two or three sacral vertebrae?

["King, Norm" <nking.ucs@smtp.usi.edu>]

I am confused about an article in SCIENCE, v. 258 (13 Nov., 1992), p. 
1137-1140, by Paul Sereno and Fernando Novas.  The article is about 
insight into basal dinosaur phylogeny from discovery of an essentially 
complete skeleton, with skull, of _Herrerasaurus ischigualastensis_.

On p. 1138 (2nd column, 2nd paragraph), they state that a valid 
synapomorphy of the Dinosauria is the addition of a dorsal vertebra to 
the sacrum.  That would indicate that dinosaurs have three sacral 
vertebrae.

However, on p. 1139, they state that there are only two fully 
incorporated sacral vertebrae in _Herrerasaurus_ and _Staurikosaurus_.  
In fact, they used that observation to conclude that full incorporation 
into the sacrum of at least one additional dorsal vertebra must have 
occurred independently in ornithischians and saurischians (i.e., after 
the time of _Herrerasaurus_ and _Staurikosaurus_).

There seems to be a contradiction here.  I have tentatively concluded 
that "full incorporation" is the key phrase.  In other words, should 
their statement on p. 1138 be interpreted as meaning that the additional 
dorsal vertebra in the sacrum of the Dinosauria doesn't necessarily have 
to be _fully incorporated_, thus allowing _Herrerasaurus_ and 
_Staurikosaurus_ to qualify as dinosaurs?

Bottom line question:  What IS the minimum sacral vertebrae count for 
dinosaurs?

Thanks in advance for replies.


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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