Brontosauria (was Re: Graves museum exhibits)

["Timothy Williams" <twilliams_alpha@hotmail.com>]

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.


Tim

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