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Re: Theropod hips

Larry Febo wrote:

<OK. I`ve been pondering the theropod hip recently, and need some
expert help. I have to admitt that I`m not that familiar with
directional forces produced by muscles, ligaments etc. In viewing the
"triradiate" hip of earlier theropods (pre- maniraptoran), could the
muscle-tendon attachments be described as "restraining" the head of the
femur into the acetabulm,directing force on the balljoint into the
socket...preventing dislocation? Could one say this function exists in
addition to swinging the leg to and fro?>

  I certainly can't call myself an expert, because my knowledge is not
based on personal examination, and I am still trying to understand the
forces that act on the leg myself. However, I can certainly say that
the two sections in the _Encyclopedia of Dinosaurs_ (Currie and Padian)
titled "Hindleg and Foot" and "Pelvis, Comparative Anatomy" and the
"Osteology" Chapter of _Complete Dinosaur_ (Farlow and Brett-Surman)
may be certainly enlightening. In extension, the various Sereno, Novas,
and Arcucci papers in the 1993-1994 _JVP_ papers discuss the hip of
dinosaurs and outgroups rather well, though they do not go into detail
on pterosaurs.

  I would like to add my input, however. The major femoral motivators
in the hip are the mm. iliofemoralis and caudofemoralis longus. There
are also weaker muscles that pull the leg sideways, and in the course
of avian evolution, the m. caudofemoralis longus has become weaker and
now pulls the femur around on its axis as well as back. Similarly, the
mm. pubo-ischo-femoralis internus and externus muscles are weak
effectors in pulling the leg foreward, backward, but primarily towards
the midline. In the full extension of these muscles, it is possible in
crocs and birds to dislocate the hips, as in mammals. They are not
absolutely restrictive, and neither is cartillage, and one stronger
muscle or excessive outside force (tripping, for instance) can cause a
muscle or tendon to tear or pull loose from the bone. However, there
are several m\bony stops in the hip, the primary one being the
antitrochanter, which Matt has explained just a little bit ago, and
primarily prevents over-extension. Another is the acetabulum itself: if
it is perforated, the femoral head fits further medial within it,
providing some stability, and is one indicator of a parasagittal
stance; on the other hand, the supracetabular crest provides the femur
with a strong support during parasagittal movement, preventing the
femur from sliding outward as it blocks the trochanters, unless the
femur is flexed, which is permissible in crocs and birds very easily.
The easiest transistory case is probably *Herrerasaurus*, or
*Marasuchus* (but I say H because the acetabulum is partially open,
rather than M).

  Just my comments,

Jaime A. Headden

  Where the Wind Comes Sweeping Down the Pampas!!!!

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