RE: hand rotation / prosauropods

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

Nick Longrich wrote: 

> The other thing is that if you make Prosauropoda monophyletic, 
> where in the heck are the sauropods coming from? 

Under the monophyletic-Prosauropoda scenario, the Prosauropoda and Sauropoda
are sister taxa within a monophyletic Sauropodomorpha.  I would add that
intermediate scenarios are possible (and have actually been proposed), in
which a subset of prosauropods (e.g. thecodontosaurids +/- anchisaurids) are
basal to a clade comprising "advanced" prosauropods (Plateosauria) and
sauropods.

> While I think Feduccia has abused the ghost lineage argument, the 
> timing of appearance of  organisms in the fossil record does provide 
> some imperfect signal and an additional line of evidence against which 
> our phylogenies can and should be tested. 

I think the Feducciaries have been hoisted by their own petard on that
score.  Anyway, as to the early evolution of sauropods, there is the
discovery of _Isanosaurus_ from the Late Triassic (as David mentioned), a
fairly impressively-sized sauropod.

> In particular, given that sauropod ancestors were likely (a) 
> relatively large, and (b) relatively abundant, being herbivores, it 
> seems unlikely that they would have escaped preservation for so long.

I think we can be certain that sauropod ancestors were herbivores, but I
don't think it can be assumed that they were large.  There is a theory (I
can't recall who proposed it) that the morphology of sauropods shows many
traits in common with juvenile prosauropods.



Tim


------------------------------------------------------------ 


Timothy J. Williams, Ph.D. 

USDA-ARS Researcher 
Agronomy Hall 
Iowa State University 
Ames IA 50014 

Phone: 515 294 9233 
Fax:   515 294 9359