Re: synapomorphies not created equal

["Mickey Mortimer" <Mickey_Mortimer11@msn.com>]

George Olshevsky wrote-

> For example, two apomorphies that are said to unite Ornithischia are the
> presence of a predentary bone and the presence of an opisthopubic pelvis.
One
> of these must have appeared first, but the fossil record is too poor to
tell
> which. Suppose the predentary appeared first. Then it's not a synapomorphy
of
> Ornithischia; it's a plesiomorphy, because there was at least one
> non-ornithischian animal that had a predentary but not the opisthopubic
> pelvis. (Likewise if the opisthopubic pelvis appeared first.)

Not exactly.  Ornithischia is a stem group (all closer to Triceratops than
Neornithes), so as long as a character isn't present in Dinosauria or more
inclusive groups, it's a synapomorphy of Ornithischia.  Now a character that
appeared after the first ornithischian taxon (eg. opithopubic pelvis), would
be a synapomorphy of an ornithischian subgroup.

Mickey Mortimer