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Re: Sauropod spines



In a message dated 95-09-19 22:18:36 EDT, zooamy@zoo.latrobe.edu.au (Adam
Yates) writes:

>Dermal armour primitive for Dinosauria? I don't think so. The loss of 
>osteoderms is a character that diagnoses the Ornithodira. It is reversed 
>in some dinosaur clades: Titanosauridae, Thyreophora and Ceratosaurus. 
>However there is no evidence that these are retaining the primitive 
>condition of the ancestral dinosaur. Indeed many basal dinosaurs 
>(Eoraptor, Herrerasaurus, Prosauropods, Lesothosaurus)  
>and close dinosaur outgroups (Marasuchus) are known from relatively 
>complete skeletal remains that don't include any trace of osteoderms. 
>What primitive ornithischian are you refering to when you say that 
>armour was present in Lesothosauria? If you are reffering to 
>Scutellosaurus then this is a thyreophoran, the one and only (to my 
>knowledge) group of ornithischians to posses armour (either supported by 
>osteoderms or not).

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.