Re: Def. of Ornithischia

["Thomas R. Holtz, Jr." <th81@umail.umd.edu>]

At 01:01 PM 6/11/97 -0400, you wrote:
>   What is the official phylogenetic definition of Ornithischia?

Using the principle of priority, under phylogenetic taxonomy the definition
of Ornithischia is "Triceratops and all taxa sharing a more recent common
ancestor with Triceratops than with birds": Padian and May, 1993.  (Of
course, this was before I suggested to Padian that Iguanodon would have been
historically a better choice...).

Thus, if sauropodomorphs DO share a more recent common ancestor with
ceratopsians, ornithopods, etc. than with theropods, than Sauropodomorpha is
a subclade of Ornithischia.  (Of course, "Phytodinosauria" could be defined
phylogenetically as "all descendants of the most recent common ancestor of
Triceratops and Diplodocus" or something similar, so that it could be
retained as a valid taxon).

While on this subject, the node joining Pisanosaurus and Genasauria (or
Lesothosaurus and Genasauria) has not been named.  "Predentata" might be
used as a node-based taxon using either of those definitions.

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