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Brontosauria (was Re: Graves museum exhibits)

T. Mike Keesey wrote:

Since DinoGeorge's Brontosauria is essentially Prosauropoda + Sauropoda, I
have on the Dinosauricon cladistically translated it as a node containing
the stem-based groups Prosauropoda and Sauropoda (as defined by Wilson and
Sereno, respectively anchored by _Plateosaurus_ and _Saltasaurus_). Thus
Brontosauria would be a group containing advanced sauropodomorphs,
possibly excluding things like _Thecodontosaurus_.

Surprisingly, things like _Thecodontosaurus_ and _Anchisaurus_ may actually be CLOSER to the origin of sauropods than the big "broad-footed" prosauropods like _Plateosaurus_ and _Riojasaurus_. At least, that's what Sereno and others have found recently. For example, the cervical vertebrae of _Riojasaurus_ actually show very little elongation (a very un-sauropodomorph character). If melanorosaurids (like _Riojasaurus_) were obligate quadrupeds then this posture probably evolved in prosauropods independently of the sauropods. Maybe more than once, judging from _Blikanasaurus_'s curious (and extremely heavily-constructed) hindlimbs.

The trouble is too, with certain prosauropod specimens (maybe _Thecodontosaurus_) is that their bipedalism may be a juvenile character. Young prosauropods may be slender-footed and bipedal, adults more broad-footed and quadrupedal. (This is also seen in _Tenontosaurus_ among ornithischians - young'uns were a lot like dryosaurs in cursoriality, adults more like iguanodontids.) Certain "primitive" characters seen in _Thecodontosaurus_ may be juvenile traits. (Remember _Efraasia_...?) A re-description of _Thecodontosaurus_ is apparently on the way (in _JVP_ I think) which hopefully will sort all this out.


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