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Re: Ankylosaurs, Stegosaurs, and other fun things..



On Tue, 2 Jul 2002 19:55:53   
 Dinogeorge wrote:
>In a message dated 7/2/02 6:04:16 PM EST, TiJaWi@agron.iastate.edu writes:
>
><< However, there are folks (including two or more on the DML) who hold strong
> views to the contrary, and do not regard stegosaurs and ankylosaurs as
> especially closely related, and regard the Thyreophora as polyphyletic. >>

Certainly possible, IMHO, although I haven't examined the situation closely 
enough to push for a full-scale demolition of Thyreophora.  About 20 years ago 
Walter Coombs composed a list of characters that united ankylosaurids and 
pachycephalosaurs, including the tendency to close the supratemporal fenestra 
(which likely is convergence, but who knows), the everted dorsal margin of the 
preacetabular process, the armoured or armour-like texture of the dorsal skull 
roof (also likely easy to be prone to convergence), the ossification of an 
interorbital septum, and contact between the ischium and ilium on the anterior 
side of the acetabulum with the pubis excluded from the acetabular margin.  

Most of these were later dismissed because of qualms with topology, homology, 
etc.  But, considering that Marginocephalia is united by a whopping three 
well-defined characters, who knows??  The most interesting of these characters 
is the last one I listed.  Sereno reported that it isn't seen in any 
non-ankylosaurid or pachycephalosaur ornithischian, although it might not be 
found in _Stenopelix_, a primitive pachycephalosaur.  If this is the case, then 
the character is basically debunked.

Marginocephalia is now (at least as of Sereno's 2001 paper in _The Age of 
Dinosaurs in Russia and Mongolia_) united by three synapomorphies: 
parietosquamosal shelf extension obscuring the occiput in dorsal view, median 
contact between the maxillae that excludes the premaxillae from the anterior 
margin of the external nares, and a short postpubic process lacking the distal 
pubic symphysis.  But, the first character was criticized, and in _The 
Dinosauria_, Dodson and Currie found the second to be pleisomorphic.  The third 
synapomorphy was similarly criticized, but better outgroup comparisons have 
strengthened it.

So, by no means is Thyreophora or Marginocephalia secured.  I would be 
interested in any discussion of the above characters, as my knowledge of them 
really only extends to what I have seen in the literature.

Steve

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