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(This message is in reply to Amy Tilescia's question about the
"official" definition of"dinosaur")
I don't pretend to be an expert, but the definition of
"dinosaur" still seems to be a bone (no pun intended) of
contention among paleontologists. Many experts
believe that "dinosaur" is not a natural group, and just
talk about "archosaurs" (which includes dinosaurs, pterosaurs,
crocodilians, thecodonts, and birds). The definition of
archosaurs is very clear-cut--it is the clade of all
vertebrates that have a diapsid (double-holed) skull, plus
an additional hole in the snout, forward of the eye sockets.
The definition of dinosaurs is much more controversial, and
as I say, some paleontologists say there is none. However,
others say they have found a "homologue" (defining
characteristic) for the Dinosauria. Robert Bakker defines
the dinosaurs as the clade of archosaurs that possessed a
"wicked claw mounted on a twisted bone." This
"twist-thumb claw" sounds perfectly reasonable, except that
I'm not sure such a large and important taxonomic dynasty
should be founded on such a trifling skeletal feature. And
as this definition for the Dinosauria also takes in birds, it
also creates a problem for those who can't bring themselves
to call the little hummingbirds in their backyard dinosaurs.
At the risk of rattling on too long (as I sometimes do on
subjects I'm so intrigued by like dinosaurs) I'll stop here.
I hope I've at least given you an idea of some of the ideas
that are "floating around".
P.S. I imagine that others in the mailing list will want to
comment on what I've said so I'm senting a copy of this message
to the mailing list address as well.