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Yet more on pterosaur quad arm posture

In a message dated 6/27/13 3:26:25 AM, mark.witton@port.ac.uk writes:

<< 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.


This is a remarkable statement. Including how citing a list of persons as 
doing a particular thing is useless if it is none of them or others have 
backed up with proper documentation. And as is becoming increasingly clear they 
have not come close to doing. 

Do not know what the "Paul (2011) reconstruction of Q. n." is refering to. 
Aside from not remembering publishing a pterosaur illustration that year, I 
restore azhdarchids with narrow trackways. 

There seems to have arisen a strange opinion that a narrow trackway 
precludes a sprawling humerus. This is of course long been known to be 
including the Gambaryan & Kuznetsov paper this year in J Zool illustrating 
the combination of a sprawling humerus and narrow trackway in echidnas. And 
that in an animal in which the humerus makes up around half the arm length. In 
azhdarchids the radius-ulna+metacarpal length ratio is about 4. So with the 
humerus sprawling, just flex the elbow about 95-100 degrees ventrally and 
medially and you get as narrow a trackway as you want. Duh. Seriously, this 
is obvious, what is this argument about? If Witton etc saw an echnidna 
trackway they would insist it is erect limbed, but it ain't. 

To determine limb posture trackways are important, and when the trackway 
gauge is very wide it must be sprawling, but when it is narrow joint 
articulations etc must be examined to determine humerus posture. 

We know quad dinosaurs had erect arms not just because of their narrow 
trackways, but because the shoulder glenoid faces predominantly 
ventro-posteriorly, with the scapula component often facing a little medially, 
precluding a 
sprawling humerous posture (as I have published in the technical 
literature). It is not an exactly parasagital gait, the elbow is bowed out some 
(as is 
common in mammals), and the hands are more widely separated than is common 
in mammals. But it is erect, not sprawling. 

The primary function of the super specialized pterosaur arms was flight. 
That was what they were mainly adapted for. Ground locomotion was a secondary 
function that had to be accomodated to the flight adaptations. 

We have no doubt that the pterosaur humerus could sprawl because that is 
what it had to do in flight, and becasue the shoulder glenoid faces laterally 
as in sprawlers, not postero-ventrally. So far no one has shown that any 
pterosaur could position its humerus in an erect posture. Vague armchair 
assertions that a laterally facing shoulder joint will also allow an ungulate 
posture will not do. Until someone publishes a detailed diagram showing it can 
be done, or mounts a cast based on well preserved bones -- anyone want to 
point out an example? -- it is very dubious speculation at best, and very 
likely wrong. 

No to "the "skewed joint at the end of the metacarpal deflects the distal 
wing laterally when the arm is folded up." The flexion at the joint of the 
base of the wing finger had to operate in the horizontal plane during flight, 
so the outer wing could be swept back during say diving flight. If the wing 
finger base acted as W suggests then sweeping the outer wing strongly back 
during flight would also cause it to arc upwards, partly flat on to the air 
flow, which would be aerodynamic madness. That's why nothing like that 
happens with the outer wing of birds. Wellnhofer 1991 says something obscure 
the wing finger base joint causing the outer wing to twist for folding the 
outer wing so its top side faces laterally, but that does not match his own 
figure on the previous page, or on p 137.

Take a look at that last figure. And p 156 of the Wellnhofer books. Those 
quad poses get the articulations of the joints and orientations of the wing 
elements right -- except for the short fingers. Humerus. Sprawling (as also 
in flight, same as in birds). Humerus condyles for radius and ulna. Directed 
ventrally (so rest of wing can be downstroked in flight at elbow because 
humerus cannot depress much below horizontal -- the lack of down flexion in 
Wellnhofer 87 p 153 is way off, as is the flexion at the wrist -- also 
streamlines joint, same as in birds). Radius anterior and maybe a little medial 
ulna (presents minimal profile to airflow when flying, same as in birds). 
Slender metacarpals anterior to stout one (also maximizes streamlining by 
keeping leading edge of hand directed anteriorly as in birds). Folded outer 
directed posteriorly to clear body, and in same plane as 
radius-ulna-metacarpus (basically same as in birds). 

