Re: Dinosauria

[Stan Friesen <swf@ElSegundoCA.NCR.COM>]

 From: Saurischia@aol.com

 > >I've seen often stated, I think even in this mailing list, that there
 > >were no aquatic dinos, yet some exist even today (penguins). ...
 > 
 > Traditionally, my understanding was that "dinosaurs" were extinct 
 > Archosaurian Diapsids, which would include Thecodonts, Pterosaurs, 
 > Saurischians, and Ornithischians, (and Placodonts?), excluding only the 
 > Crocodylia. 

Strictly speaking the term applies to the Ornithiscia and Saurischia
only.  This *excludes* thecodonts and pterosaurs as well as crocodilians.

 > But am I to understand that birds, or more accurately, 
 > animals in the class Aves, are also members of dinosauria? 

Under the *cladistic* definition of Saurischia they must be included
in the group, and thus in the dinosaurs.

This is because the under cladistic taxonomy a taxon must include
ALL descendents of the most recent common ancestor, and birds are
almost certainly derived from theropod dinosaurs (and thus ARE
theropod dinosaurs to a cladist).

Of course if you allow that a taxonomic group can exclude some
descendent groups, then birds need not be considered dinosaurs.
rm 

swf@elsegundoca.ncr.com         sarima@ix.netcom.com

The peace of God be with you.