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Re: Sauropod spines
In a message dated 95-09-20 02:42:48 EDT, email@example.com (Adam
Yates) writes (first quoting my own posting):
>> 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
>> to go into just yet.
>> Among phytodinosaurs, dermal armor was likely present in the
>> _Echinodon_, for one, and it was certainly present in the ancestral
>> ankylosaurian and ancestral stegosaur (which are not the same thing,
>> Thyreophora is paraphyletic after all). It was also present in the
>> _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
>> All dermal armor was secondarily lost in Pterosauria--an adaptation for
>> flight, of course.
>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.
Hence my use of the term "likely," rather than, say, "certainly." I would
tend to agree that _Echinodon_ dermal scutes are a bad example, but they
_are_ cited in the literature. The jaws, however, are clearly lesothosaurian
based on the cheek teeth. No known thyreophoran had premaxillary and dentary
fangs, so if _Echinodon_ is to be considered such, the fangs are
autapomorphic. On the other hand, premaxillary and dentary fangs _are_ known
in heterodontosaurians, so it was more likely some kind of primitive
heterodontosaurian--though the fang arrangement was unusual.
>Do you have a reference for the paraphyletic nature of the Thyreophora?,
>this is news to me.
Thyreophora is paraphyletic if you include the common lesothosaurian ancestor
of ankylosaurs and stegosaurs in it, for then you must also include at least
the heterodontosaurians (i.e., heterodontosaurids + pachycephalosaurians +
ceratopians) in the group as well. If you don't, then Thyreophora is worse
than paraphyletic; it is diphyletic. Stegosaur pelves, studied (to death) in
my forthcoming _Historical Dinosaurology_ #1, show several good
synapomorphies with cerapodan pelves, virtually none with ankylosaurian
pelves (in fact, _Stegosaurus_ was once suggested to have been related to
_Camptosaurus_[!] by at least one authority--Colbert, if I remember
correctly). Stegosaur hind feet, with reduced or absent metatarsals I and V,
are much closer to cerapodan (heterodontosaurian + ornithopod) hind feet than
to any ankylosaur feet, which retain a robust metatarsal I and digit I and
strongly suggest a bipedal, cursorial common ancestor for Stegosauria.
Stegosaur skulls (of which we have only about a dozen examples) and teeth
were markedly different from ankylosaur skulls, even the skulls of such
primitive ankylosaurs as _Scutellosaurus_ and _Scelidosaurus_. Whatever
resemblances remain between the two groups are due either to plesiomorphy or
convergence within similar adaptive character suites (i.e., heavy graviportal
body plan, etc.).
> 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?
No--the dermal ossifications described by Steve Czerkas at the 1993 SVP
meeting. (Anybody--has he published on this yet? I know Greg Paul has started
to include them in his pictures of duckbilled dinosaurs.)
>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.
It's not called _Thescelosaurus neglectus_ for nothing... :-)
"Reverted to the ancestral archosaur condition, of possessing osteoderms"?
After (in the standard phylogeny) losing the osteoderms? Well, it's easier
than re-evolving a new digit, complete with phalanges, I suppose. Could it
have happened six or seven times independently (in sauropods, ankylosaurs,
stegosaurs, _Ceratosaurus_, _Thescelosaurus_, hadrosaurs, etc.)? I'm not
nearly so sure.
> 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,
Not Theropoda. To be in my version of Theropoda, you have to have the
retroverted hallux and a functionally tridactyl foot (among other things).
The little guys belong in a smaller, more primitive dinosaur group called
Lagosuchia. Here the foot was still functionally tetradactyl (or tridactyl
without a retroverted hallux), with digit V strongly reduced or absent and
metatarsal V reduced. In Dinosauria, the primitive pedal digital count was
five, all functional, as retained in all sauropods. You've got to expand your
notion of "dinosaur" to include some very small forms, such as lagosuchians
and even _Longisquama_, in order to hold any hope of (1) retaining dinosaur
monophyly and (2) explaining the origin and early (pre-archaeopterygid)
evolution of birds.
>Still its good to see a healthy diversity of opinions.