Re: Brachiosaur Forelimb Proportions (Was: Isle of Wight)

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

Mike Taylor writes:

" ... The oddness is in the cross-section of the
humerus.  Stupidly, I didn't take any photos, even though I had my
camera with me; and I don't have accurate measurements; but folks,
that bone is weird.  It's more like a blade than a tube.  My rough
estimate would be that it's something like six inches wide
medio-laterally, but no more than an inch and half think
cranio-caudally.

"So two questions.  Firstly, isn't that just wrong for a brachiosaurid
humerus?"

I didn't work much with Brachiosaurus in my dissertation, but all sauropod 
humeri are rather flat anteroposteriorly (craniocaudally).  Without seeing 
this specimen, perhaps it is somewhat crushed, but sauropod humeri tend to 
be flattened anteroposteriorly.  Although cross-sectional shape will vary 
somewhat from specimen to specimen, the humeri and femora of sauropods tend 
to have an elliptical cross-section, although usually more pronounced in the 
femur than humerus.


Mike continues:
"And secondly, surely such a medio-laterally wide blade shape just
makes no biomechanical sense?  You'd think that if the humerus was
going to be significantly bigger in one dimension than another, then
it would be cranio-caudally, so that it could better absorb the forces
of acceleration and deceleration when starting and stopping walking."

Well, having an anteroposteriorly flattened humerus gives it an elliptical 
cross-section.  An elliptical cross-section is good for dealing with torque 
(i.e., twisting forces).  The humeri and femora of most sauropods are loaded 
off-center, unlike what you see in elephants where the loading is 
more-or-less straight down from the head of humerus/femur through the shaft. 
 This is because elephants have humeral and femoral heads that are directed 
almost directly dorsally (i.e., straight up).  Sauropod humeri and femora 
have heads that are directed more medially away from the mid-line of the 
shaft (more pronounced in the femur, but still you can see this to some 
extent in the humerus).  If you load the humeral or femoral head off-center 
when you're walking, you would generate torques (twisting forces).  It turns 
out that an elliptical cross-section is better able to handle these forces 
without breaking than a round cross-section.  Even in the human femur, which 
is also loaded off-center, you get a more elliptical cross-section than you 
see in the humerus.

Hope this helps! =)

Remember: sauropods are not elephants; sauropods are not big 
reptile-mammals; they are dinosaurs and do things differently. =)

Matt Bonnan

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