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Re: Theropod hips



-----Original Message-----
From: Jaime A. Headden <qilongia@yahoo.com>
To: dinosaur@usc.edu <dinosaur@usc.edu>
Cc: larryf@capital.net <larryf@capital.net>
Date: Wednesday, December 13, 2000 11:31 PM
Subject: Re: Theropod hips

>
>  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,


Thanks for the response and refs. They were informative. I also came upon
this... (from G.Paul "Predatory Dinosaurs" pg. 118)..."What the elongated
pubis did is something of a mystery. The muscles the pubis and it`s boot
anchored helped hold the hind limbs close to the body, but other erect-gait
animals do not have these muscles."

I`m glad I came across this, because I have very little knowledge of the
anatomical details, but believe I have a possible explaination for this
theropod feature. I was putting together a statement trying to describe
enviornmental reasons for differences in pterosaur and theropod hip
structure, and here is what I came up with:


I`ve been looking into pterosaur and theropod hips, and trying to evaluate
these differences in terms of enviornmental conditions that may have
prevailed.Here are some of my recent thoughts on the subject...

Why pterosaur and theropod hips differ (keep in mind a BCF viewpoint):

 The pterosaur body plan is "top heavy". When they landed on their feet they
had a strong tendancy to flop over onto their forepaws as well. This is why
they have no clavicles. Such fragile bones would have broken if they were
present. Pterosaurs may have landed on cycad branches which would have
offered a broader landing pad, and a springiness that would lessen the
schock of landing.There was no evolutionary "incentive" to evolve a deep hip
socket.

Early birds evolved in a colder enviornment, and had to land on the
relatively stiff branches of Gymnosperm trees. The landing pad was a slender
branch, and any foreward momentum would have caused the bird to overshoot
it`s mark, and rotate foreward and off the branch. The center of gravity had
to be aligned precisly to prevent this. The forces of the landing were
transmitted directly to the acetabulum, and therefor the socket had to be
deep.Muscles and ligaments attached from the tip of the pubis, and the
middle of the femur would tend to prevent the limb from moving outside of
the parasaggital plane, (thus preventing possible dislocation at the knee or
hip socket).The long pubis of theropods puts the distal end of this bone in
an excellent position to perform this function.Plus by it`s connection in
the midline by the pubic symphysis, makes an excellant triangular brace
against the pelvic girdle.



It is interesting to note, looking at the tyrannosaur skeleton (for
instance), that a flighted form with this body plan would have it`s pubic
bone pretty much "inline" with the path of approach to a landing pad. The
legs extended in anticipation of obtaining a perch, would tend to bring the
femur pretty much in line with the pubic bone as well. Therefor, the muscle
attachment between pubis and femur contracted to it`s fullest would bring
this alignment about. Once contact with the branch is made, and knee flexion
begins, this muscle becomes somewhat stretched, thus absorbing some of the
energy of the landing, while still maintaining a parasaggital orientation of
the femur.


Later birds were able to rotate the pubis backwards, because they developed
better rapid flapping capabilities which allowed a degree of "hovering" and
consequent decrease in the landing speeds, thus negating the need for strong
ligament restraint ventral to the acetabulum.

So guys,.....whadda ya think???