RE: Opisthocoelicaudia (was Re: Titanosaurids)

["Williams, Tim" <TiJaWi@agron.iastate.edu>]

Rutger Jansma wrote:

>The chevrons eliminate both referall to the Chinese
>"Cetiosaurids" (like Shunosaurus or Mamenchisaurus) or Diplodocid,
>although the option should not be excluded. It's tail was stiffened by >the
reversal of it's tailvertebrae, making it's skidlike chevrons >useless for
propping up on his tail, so they possibly disolved or >something. 

Several things came to mind when reading this post:

(1) "Skid-like" chevrons have a wider distribution within the Sauropoda than
just the diplodocoids, _Mamenchisaurus_ and "shunosaurines".  Their purpose
may not have been to "prop up" the sauropod into a bipedal (or "tripodal")
stance (I don't ever recall that purpose being attributed to the forked
chevrons).  One suggestion is that they protected the underside of the tail
if it came into contact with the ground.

(2) There is titanosaur material from South America that shows a combination
of opisthocoelous and non-opisthocoelous caudal vertebrae.  Considering what
we now know about homoeotic genes and pathways, the possibility of a salient
switch from a strongly procoelous --> strongly opisthocoelous condition in
caudal centra should not be dismissed out of hand.

(3) Several papers on sauropod phylogeny published in the last few years
have presented a very strong case for assigning _Opisthocoelicaudia_ to the
Titanosauridae.  There's a lot of common features in the appendicular
skeleton - don't get distracted by that weird tail.  Relevant authors
include Upchurch; Wilson and Sereno; Calvo and Salgado; and others.

>That's pretty mean, there was on OPUS dinosaur it was possibly able to
>dive into the water, could this be one the things you know? 

Tracy's lips are sealed!  :-)  By the way, what's this "diving dinosaur" you
mentioned?



Tim