Re: "real" opisthopuby (very speculative)

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

Ken Kinman (kinman@hotmail.com) wrote:

<Is it possible that the prepubic process is a primitive character in 
Ornithischia, and that its
gradual loss is derived.>

  This is very unlikely. The given fossils which would indicate polarity of the 
production of the
dervied pubic condition are the most basal, including basal thryeophores like 
*Scelidosaurus*,
*Lesothosaurus*, *Pisanosaurus*, *Leptoceratops*, and *Heterodontosaurus*. Most 
of these forms
either lack the process entirely, or have mearly a small process or nubbin to 
account for. Because
the basal most marginocephalians lack a prepubic process entirely, it is most 
parsimonious and,
dare I say it, logical, to state that the plesiomorphic condition is abscence 
and the presence of
process in higher marginocephalians dervied in both Pachycephalosauria _and_ 
Ceratopsia. The basal
ornithopod process is very small, spine-like, and short, becoming for robust 
and larger in higher
forms, such as the Iguanodontia. Whereas ankylosaurians lack the process, 
stegosaurs have it, as
does *Scelidosaurus*, but *Scutellosaurus* does not. That basal ornithischians 
lack the process
and it is present only in higher groups (Ornithopoda, Pachycephalosauria, 
Coronosauria
[Ceratopsia], and Stegosauria) suggests that the plesiomorphic condition is the 
absence, and that
ankylosaurs may have retained this condition, whereas stegosaurs developed the 
process. That the
process is triply convergent is not a surprise given the nature of the basal 
forms which lack it.
*Pisanosaurus* is often cited (e.g., Bonaparte, 1986; this list) as being in 
the process of
transforming a propubic pelvis into a meso- or opisthopubic pelvis. The pubes 
are nearly
vertically oriented in basal dinosaurs like *Eoraptor,* *Guaibasaurus* 
(Saurischia), and
*Saturnalia* (Sauropodomorpha), and even some ceolophysoids may have evidenced 
the mesopubic
condition in having a vertical pubis, such as *Gojirasaurus* [as suggested by 
the angle between
cranial margin of the pubis and the facet for the ilium on the iliac peduncle, 
and by the strait
pubis], that is is only logical to suggest that the basal ornithischians would 
have one as well.
 
<And here's some wild speculation to chew on (spit it out if you don't like the 
taste)----- what
if the prepubic process IS homologous to the pubes in other dinosaurs, and that 
the postpubic
process is the real neomorph? The postpubic process could have gradually 
lengthened in parallel to
the ischium.>

  And on a similar vein to the above, I would reply that the osteological data 
is firmly against
this. The number one reason would be the position of the pubic foramen directly 
caudal to the
pubic shaft, and not between shaft and prepubic process. (Though historical 
usage suggests
retaining the term "postpubic" for the main shaft of the pubis in 
ornithischians, this is a _black
and white_ situation prompted by naming the propubic process, and is 
misleading: the main shaft of
the pubic did not disappear to which a growth of bone, as a new process, 
developed caudally, as in
the prepubic process cranially. Such, the term "pubic shaft" as in saurischians 
and other
archosaurs may be properly applied to the "postpubic process of the pubis;" it 
is neither a new
process nor is it posterior to the pubis itself.) Paleontological evidence 
(that is, _fossils_)
indicates that the main body of the pubis rotated caudally, the pubic boot 
flattened
mediolaterally, and expanded dorsally/proximally include up to 1/3 the pubic 
shaft, as in
*Hypsilophodon* and othneiliine "hypsilophodontians." Fairly early in 
ornithischian evolution, the
pubic foramen opened ventrally, but never disappeared. Extreme opisthopuby, 
seen only in
non-lambeosaurine hadrosaurids (which have very slender, distally tapered pubic 
shafts) approaches
the horizontal in orientation.


=====
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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