Some of you may be bouncing off walls saying but the trackway gauge is wide 
in the figures. Stop, calm down, and think about it. Be Zen. Note how the 
wide hand gauge is because the humeri are directed somewhat dorsally (as 
during the beginning of the flight upstroke), while the elbow flexion is about 
90 degrees, so the arms slope down and out, placing the hands laterally for 
no particular reason. Here's how to solve the problem. Simply depress the 
humerus about 20 degrees until it is horizontal, and/or flex the elbow more as 
needed, swinging the hands medially until they are nearly on the midline. 
There you go, a sprawling humerus with a narrow trackway. Ta-da! While at it, 
rotate the humerus along its long axis so the rest of the arm is more 
vertical rather than directed strongly forward as shown in both figures, and it 
ready to walk, baby. 

Also change the orientation of the fingers. They are shown directed 
forwards, but that cannot be correct because the distal line of the small 
metcarpals runs antero-posteriorly (this is like trying the wrench the fingers 
your left hand hard to the right, on the same plane as the metacarpus and 90 
degrees relative to their true extension-flexion arcs - ouch!). With the 
fingers correctly extending dorsally relative to the plane of the metacarpals 
they will splay laterally.  

Here's another way to look at it. Take the two sprawling quad figures. Keep 
the radius-ulna-metacarpus complex articulation and orientation realtive to 
the body exactly the same. But depress the humerus a little until it is 
horizontal. And extend the wing finger. You now have the flight downstroke 
(with the elements presenting minimal frontal profile). Which we know worked 
just fine (do the same with the bones from your handy dandy bird wing, they 
have the same articulations and orientations in the downstroke). Now, refold 
the wing finger, and you have the basic quad pose, except flex the elbow a 
little more (remember, it can flex until the radius-ulna are nearly folded 
against the humerus as in birds and us, so getting the walking posture is easy) 
to get that narrow trackway we all know and love. Neat, isn't it. All that 
has to then be done to walk on the arm is to rotate the humerus on the 
shoulder joint (presumably rolling it along its long axis [also used to vary 
wing's angle of attack relative to the body in birds and pterosaurs according 
to Wellnhofer] and perhaps fore and aft horizontally some [used to help 
vary wing sweep]) to achieve a stride, pretty long because the 
radius-ulna+metacarpal is so long so only a little motion of the humerus will 
do it. There 
is little action at the elbow. It's all so easy, no fuss, no bother, using 
the same basic bone posture and muscle actions as in flight for quad walking, 
no strange if not impossible articulations or rotations. Because the same 
muscles are used for walking as for flapping, no need to have distinct walking 
versus flying muscles that add weight. 

If we instead try swinging the humerus back approaching 90 degrees (in 
dorsal view) into an ungulate posture, aside from probably disarticulating the 
shoulder joint, the outer wing is going to be directed right into the body, 
unless there is the improbable rotation at the elbow or along the long axis 
of the radius-ulna that look locked together along their shared plane (these 
are not feasible in birds, chances are low that pterosaurs could do them) 
that is undocumented arm chair speculation. The erect arm idea is directly 
self contradictory. The wing finger cannot collapse less than about 25 degrees 
tight on the metacarpus. So the more tucked in the eblow, the more the outer 
wing jabs into the body. And what for? Why have the arm for some mysterious 
and illogical reason going all erect what with the outerwing banging into 
the body and adding weight for muscles that work the humerus fore and aft and 
what all. That when keeping the humerus parsimoniously exploits the flight 
posture and avoids also those problems. Anatomically probably impossible and 
does not make functional sense. 

Take a gander at the photo of the complete Santanadactylus wing p 125 
Wellnhofer. It shows the arm in flight posture in dorsal view. If the winger 
finger is swept back nearly 90 degrees it will remain in the same plane, 
pointing directly down in the photo, no? The photo also shows the arm in close 
posterior view if the humerus is assumed to be in an erect walking posture 
(particularly the ulna-radius-metacarpus, there would be some flexion of the 
elbow). So what will happen to the wing finger when it is folded? It pokes 
right smack into the middle of the body, doesn't it. Anatomical nonsense, 
agreed? And there is no way to swing the folded outer wing outwards to clear 
body by radius-ulna rotation or other joint rotations in such a locked up 
system. How to get the outer wing to fold posteriorly? Easy. Flex the elbow 
about 90 degrees, swing the humerus laterally until it is sprawling, and the 
folded outer wing is clear of the trunk, right? 

