Re: "real" opisthopuby

["Jaime A. Headden" <qilongia@yahoo.com>]

  I think Tom is right in saying that opisthopuby in maniraptorans and in 
ornithischians may not
be the same, but there are probably similarities. Understanding what causes 
opisthopuby (or what
correlates with it) in ornithischians may help clarify the issue. Who knows, 
maybe Steve and John
can look at them next, or an enterprising young paleo-head can use this as a 
reason for getting a
research paper started. Maybe that's me... I had a theory regarding 
maniraptoran opisthopuby that
had aspects of ornithischian opisthopuby tied in to it ... not to mention 
another group of
amniotes with _technical_ opisthopuby that no one thinks about....

  Results indicated from other authors (published) suggest that retroversion of 
the pubic shaft
occured in ornithischians for a totally different reason (gut expansion) than 
it did in
maniraptorans (balance and locomotor module decoupling). But this was 
initially. (My conjecture)
not too far into their evolution, ornithiscians, in three branches, modified 
the primitive
condition in developing a prepubic process, which became the effective pubic 
shaft relative to
saurischians and even basal ornithischians.

  So what do they have in common? The actual state of the bone.

  Opisthopuby is generally taken as having the pubis being inclined 
caudoventrally from the ilium,
and caudal to the vertical by 5 or so degrees or more. Recurvature of the shaft 
may influence this
somewhat, and this is where the conflict may be likely in the state of 
maniraptorans for which
*strong* recurvature is present, but the pubis is oriented in three different 
directions along its
length in lateral view. Propuby may be defined as the direct corrolary 
opposite, but don't quote
me on that ... people take propuby for granted, frankly, and there are 
theropods that people
assume are propubic that probably aren't. As for "mesopuby," this is an 
undefined term coined
about two years ago on this list, used for a condition that is generally 
accepted, but _has never
been defined in the literature_. Thus, it really has only a vernacular usage. 
Hopefully in the
near future, this term can be defined and applied to dinosaurian pelves, and 
possibly more broadly
to various amniote groups for which it may serve to aid in studies on pelvic 
evolution [pers.
obs.] but that's down the road.

  Hope this helps,

=====
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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