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Re: General Biomechanics and Therizinosaur hips



Dr. Martin Baeker (martin.baeker@tu-bs.de) wrote:

<As an example, whenever I look at the hip of a therzinosaur(e.g.
http://dinosauricon.com/images/therizinosaurid-ma.html), it has two very
peculiar features: The pubis points backwards and joins with the ischium,
which in itself is not unusual, but the two are somehow connected by a
bridge (however you say that in latin) and there is this more or less
rectangular hole between them.>

  That bridge, if I am interpreting the description correctly, is the
co-ossification of the obturator process of the ischium to the posterior
margin of the distal pubis. This region in the pelvis of *Nanshiungosaurus
brevispinus* is unknown, despite reconstructions.

<And at the front of the illium there is this downward pointing (is that
ventrally directed?) process, so that on the whole the illium looks as if
someone with a narrow snout has bitten of a chunk. 

Obviously, these fetaures are good for classifying - but probably
evolution did not want to accomodate plaeontologists by preoducing them.
So what are they good for?>

  Well, these features are known to some degree in other animals. In the
case of the ilium, this is actually a _normally_ dorsoventrally expanded
preacetabular blade, that is then turned sideways. This is most apparent
in dorsal view, where the iliac width is nearly as great as the length of
the sacrum, if not more so. This is a result, as I see it, of the
expansion of the gut laterally, seen in many larger mammals, and of
particular similarity, giant ground sloths (look at *Megatherium*, for
example) and elephants.
 
  In this manner, books on biomechanics would not be of too much help, as
the research involving the features of the ilium would be found in recent
publications and papers, not integrated into book form as of yet. I can
suggest work by Steve Gatesy, John Hutchinson, Al Romer (who also did work
on crocodile pelvises), and Greg Paul ... who did devote a chapter of his
book _Predatory Dinosaurs of the World_, New York Academy of Sciences
Press (New York), to the subject of reconstructing musculature, limb
design, and another to various limb-related biomechanics. Those would be
the best resources I could suggest. Work by Coombs and Romer on the hips
of ornithischian dinosaurs will help in realizing the structure and
relationship of muscles and lines of force on the limbs as a result of
reverse-oriented pubes (opisthopuby), which is of great relevance to
therizinosaurs (= segnosaurs), and Gatesy and Hutchinson, as well as Paul,
adapted these theories to the study of _theropod_ opisthopuby in
dromaeosaurids and birds. Only recently have segnosaurs been conceived
irrefutably as theropods, have they been considered in any strong manner
to the condition of musculature, but Barsbold and Perle have both
published on the condition of the hips, and Barsbold's monograph in 1983
(which either Tracy or I can provide if you need it, as well an English
version available (without figures) at
http://www.uhmc.sunysb.edu/anatomicalsci/paleo/ -- otherwise, I'd be happy
to photocopy the figures for anyone who wants them) is of extreme
importance of placing the hips of segnosaurs in context with theropods and
the origin of birds, including anatomical transformations.

=====
Jaime A. Headden

  Little steps are often the hardest to take.  We are too used to making leaps 
in the face of adversity, that a simple skip is so hard to do.  We should all 
learn to walk soft, walk small, see the world around us rather than zoom by it.

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