerus is going to be posed the way it is in a giraffe, then the wing
> finger should be folded medially, with the outer wing tangled up with the 
> body.
> Or, if the wing finger is going to be folded aft, then the humerus has to
> be strongly sprawled laterally. Because the latter is also entirely
> compatible with the shoulder joint, and the former is very dubious 
> articulation wise
> and messes up the aerodynamics and action of the wing, the latter should be
> correct.
>
> Because the short humerus is so short relative to the very long
> radius-ulna-metacarpus in azdarchids, and the elbow can easily flex 90+ 
> degrees
> ventrally, the hands can be nearly on the trackway midline, giving the false
> impression of erect arms. Since do not have access to good azhdarchid casts is
> difficult to parse out the details. Such as whether the humerus was probably
> swept back somewhat rather than held exactly straight out. How much of the
> lower arm action was achieved by stroking the horizontal humerus fore and aft
> on the horizontal plane versus rotating the humerus along its long axis.
> Finger 3 seems to be directed aft in trackways, that looks like the way it 
> works
> when the wing finger is folded aft rather than medially, hard to be exact
> about it since articulated pterosaur hands are usually flattened and I have
> not seen a detailed description of the finger joint articulations in
> pterosaurs. It is possible that pterosaurs did not run quadrupedally, may 
> have gone
> bipedal to do that.
>
> But wait, there’s more! By keeping the humerus sprawling both when flying
> and when quadrupedal, pterosaurs would have been fairly conservative in using
> the same muscle groups in a fairly similar manner in both modes of
> locomotion. The much more radical switching from sprawling flight to erect 
> walking
> and running would have been inefficient and functionally problematic at best.
>
>
> A body posture advantage of the horizontal humerus is that it lowers the
> shoulder in azhdarchids. This is optimal because even just the
> radius-ulna-metacarpus length was considerably greater than that of the leg, 
> so the body
> would not be as awkwardly tilted upwards. That would have made it easier for
> the beak tip (especially a short one) to reach the ground.
>
> So with a horizontal humerus, and probably a relatively short neck, 150-250
> kg Q. n. was probably markedly shorter than your 1000-2000 kg giraffe.
>
> GSPaul
>
> </HTML>



-- 
Dr. Mark Witton
www.markwitton.com
Palaeobiology Research Group
School of Earth and Environmental Sciences
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