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THYREOPHORES, STEGOSAURS, AND PREPUBES
A few days ago, I wrote:
<< I am genuinely curious about the apparently "major" synapomorphies that
Geoge has demonstrating that stegosaurs are closer to ceraopods than to
To which, George Olshevsky replied:
<<In due time, in due time.>>
I would like to urge George to let the cat out of the bag here on Stegosaur +
Cerapod synapomorphies. I don't like arguing against secret characters.
George also wrote (about the perforate/non-perforate acetabulum):
<<This is not a character I would use. The acetabulum is partly closed in all
stegosaurs by an acetabular pocket off the pubis--an enlargement of the pubic
peduncle--which is a good stegosaur apomorphy (literally all known stegos
have it). There is nothing like this in any ornithopod, or in any other
ornithischians for that matter.>>
<<In ankylosaurs the acetabulum is secondarily closed off by invasion of the
ilium while the pubis shrinks to practically nothing. All stegos, however,
have a prominent prepubic process and a long postpubic process, which are
much more like those of some marginocephalians and some ornithopods than of
The pre-pubis of stegosaurs is probably a convergent feature between
stegosaurs and some cerapodans. I would venture a guess that it had
something to do with being big, and is of dubius phylogenetic significance.
Notice that the process is absent in the very small heterodontosaurids which
are probably basal to the Marginocephalia, but present in various degrees
among true marginocephalians, and especially pronounced in the gigantic
Also, look at ornithopods, whos basal-most (and smallest) members have the
smallest pre-pubes. Some basal Iguanodontians actually go through
ontogenetic changes where a baby looks like it has a "hypsilophodont"
prepubis, but then grows into a large, paddle-shaped iguanodontian prepubis.
The point I am getting at here is this: the ornithischian pre-pubis was
functional in nature and was relatively more pronounced in physically larger
animals. Lets everyone think for a second and remember what the pubis DOES:
It's an anchoring point for abdominal muscles, and an enlarged anchor point
closer to the sternal area would be stronger than, say a paddle-shaped pubic
boot, and would exert more leverage and be less prone to torque and breaking.
<<[ankylosaurs] which as far as I can tell >never< evolved a true prepubic
process. The ankylosaur pelvis and the stegosaur pelvis must have evolved
independently from some very early, generalized ornithischian pelvis and do
not necessarily share any derived features.>>
It is true that ankyloaurs never evolved a pre-pubis, but that's because the
pubis was almost reduced into oblivion. The very large pre-acetabular
processes of the illia may have functioned as prepubis analogs in
ankylosaurs, but maybe not.. One thing that stegosaurs and ankylosaurs do
share in their pelves is the loss of the obturator process of the ischium,
which is a derived character state. Lesothosaurus has one, Herrerasaurus has
one, and so does Plateosaurus. The loss in ankylosaurs and stegosaurs could
be another thyreophore synapomorphy.
<<Stegosaurs represent a distinct branch of Ornithischia that may or may not
have anything to do with the ankylosaurs. They differ from ankylosaurs in
practically all aspects of their skeletal anatomy, so if they do share a few
minor similarities these may or may not be meaningful.>>
I am still waiting to see what conflicting characters you have George. Maybe
you're right, maybe the characters are "minor" or "not meaningful" but for
christs sake, tell me WHY. Don't just answer with something that amounts to
"because I said so," because THAT will not be welcomed.