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Re: ANKYLOSAURS, STEGOSAURS, AND PSITTACOSAURS



On Wed, 3 Jul 2002 20:58:30   
 Jaime A. Headden wrote:

>Otherwise, though continuously suggested as stem-marginocephalians,...

I know this idea has been thrown around a lot on the list, and is favored in 
_Dinosaurs of the Isle of Wight_, but has there been one solid cladistic 
analysis that has shown it?  

>...heterodontosaurs have also been allied to ornithopods upon nearly one
>feature, the contact between premaxilla and lachrymal. This has been
>Sereno's contribution to the group. 

I don't know much about this character...but this aliance is based just on the 
contact of the two bones?  The lacrimal and premaxilla contact each other in 
psittacosaurs (actually, there is a very broad contact).  This contact seems to 
be absent in the few pachycephalosaurs that I have examined.  And, it appears 
to be present in many ornithopods, I take it.  With that being said, what makes 
this character a synapomorphy?  It might just be a pleisomorphy that was 
modified independently in pachycephalosaurs and neoceratopsians (once again, 
I'm unsure of the state in _Chaoyangsaurus_ and _Liaoceratops_).  What's more 
interesting to me is the jugal-premaxilla contact that might be a synapomorphy 
that links various species of _Psittacosaurus_.

>Psittacosaurus are themselves united
>on their short snouts, which can be considered a character complex
>involving reduced dentition count, contact between otherwise unaffiliated
>snout bones, closure of the antorbital fenestra into a foramen or
>completely absent, elevation of the external nares, and so forth. These
>can be both diagnostic to the group and considered expected in a
>short-snouted form, thus are mechanically widespread. I have a paper in
>prep on this very issue.

I've been thinking about this.  It's good to hear your views.  Many of these 
characters are certainly diagnostic, but many might be linked in an overall 
functional adaptation.  I look forward to your paper!

>  All other forms possess a distinct cranial form and similar pelvic form.
>Including almost or compeletely absent prepubic process. There are really
>few cranial features that can be shown to group one or two psittacosaur
>taxa together, 

See, this is something that is often mentioned, and might very well be the 
case.  But, it now might be possible to discern polarity of certain characters 
that Sereno could not have when his dissertation was written.  It's been 
something like 15 years since his dissertation was published, and it's been 
over a decade since his papers in _The Dinosauria_ and _Dinosaur Systematics_ 
appeared.  We now have the basal neoceratopsian _Liaoceratops_ and the strange 
(basal neoceratopsian, psittacosaurid, or basal ceratopsian) _Chaoyangsaurus_, 
and these species can be used to perhaps determine polarity of some characters 
that were very suggestive, but up until now unable to really examine in a 
cladistic context.

>but for the majority, a grade can be shown to progress from
>sattayarki to sinensis and meileyingensis, and the topology of characters
>suggest that there may be wisdom in recognizing a few polyspecific taxa
>commonly referred to as "genera". 

That is if we recognize _P. sattayarki_ as valid.  Sereno seems to think not.  
I have no opinion yet, as I have not yet read its description.  However, if 
indeed the type specimen comes from two different individuals, and if indeed 
the dentary it fragmented, broken, and worn, then it is very difficult to place 
this fossil in its own species.

>Resolution may offer the often appealing
>idea of naming a new genus to every species, but this resolves nothing of
>the relationships of the species, and all species must be treated equally
>as sets of independant taxa.

There is no need for a new "genus" for each species, but it might clean things 
up a bit if we could separate two or three genera.  But, separating the species 
into genera is only one thing.  As you mention, that tells little about 
interrelationships between the species, which is what I'm very interested in.  

>  However, this is not my thing; more Pete's territory than mine, and this
>is why I refrained from comment until he. Nonetheless, there is work in
>press on this issue. Stay tuned.

Nice to hear.  Anything else to add, Pete?

>  As for me, I am more interested in the sequence of transformation of an
>essentially heterodont dentition to the homodont dentition of ceratopsians
>proper and higher diplodont pachycephalosaurs. There are also ecological
>implcations, and these are in prep as well....

Ah, I'm already anxious to read your work.  Best of luck in finishing the 
paper, and, by all means, try to get them published as soon as possible :-))

Steve

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Steve Brusatte-DINO LAND PALEONTOLOGY
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