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Re: Re: two or three sacral vertebrae?



>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_.

[material deleted for space's sake]

> 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?

It depends on what you call a dinosaur.  IF herrerasaurids such as
_Herrerasaurus_, _Staurikosaurus_, and _Chindesaurus_ are true dinosaurs,
then the full incorporation of a third sacrum is not a dinosaurian
synapomorphy (or alternatively, the "disincorporation" of the third is a
synapmorphy of Herrerasauridae).  If herrerasaurids are the sister taxa to
Dinosauria, then three fully incorporated is the basal condition for all
dinosaurs.

No matter what is the position of Herrerasauridae, the five fully
incorporated sacrals in ornithischians and neotheropods is clearly convergent.

[Historically, the incorporation of extra sacrals has been very important in
dinosaur studies.  This was one of a handful of characters that Owen
recognized in the original diagnosis of Dinosauria.]


Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661