Re: sauropod arm articulations

["Matthew Bonnan" <mbonnan@hotmail.com>]

It's been a long day, but I'll see what I can answer now regarding my new 
sauropod manus paper.

G Paul:

"In the latest JVP Matt Bonnan restores the articulation of the arms of
sauropods, with special emphasis on the elbow in which he argues that the 
radius is
positioned more medially than normal. This got me curious so I got out the
figure I published of the elbow articulation of Brachiosaurus in the Dinos 
Past 7
Present vol 2 back in 87. Lo and behold my articulation diagram is
essentially identical to Bonnan's in fig 9B of Apatosaurus. I used the same 
articulation
because the position of the olecranon process relative to the medial and
lateral articulations of the ulna for the humerus forced it. Also because 
this
pronated the hand as per Bonnan's observation."

Greg, I have always been envious of your artwork, and in fact that 
particular Brachiosaurus forelimb you have in the 1987 paper inspired me to 
illustrate the Apatosaurus forelimb in the same style.  But beyond this our 
two articulations and outcomes are not really all that similar.  For 
example, your illustration clearly shows the radius crossing the ulna, for 
instance, where I have shown by physically articulating sauropod skeletons 
that this cannot be the case.  You can see this well in the photograph of 
the Camarasaurus forelimb articulated.

"The main difference between the over all pose of the arms is that Bonnan
rotates the entire arm along its long axis so the anterior surface faces 
more
medially. This would seem to create problems in flexing the arm joints 
during the
recovery stroke, but it fits the trackway evidence."

Well, as I point out in the paper, the long axis of forelimb would be 
rotated ~30 degrees because the coracoid curves inward at this angle and, 
with it, the glenoid shoulder socket.  Therefore, if the scapular blade lies 
against the ribcage, the forelimb is "automatically" rotated into this 
position.  As far as flexing the arms joints during the recovery stroke, if 
you saw Ray Wilhite's excellent Romer Prize talk at SVP last year, or saw 
his dissertation on-line or his Pal. Electronica paper, you see that he also 
has a similar forelimb arrangement (forelimb rotated inward) but that this 
seems to work just fine.  Phillip Platt, the creator of  the model I refer 
to in my paper, has also come to a similar conclusion independent of both me 
and Ray from his own tinkering.  In fact, considering that the ribcage 
expands as you follow the scapcoracoid back from the sternal region, it 
almost "makes sense" that the arm would swing posterolaterally instead of 
being resticted to a purely posterior movement which might drive it 
(especially the elbow) into the ribcage.

"Bonnan argues that sauropod wrists could flex significantly, but to a 
limited
degree."

Well, of course in the paper I don't just argue this, I actually show a real 
sauropod radius articulated with a real carpus and manus.  And, having 
examined the articulations between the carpus bones and the radius and ulna 
in various neosauropods, I can't help but conclude, even with cartilage and 
soft tissues present, that sauropods could really flex their manus more than 
30 to 40 degrees.

"I wonder about this because the horse wrist can flex only a few
degrees with everything in proper articulation, but up to 120 degrees during 
the
recovery stroke, and almost 180 when folded up while lying on the ground.
Massive disarticulation of the carpals is involved."

Greg, I have not had an opportunity to do what you suggest here with a 
horse's wrist, but I have difficulty accepting that wrist mobility in horses 
literally involves massive disarticulation of the carpals.  I'm really 
curious about this: what keeps the bones together?  Obviously joints are 
involved, but it's not as if the carpus is just a sack of loose bones that 
can slide totally apart and then articulate back together, right?  You're 
not really saying that, are you?  I suspect no, but I wouldn't mind some 
clarification.

"I suspect sauropod wrists could
flex to nearly 90 degrees to clear the hand from the ground during the
recovery stroke."

Well, this assumes: 1) that the preserved articular morphology is relatively 
useless in telling us about how the sauropod wrist worked and 2) that 
sauropod wrists worked like those of elephants -- which don't disarticulate 
when they "flick" back.  From personal examination in both a dissection and 
from bones, the carpus of an elephant is much taller than that of sauropod.  
They can apparently "flick" their wrists at 90 degrees because of 
well-developed articular surfaces on their proximal carpal bones that allow 
them to slide around the ulna/radius.  I don't agree with you here based on 
the osteology.  I believe my paper's photographs of the wrist bones in 
articulation with the radius, the figures, and the text detail how I arrived 
at my estimation of sauropod wrist flexion in fairly good detail based on 
the morphology.  Maybe there was more motion, but I can only work with what 
I have -- the bones.  That's at least setting some limit based on the 
morphology instead of a logical posit.

"All restorations showing the scapula-coracoid subhorizontal are grossly
incorrect. In virtually all tetrapods the scapula is sub-vertical, vertical, 
or
even  tilted forward. Rotating quadrupedal dinosaur scapula-coracoids to 
nearly
horizontal forces the sternal complex to be nearly vertical, which is absurd
since in tetrapods the sternum is subhorizontal."

