Re: Details on SVP 2002 Part 1

["T. Michael Keesey" <mightyodinn@yahoo.com>]

--- Aegyptiacus@aol.com wrote:
> Well, I'm no specialist at all, even less a scientist, but... You say (if 
> I've understood clearly) that a common ancestor of Ornitischian and 
> Saurischians could be something close to *Eoraptor*;IIRC, some phylogenies 
> place *E.* close to basal Theropods (ie Saurischians). Then my question is : 
> is there a real sistership between Ornies and Sauries (sister taxa) or do 
> Ornies come from Sauries, or is there an unnamed clade for their common 
> ancestor (if this one is not *E.*).

A clade cannot consist of a common ancestor; by definition, a clade is an
ancestor plus all of its desendants.

_Saurischia_ can be defined as the first ancestor of _Megalosaurus bucklandi_
which is not also an ancestor of _Iguanodon bernissartensis_, plus all of that
ancestor's descendants. _Ornithischia_ can be defined as the first ancestor of 
_Iguanodon bernissartensis_ which is not also an ancestor of _Megalosaurus
bucklandi_, plus all of that ancestor's descendants. Thus, by definition, the
two are sister groups. What is included in either group may be revised by
further research (e.g., _Herrerasauridae_ has been placed outside either group,
and hence outside _Dinosauria_, by some authors, but now it looks as though it 
belongs to basal _Saurischia_), but the definitions stand.

=====
=====> T. Michael Keesey <keesey@bigfoot.com>
=====> The Dinosauricon <http://dinosauricon.com>
=====> BloodySteak <http://bloodysteak.com>
=====> Instant Messenger <Ric Blayze>
=====

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