Opisthocoelicaudia and cladistics (was Re: Titanosaurids) (long)

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

George wrote:

<< 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_. 

>No, that's >dozens< of characters, since there are lots of dorsals,
>caudals, and sacrals that are different. 

Does that mean we have to count every individual feather of
_Archaeopterx_ to find out how many characters it shares with
modern birds?  Jeez Louise!

>The appendicular characters are minor and 
>insignificant, and can easily come about through convergence, because
>all sauropods had to walk and carry great body mass. 

This cuts both ways.  The bifid neural spines had a purpose too (yeah,
that's right - they weren't just there for decoration).  The functions
ascribed to bifurcation of the neural spine imply that this character be
expressed serially down the column (or a section of it), so I don't think
it's valid to score each individual vertebra as an isolated character.

>The big problem with cladistics is that there is no way to weight
>characters, so it becomes a phony one-character-one-vote situation. You
>can't equate a character that appears everywhere along the spine with a
>character that is a single small lump on a long bone. Cladistic
analysis >is not necessarily a democracy. 

Nor should phylogeny be ruled by characters we happen to like the most.
Then we're back to birds evolving from megalancosaurids, simply because "my
sixth sense tells me" that long arms and a triangular skull of
_Megalancosaurus_ carry more weight than the 135 characters that place Aves
within the Theropoda.  

Certainly, this is an extreme example (and I'm not accusing George of
subscribing to such nonsense).  But it illustrates the point that we
shouldn't pick and choose our characters - unless we have damn good evidence
that they're developmentally or functionally linked.

 
Rutger Jansma wrote:

>Because of it's stiff tail, Opisthocoelicaudia wasn't able to contact
>the ground, leaving the specially adapted chevrons useless. If
>something is useless in nature, look at the example of the monkey-tail,
>it gets reduced and eventually get lost. 

You may well be right in this explanation.

>It isn't something rare in nature, flightless
>birds did the same, the lost their power to fly because their wings
>became to short to provide any lift.

I think your cause and effect are transposed.  The wings become shorter
because the bird no longer needs to fly.

>And again, if you read the post more closely, it's not just the tail I
>was talking about, it also concerns the sacral vertebrae:
>"titanosaur-sacra have a extra dorsosacral in their formation, but 
>when you look at Opisto-guy here, he included a caudal vertebrae in it's
>sacrum! Totally different configuration=totally different ancestry!" 

Again, this is ONE character.  Even if this interpretation is correct (I'd
have to check my references to see if all sauropod workers are unanimous on
this point), when it comes to the number of vertebrae incorporated into the
sacrum, there is no reason that a previous condition can be un-done. Here's
one possible scenario: (1) the dorsosacral was detached from the sacrum; (2)
a caudal was recruited into the sacrum.  This could occur as two separate
steps, or as a single transformation.

Stranger things have happened.  You cannot assume _a priori_ that different
homologies for the six sacral vertebrae imply an independent ancestry.

>Diving dinosaurs are probably something we could only fantasize about
>untill some evidence has come to light. 

Fantasize no more!  We already know about those sexy hesperornithiforms,
penguins, puffins, ducks, loons ... :-)


Tracy Ford wrote:

>To me, it's pretty ludicrous that there is only one group of Late
>Cretaceous Sauropods. 

Morphologically speaking, the Titanosaurifomes is very diverse - especially
when it includes everything from _Brachiosaurus_ to _Pleurocoelus_ to
_Andesaurus_ to _Phuwiangosaurus_ to _Aeolosaurus_ to _"Titanosaurus"
colberti_" to _Saltasaurus_ to _Opisthocoelicaudia_ (yes, even him).  I
think the success of the Titanosauriformes in the Late Cretaceous is
analogous to the success enjoyed by the Maniraptoriformes at around the same
time - except that the dominance of titanosauriforms was global.  (Maybe it
was for maniraptoriforms as well!)

Having said that, there are possible non-titanosauriforms from the Late
Cretaceous, such as _Austrosaurus_ (which Molnar has called a
titanosauriform, though the evidence isn't compelling), _Campylodoniscus_,
and the _Antarctosaurus_ jaw material.

>The Diplodocid looking titanosaurs,are IMHO diplodocids, the Camarasaur
>looking titanosaurs are IMHO camarasaurds, etc. There is new evidence,
>still being worked on, that will...Ok, I won't be mean... 

If new evidence backs that up, terrific!  The more the merrier, when it
comes to sauropod diversity.



Tim