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>_Lagosuchus_ was close to, but not quite, a dinosaur. I often see the
>character "perforated acetabulum" as defining dinosaur, but _Lagosuchus_
>had that trait (and, yes--I know there are even other non-dinosaurian
>reptiles having perforated acetabula). Then, I've seen three sacral
>vertebrae specified for dinosaur, but apparently _Herrerasaurus_ and
>_Eoraptor_ still had only two "fully incorporated" (as labeled by Sereno
>and Novas) sacral vertebrae.
>So, I'm not sure what to say it REALLY is that shows _Lagosuchus_ is not
>a dinosaur. Can anyone help me with this one? Thanks in advance.
_Lagosuchus_ (or _Marasuchus_) is not a dinosaur because the most
parsimonious distribution of derived character states yields a position
outside of the node joining birds and _Triceratops_ (or _Megalosaurus_ and
_Iguanodon_). It is very close to that group, but has not yet been
demonstrated to lie within that clade (and hence is not a dinosaur).
Thomas R. Holtz, Jr.
Vertebrate Paleontologist Webpage: http://www.geol.umd.edu
Dept. of Geology Email:email@example.com
University of Maryland Phone:301-405-4084
College Park, MD 20742 Fax: 301-314-9661