dinosaur hip mechanics (was Re: Big Dinosaur Print)

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

Dan:

The acetabulum (hip socket) of many dinosaurs has a process called the 
antitrochanter, essentially a bump on the posterior side of the acetabulum.  
On the posterior (back) aspect of the femur (thigh bone) is a notch or fossa 
into which the antitrochanter articulates.  It is physcially impossible in 
many dinosaurs (not all, but many) to rotate the femur much past vertical if 
the body is horizontal.  In other words, in a theropod with a horizontal 
vertebral column, the femur can swing forward in flexion (Nick Hotton III 
had reported a value of approximately 60 degrees or so in his 1980 paper in 
the "A Cold Look at the Warm-Blooded Dinosaurs" book) but is physically 
stopped by the antitrochanter once it passes a vertical orientation -- in 
other words, like ourselves, many dinosaurs appear to have had limited 
femoral extension.

Furthermore, as Greg Paul and others have pointed out, rocking the body into 
a more vertical position would have strained the pelvic musculature, 
especially the adductor muscles that hold the femora close to the body.

I am not a theropod expert, however, and I'm sure someone else on the list 
could enlighten us further on the extent of theropod dinosaur leg movements. 
 From what I can tell from my studies of the hip joint and should joint in 
sauropods, motion was relatively restricted (big surprise!), but many 
sauropods have a reduced or almost absent antitrochanter.  It seems likely 
that sauropods had a more pendulum-like leg motion than theropods because of 
this, but wait for the papers -- oh, how much I wish I had this stuff out 
now! =)

I may as well take this opportunity to announce that I have accepted an 
assistant professor at Western Illinois University in their biology this 
fall (as a functional morphologist) and will be defending the dissertation 
on sauropod locomotion in April.  I will fill you all in on my new address 
when the great move is finished.  Anyway, now you all have to bow before my 
power as a member of the academic community -- just kidding!!!! =) =P

As the geologist Jonathan Cooley once related to me, "In a Ph.D. program you 
learn more and more about less and less until pretty soon you know 
everything about nothing." =)

Matt Bonnan
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