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Re: Sauropod spines




On Tue, 19 Sep 1995 Dinogeorge@aol.com wrote:

> 
> Ah, yes. The theropods. By the time we reach _Eoraptor_ et al. in the
> theropodomorph cladogram, the dermal armor has become pre-feathers (and
> farther up, real feathers), which are known only in the basitheropod
> _Longisquama_ so far. But that's a whole other story that I'm not quite ready
> to go into just yet.
> 
> Among phytodinosaurs, dermal armor was likely present in the lesothosaurian
> _Echinodon_, for one, and it was certainly present in the ancestral
> ankylosaurian and ancestral stegosaur (which are not the same thing, because
> Thyreophora is paraphyletic after all). It was also present in the ornithopod
> _Thescelosaurus_, and vestigial dermal armor decorated the ridges of the
> necks, backs, and tails of hadrosaurs and lambeosaurs. The stuff crops up
> almost everywhere in Phytodinosauria, just not laid on as thickly as in the
> thyreophorans.
> 
> All dermal armor was secondarily lost in Pterosauria--an adaptation for
> flight, of course.
> 
> G.O.
> 
As far as I am aware the problematic Echinodon is known from dispersed 
individual bones and therefore it is difficult to assert that any 
osteoderms found in the same deposits actually belong to this genus. In 
any case I thought that the jaw displayed thyreophoran affinities anyway. 
Do you have a reference for the paraphyletic nature of the Thyreophora?, 
this is news to me. When you say the backs and necks of hadrosaurs are 
decorated with the remnants of dermal armour are you reffering to the 
skin folds and frills? If so I would seriously doubt that they are 
homologous with the dermal armour of other archosaurs. I am aware of the 
small non-bony turbecules found under the base of the tail, however when 
I speak of armour I am reffering to scutes supported by bony osteoderms. 
There is simply too little evidence to argue about the distribution of 
enlarged scales that are not supported by osteoderms. They may have been 
present in the ancestral dinosaur, who knows? I admit I had neglected 
Thescelosaurus in my original posting but I would regard this animal as 
another on the list of dinosaurs that had reverted to the ancestral 
archosaur condition, of possesing osteoderms. Obviously your 
interpretation of character state distributions will depend on the 
phylogeny you are working from. I find your inclusion of Marasuchus (and 
presumably Lagerpeton as well) within Dinosauria, let alone Theropoda, 
puzzling. Still its good to see a healthy diversity of opinions.

Adam Yates