Regarding Bennett's 97 restoration of pterowalking. It's wrong. Bill says 
he did not attempt to restore the actual joint action in the paper, so his 
posture is intuitive not scientific. The first clue that something is not 
right is that with the humerus erect the wing finger is folding on the 
outside(!) of the arm, which is opposite of what would actually happen. Why the 
degree error? Because the proximal end of the radius in both pterosaurs 
figured is shown articulating with the ulna condyle of the humerus. (There is 
way to get the left radius so far medial with the left humerus in the erect 
posture shown. Get your bird bones and try it. To get the radius in 
something close to that position the humerus has to be nearly horizontal and 
directed very strongly anteriorly, with the distal humerus condyles directed 
medio-ventrally posteriorly, all of which is impossible. In the figure the left 
elbow is also hyperextended well beyond the articulations. To get the right 
radius in the exact position and orientation shown requires having the humerus 
directed nearly vertically upwards. I'm not kidding, get your bird wing 
bones and ye shall see.) If the proximal ends of the radius and ulna are 
articulated properly the outer wing is directed inwards and banging into the 
belly. Also incorrect is the articulation of the short fingers, they are 
90s degrees relative to their actual position on the metacarpus so the palm 
faces medially as in the trackways. With the orientation of the radius, ulna 
and metacarpus as shown in the fingers, the fingers would be splayed 
anteriorly. To actually get the fingers splayed out laterally as per the 
the radius needs to be anterior to the ulna, with the slender metacarpals 
leading the stout one. That happened automatically with, and only with, the 
humerus sprawling. Also incorrect is the orientation of the wing finger 
relative to the metacarpus. In the view shown in the figures we should be 
the leading edge directed laterally, towards the viewer. Instead it is 
directed posteriorly (which gives the illusion of proper outerwing folding when 
walking), and that would again actually happen only with the radius and 
slender metacarpals on the leading edge during walking which requires a 
humerus (in case you have not gotten the message yet).  

The fact is that most folks have not been paying into to the details. Some 
have posed the wing elements pretty much properly and have the humerus 
sprawling as it should be, but ignored the trackways so the elbow is too 
and the hands separated too far (a common flaw of quad mounted skeletons). 
Many have been paying hardly any attention to  how the joints articulate and 
how the arm must work as a wing and assuming a narrow trackway demands an 
erect humerus when a little geometry and some modern animals proves 
otherwise. I would do something about this, but it is a major project that 
include obtaining casts of the Santanadactylus arms (cannot even get the 
Leonardi & Borgomanero 87 paper throught the JHU system). What should happen is 
that I should be the reviewer of all papers on the subject of archosaur 
articulation since I have been at this so long and have a better ability to 
mentally visualize functions in 3-D than most it seems (have already shot down 
couple of bad articulation papers in peer review for major bio journals). That 
would eliminate a lot of nonsense. Seriously, send the manuscripts to me 
and I'll determine if they make sense or not. 

So if one wishes to continue to put ungulate posed arms on pterosaurs, get 
out there and show how it actually works with nifty detailed diagrams 
including of the joint articulations and/or a mounted cast. With the joints 
actually articulated as they should be. Preferably in the technical literature. 
And when it does not work out and you instead publish a paper showing the 
humerus was sprawling as the hands were nearly on the midline, then be sure to 
credit my correct illustrations. There has been way too much failure to 
credit priority of late.  

By the way, as noted by Wellnhofer pterosaurs could not elevate the humerus 
vertically as is normal in modern flying birds. It is very likely that most 
if not all pterosaurs had good climbing flight abilities. So the inability 
of basal dinosaurian fliers like Archaeopteryx and Microraptor to fully 
elevate th humerus does not necessarily refute their ability to be good powered 
fliers. Contemplate it.