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RE: titanosaurs



Dinogeorge wrote:

> The point to remember is that if Opisthocoelicaudia is a titanosaur, it
> is the only one with a series of bifid neural spines. 

> How odd to find 
> this anomaly in the >same< genus that also features a caudal series 
> that has exactly the opposite anatomy of titanosaur caudals. 

True, opisthocoely could be said to represent the opposite condition to
procoely.  However, as Jaime noted, the opisthocoelous caudals of
_Opisthocoelicaudia_ lack a ball-shaped anterior articular face - instead
this surface is rather flat or slightly convex.  Thus they are not really
the opposite condition of the pronounced "cup-and-balled" procoelous
morphology of derived titanosaurs.

Again, the terms "opisthocoelous", "procoelous" and "amphicoelous" are
thrown around a lot, and sometimes with little regard for the great
variation seen in the degree of convexity and concavity of the front and
back articular faces of vertebrae.  Amphicoely grades into amphiplaty and
then into platycoely; but conditions described as "procoelous" have been
used to include incipient expressions of the ball and/or cup (e.g. basal
titanosaurs), as well as the pronounced procoelous condition (e.g. derived
titanosaurs).  The same applies to "opisthocoely"; by itself, it is not an
explicit term. 

Also, although "procoelous caudals" gets cited as a diagnostic titanosaur
character, other caudal articular morphologies are expressed across the
Titanosauria - even among derived titanosaurs.  Many recent studies (and
many old studies recently recognized) show that there is great variation in
caudal morphology among titanosaurs.  _Opisthocoelicaudia_ and its short,
mostly opisthocoelous caudal series represents an extreme condition; but it
is not "off the map" with regard to titanosaur caudal morphology.  Among
titanosaurs, opisthocoelous caudals are not unique to
_Opisthocoelicaudia_....

e.g. http://www.cmnh.org/dinoarch/2002Jan/msg00924.html

> How odd to find bifid neural spines in a dinosaur from the same general 
> region as a putatively independent group of sauropods (e.g., Euhelopus, 
> Mamenchisaurus, Omeisaurus) known to have a series of bifid neural 
> spines. 

Evolution happens.  Diplodocoids also have bifid neural spines (especially
pronounced in dicraeosaurids), so it's not a uniquely Asian phenomenon.


> It seems quite reasonable to me that Opisthocoelicaudia derived from 
> some kind of euhelopodid and acquired its suite of titanosaur 
> characters convergently 

Ahh, what is "reasonable" to you, may not be reasonable to Joe Schmo.  This
is why we have analyses - to compare and test different scenarios.  




Tim