[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: How did dromaeosaurs use their arms?




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
________________________________________________________________________
Get Your Private, Free E-mail from MSN Hotmail at http://www.hotmail.com