Re: What is a Dinosaur?

["David Marjanovic" <david.marjanovic@gmx.at>]

> Comments welcome as always.

You write:

"It's nice to see that Owen's fused sacrum is still there, number two on the
character list - even if it's been somewhat diluted by the discovery of more
basal dinosaurs. But it's important to understand that this character is now
only a part of the diagnosis, not the definition."

Less nice is recent work by Galton that shows that this character is wrong.
B-) Basal dinosaurs all have 2 sacrals, and among those prosauropods that
have 3 sometimes a dorsal and sometimes a caudal has joined the two original
sacrals.

Regarding Eoraptor:

"If it's at point B, then it's a dinosaur, but does not fall in either of
the major dinosaurian orders Saurischia and Ornithischia - it's a ``basal
dinosaur''.

If it falls at point C, then it's a basal saurischian, neither
Sauropodomorph nor Theropod."

All wrong :-) . If it's at point B, then it's a basal saurischian because
Saurischia is stem-defined. If it's at C, then it's a basal theropod because
Theropoda is likewise stem-defined.

"For what its worth, depending on what definitions you use for the various
groups in this cladogram, some of the branches do not exist. For example,
Saurischia and Ornithischia are sometimes defined as sister stem taxa
{Passer > Triceratops} (meaning Passer and everything that has a more recent
common ancestor with it than with Triceratops) and {Triceratops > Passer}
respectively. These definitions exactly partition the dinosauria, so that
there is no such thing as a ``basal dinosaur'' which is neither saurischian
nor ornithischian - so point B does not exist."

Points B and C exist. You just have put the names Saurischia and Theropoda
at nodes instead of at the stem. (Dinosauria and Archosauria are nodes and
belong at the places you show.)

There is a basal dinosaur: The Most Recent Common Ancestor. Recently it was
debated on the PhyloCode mailing list whether such ancestors are an entire
species or just one breeding pair.

Oh, and maybe you should explain Crocodylotarsi and Crurotarsi.