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RE: Family Nemegtosauridae



 

-----Original Message-----
From: TWILLIAMS@canr1.cag.uconn.edu

If I've got a handle on titanosaur evolution, the primitive condition 
is to have amphicoelous/platyan centra throughout the tail. 
_Malawisaurus_ and _Janenschia_ both show procoelous anterior 
caudals.  (I know _Janenschia_ could be an amalgam, but the 
strongly procoelous anterior caudals look titanosaurid).  
-------------------------
Yippy a sauropod discussion! Yes that is what appears to be happening in
basal titanosaur evolution.

-Tim wrote:--------------
In more derived titanosaurids like _Alamosaurus_ and _Saltasaurus_ 
procoely extends through the mid-caudals too.  According to recent 
cladistic analyses, _Opisthocoelicaudia_ belongs among these derived 
titanosaurids.  This means either that procoelous caudals evolved 
independently among certain titanosaurid genera (not so unlikely in 
my view, since some basal titanosaurs show anterior caudals that 
are only mildly procoelous), or that _Opisthocoelicaudia_'s ancestors 
were procoelous, then reverted to the amphicoelous/platyan condition, 
then became opisthocoelous.  I still think this transformation is a 
little improbable.
--------------------------

Yes it is starting to look like _Op._ is a very derived titanosaurid quite
close to Saltasaurus. This does not, however, imply that the tail must have
gone through a transformation that you have suggested above. There is a way
to get from the strongly procoelous caudals of a saltasaurine to the
opisthocoelous caudals of Op. without an intermediate platyan condition. To
understand this you need to look at the rest of the vertebral column of
sauropods not just the tail. If you look at most recent sauropod phylogenies
you will notice a trend towards increasing amounts of opisthocoely in the
presacral column as you head towards saltasaurines. The basal saurischian
condition is to have no opisthocoelous vertebrae anywhere in the column and
this is the condition found in Prosauropods. The neck  and the first one or
two trunk vertebrae have become procoelous in the most primitive known
sauropods (Shunosaurus, Barapasaurus - Vulcanodon is missing its neck of
course). Closer outgroups to the Camarasauromorpha (Omeisaurus,
Diplodocoidea) show increased strength of opistocoely in the anterior trunk
vertebrae that gradually fades out to platyan towards the posterior trunk
vertebrae. Camarasauromorphs show strong opisthocoely all the way to the
sacrum. Things get lost in the sacrum (due to fusion) but it appears that
the large anterior 'ball' that is the main feature of opisthocoely extends
all the way through the sacrum and into the first caudal of saltasaurine
titanosaurids. This results in a biconvex first caudal. Might Op. be the
ultimate expression of this trend? If so the intermediate would have
opisthocoelous anterior cadauls joined to procoelous posterior cadals via a
biconvex vertebrae (or perhaps a short transtion zone somewhere in th
midlength of the tail).
I can imagine this transformation being acheived by the extension of the
affect of some Hox gene (one that turns on the machinery that grows large
convexities on the anterior faces of the centra)  further and further back
along the antero-posterior axis of sauropod embryos. I can't test this idea
yet, but I know how to, anyone got the money to send me to central Asia to
look for the immediate outgroups of Opisthocoelocaudia? ;-)     

Adam Yates