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Re: ORNITHISCHIANS :-)



In a message dated 6/13/01 6:17:32 PM EST, Mickey_Mortimer11@email.msn.com 
writes:

<< On the other hand, there has only been one character presented that would 
support a stegosaur-cerapod group- the completely 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 
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.

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.