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the dinosaur's rear-facing shoulder socket

A rear-facing shoulder socket is often listed as a diagnostic dinosaurian 
trait (synapomorphy?).  Can anyone suggest what advantage such an 
orientation had for the earliest bipedal dinosaurs (and Marasuchus, as 
well)?  I presume it was retained in the secondarily quadrupedal 
dinosaurs because it is one way for the forelimbs to be oriented in erect 
posture.  Maybe also the forelimbs could move most efficiently (or at 
least _very_ efficiently) in a parasaggital plane with the glenoid in 
that orientation.  Any ideas?

Norman R. King                                       tel:  (812) 464-1794
Department of Geosciences                            fax:  (812) 464-1960
University of Southern Indiana
8600 University Blvd.
Evansville, IN 47712                      e-mail:  nking.ucs@smtp.usi.edu