RE: Opisthocoelicaudia (was Re: Titanosaurids)

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

Dinogeorge@aol.com wrote:

>It's not just the tail, its the sacral vertebrae and the bifid dorsal 
>vertebrae (unknown in titoanosaurs). Don't get distracted by the limbs!

Well, that's THREE characters (the first two of which may be functionally
correlated) all of which can be readily interpreted as autapomorphies of the
genus _Opisthocoelicaudia_.  There are many more characters that unite
_Opisthocoelicaudia_ with the "typical" titanosaurids.

Sure, the bifid dorsal spines are unique among titanosaurs - but whatever
way you shake the sauropod tree, this is a character that evolved more than
once in the Sauropoda.  Homoplasy is rampant in this group, and I can't see
how you can intuitively regard bifid neural spines as linking
_Opisthocoelicaudia_ with the camarasaurids simply because
_Opisthocoelicaudia_ "looks like" a camarasaurid.

_Opisthocoelicaudia_ is just an unusual titanosaur.  George, I remember (not
so long ago) you shouting from the rooftops that segnosaurs COULD NOT under
any circumstances be theropods.  After all, that primitive prosauropod-like
pes precluded them from a place in the Theropoda - despite cladistic
analyses saying otherwise.  Cladistics - bah humbug!  Then along came
_Beipiaosaurus_... and tum-de-dum, maybe segnosaurs are theropods after all.

>And if we're going to invoke homeobox genes in the formation of the tail,
>why not invoke them to explain all kinds of other character anomalies in
>cladograms as well? 

Well, a transition from cup-and-ball to ball-and-cup configuration lends
itself to this interpretation.

>It might just be, but this gives it no chance: if any
>question about a morpho-cladogram is answered by invoking hidden genetic
>causes, then the science goes right out the window.)

True.  But if such changes are demonstrable (e.g. developmentally correlated
mammal tooth characters) then this new data needs to be incorporated into
phylogenetic analyses.



Tim