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

More on azhdarchid height and quad arm posture

Back in 
 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 
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 
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 
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 
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 
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. 
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 
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 
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, 

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 
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 
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 

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 
and messes up the aerodynamics and action of the wing, the latter should be 

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 
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 
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 
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.