Re: How did dromaeosaurs use their arms?

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

Betty Cunningham writes:
> Are you suggesting dinosaurs were unable to turn until the development
> of feathers?

No.  Only that theropod femora, and their articulation with the acetabulum, 
do not permit the abduction/adduction motions of the hindlimb observed in 
mammalian predators, which have a spherical femoral head.  Dinosaurs in 
general have a stiff-action limb joint that is great in parasagittal 
(upright posture) plane moving fore and aft, but is not very good at 
abduction (moving the femur away from the body).  The physical articulation 
of the femoral head with the hip socket prevent abduction -- you would have 
to disarticulate the femur from the socket in order to do it.

Whereas in mammals the spherical head of the femur allows the hindlimbs to 
make fine adjusts to terrain and also provides a better turning radius, 
theropod limbs are more restricted.

> Are you suggesting that there were few successful dinosaur predators
> until they developed feathers?

No.  Their prey are limited by the same physical constraints too.

> Dinosaurs use their tail at every stride.  They can't do anything else
> BUT waggle their tail when they walk.  The tail would HAVE to be
> involved in turning simply because the tail POWERS the turn.

Yes, and probably the tail was used to help theropods and other dinosaurs 
turn.  However, again, the fine adjustments mammals can make, simply because 
of joint geometry, suggest that dinosaurs and mammals are doing different 
things as far as how they go about walking, running, and capturing prey.

However, the caudofemorales muscles which run off the tail and the pelvis 
and into the back of the femur are mostly involved in femoral retraction 
(i.e., swinging the femur back) during locomotion, and not necessarily in 
"turning" the animal.  The series of pelvic muscles that arise from the 
pubis and ischium were more integral in turning a theropod.

Furthermore, waggling the tail may or may not have happened during 
locomotion, depending on how stiff the tail was.

To reiterate: the basic joint geometry of theropods shows that the femur was 
mostly restricted to a parasagittal plane where it could swing fore and aft 
very nicely but could not rotate or swing away from the body as observed in 
mammals.

My original point was that the development of feathers in theropods does not 
necessarily indicate that they have an arboreal ancestor and could have 
developed for other reasons, later being exapted for flight.

Matt Bonnan
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