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Re: ORNITHISCHIANS :-)



In a message dated 6/10/01 2:42:36 AM EST, Tetanurae@aol.com writes:

<< George Olshevsky insists that the only "major" synapomorphy linking 
 stegosaurs to ankylosaurs is the presense of armor, which he insists is 
found 
 all over Orntihischia and Sauropoda.  I'll ignore for the time being the 
fact 
 that no other ornithischians have ever been found that have any armor 
 preserved, as well as the fact that no sauropods, but a few very derived 
 titanosaurs had any sort of armor.  With the sporadic disribution we see 
 here, it's somewhat similar to insisting that all sauropsids had beaks since 
 birds and turtles have beaks, and since we don'tm find them elsewhere, 
 they're just not preserved right, or cartilaginous. >>

I should clarify this position a bit. Right, we only find >armor< (that is, 
bony dermal ossifications) sporadically in phytodinosaurs. The point I 
haven't made very clearly yet is that phytodinosaurian skin (and indeed most 
dinosaurian skin that I've seen and/or read about) has a beaded texture 
something like that of Gila monster skin. The dinosaurian skin tubercles 
could very easily transform into dermal ossicles and scutes quite 
independently (and randomly, as needed) in different dinosaur groups. This 
seems to have happened with ankylosaurians and stegosaurians, as well as with 
certain titanosaurians. There are also dermal-armor-like structures, namely 
the dorsal spines of Diplodocus, known in other sauropod groups. So presence 
of dermal armor of itself does not impress me greatly as a character to 
support a phyletic relationship. (I have a similar view of the other 
characters supposedly supporting a monophyletic Thyreophora.)

>Shape< of the dermal ossifications might be a better character. E.g., 
presence of >keeled scutes< and >spines< in ankylosaurs and stegosaurs might 
indeed support a phyletic relationship. But other than that, stego and ankylo 
armor is rather different (can't really mistake one for the other, although 
spines are similar), hence my conclusion that armor probably developed 
independently in those groups, and keeled scutes & spines in both is a 
coincidence (convergence). Generally, stegos and ankylos are so different 
from each other that their common ancestor must have lived well before 
Scutellosaurus (which is a good basal ankylosaurian and has no stego 
features). All the dinos called basal thyreophorans these days are actually 
basal ankylosaurians (except Emausaurus, which is clearly a huayangosaurid 
stego). Right now I still cannot resolve the trichotomy 
ankylos+stegos+cerapodans; stegos seem as different from ankylos as they are 
from cerapodans. (Indeed, I have doubts about grouping cerapodans = 
marginocephalians + ornithopods, so that the trichotomy should really be an 
unresolvable tetrachotomy. Could build a decent case for grouping stegos and 
ornithopods, for example.)