Greg, that doesn't follow, unless you're basing the sternal thing on 
chameleons or other lizards.  You bounce back and forth between lizard and 
mammalian models for the scapulacoracoid movements and positions in your own 
work, but neither of these have to be the only models for dinosaur 
scapulocoracoid movement (or non-movement).  Birds, which are direct 
dinosaur descendants, certainly don't have such a gliding articulation with 
the sternum, and crocodylians don't either, so far as I know.  We have such 
little data on sauropod (or other dinosaur) sternals that to postulate a 
cartilaginous episternum in which the coracoid traveled in is stretching it 
a bit.  Maybe, but maybe not.  I can imagine all sorts of possible sternal 
articulations and scapcoracoid movements, but just because they seem logical 
doesn't mean that was the actual mechanism.  I certainly never say that the 
sternum was vertical in sauropods, nor did I think that the way the 
coracoids articulate with the sternum in a lizard have much bearing on what 
was going on in dinosaurs.

"It is hardly likely that
sauropods for some reason violated this basic pattern."

I disagree.  First of all, in many mammals the scapular blades are not 
oriented vertically but horizontally across the ribs and in relation to the 
forelimbs.  Many digging mammals come to mind.  Second, from a purely 
mechanical standpoint, the glenoid of sauropods does not face out, but down. 
 In fact, the glenoid of many heavy, graviportal vertebrates faces down, 
not out or back -- check out elephants.  This allows the humeral head to 
articulate with the glenoid in such a way that mostly compressive forces are 
transmitted.  Regardless, the more you tilt the shoulder blade in a sauropod 
up, the further posteriorly that glenoid is going to face.  And it gets to a 
point where the humerus would become horizontal because the humeral head of 
sauropods articulates up and back.  You cannot have a vertical 
scapulocoracoid and a vertical humerus.  I noticed this, Ray Wilhite has 
noticed this, and Phil Platt noticed this, all of us independently.   Again, 
look at the figures in my paper and read the descriptions -- it is fairly 
evident that the only way a columnar limb would have worked in a sauropod is 
for that scapulocoracoid to have been sub-horizontal -- 30-40 degrees.

"Horizontal blades are found
in flying archosaurs with strongly flexed coracoids (pterosaurs, birds and 
near
avian theropods) that articulate with the front end of the horizontal
sternum."

Don't these "violate" the basic pattern?  It seems to me that when you get 
changes in function, you're going to have changes in the orientations of the 
functioning objects.  You, of all people, are arguing for parsimony???? =)  
You've argued that some maniraptorans are flightless descendants of birds, 
but then turn around and say that scapulocoracoid position cannot change 
because it violates a basic rule?  I'm giving you a hard time here, but I'm 
still surprised!

"Bonnan argues that sauropods had straight limbs because land giants have to
in order to bear their mass in 1G."

But you're leaving out the rest of my sentence from the paper, which is 
"during the support phase."  Obivously sauropods flexed their limbs when 
they lifted them off the ground -- elephants do as much.  But when 
graviportal mammals are walking and they are supporting their weight on 
their limbs, those limbs assume a columnar position -- see Muybridge's 
pictures.  In any case, in the paper I wasn't concerned with what the legs 
did when they were off the ground because I was interested in manus 
pronation.  Since the manus prints could only be made during the support 
phase of locomotion, it seemed logical to focus on limb orientation during 
the support phase of locomotion.  In this case, the joints of sauropods 
strongly support a columnar orientation.

"This is not correct because many land giants
had flexed legs judging form their joint articulations - indricotheres (as
correctly noted by Gregory and Granger back in the 30s), recent extinct 
giant
rhinos and living rhinos, titanotheres (as detailed by Osborn in his classic
titanomonograph), ceratopsids (Paul and Christiansen in Paloebiology),
hadrosaurs, giant theropods (Paul in Gaia)."

Yeah, but sauropods are not indricotheres, rhinos, titanotheres, etc.  They 
are sauropods.  They were heavy.  They have column-like bones that leave 
little to the imagination.

"Straight versus flexed is linked to running
performance rather than sheer size."

What about Hutchinson's work with the "running" elephants?  They're 
straight-legged and yet they are capable of some pretty impressive speeds.  
I don't think you can use simple "straight vs. flexed" as a measure of 
running performance.  However, I doubt sauropods ran, at least in the 
biomechanical sense of the word.  The manus prints and the bones in the arm 
all line up if, and only if, the forelimb assumed a columnar orientation 
during the support phase of locomotion.

I encourage everyone interested to read the paper and look at the 
photographs -- that's why I have them.  We need more photos of the real 
bones in articulation.

Matt

Matthew F. Bonnan, Ph.D.
Department of Biological Sciences
Western Illinois University
Macomb, IL 61455
(309) 298-2155
mbonnan@hotmail.com
MF-Bonnan@wiu.edu
http://www.wiu.edu/users/mfb100/

_________________________________________________________________
Add MSN 8 Internet Software to your existing Internet access and enjoy 
patented spam protection and more.  Sign up now!   
http://join.msn.com/?page=dept/byoa