Re: Theropod phylogenies

["Tim Williams" <twilliams_alpha@hotmail.com>]

"Richard Forrest" <richard@plesiosaur.com> wrote:

> I don't understand: if we can't resolve the relationships of modern birds, 
> how does fossil evidence help?

Extant birds are very derived.  Each avian order has had a long time (> 65 
Ma for some/most? orders) to accumulate a long list of apomorphies.  
Moreover, the different neornithine lines ("orders") may have evolved very 
rapidly, as seems to be the case for placentals.  Thus the apomorphies that 
diagnose each order might have appeared quite suddenly.  Discovering taxa 
(i.e. fossils) at the base of each order will help to resolve the basal 
relationships of neornithine orders.  Basal taxa are more likely to preserve 
primitive character-states that can be used to link one order to another.

To give a comparable situation to illustrate the value of basal taxa:  The 
discovery of basal oviraptorosaurs (_Incisivosaurus_), basal therizinosaurs 
(_Beipiaosaurus_), basal dromaeosaurids (_Microraptor_) and basal 
troodontids (_Sinovenator_) has helped enormously in resolving the basal 
relationships of maniraptorans.  The dromaeosaurid-troodonid clade 
(Deinonychosauria) and oviraptorosaur-therizinosaur clade ("Enigmosauria") 
now appear far superior to alternative arrangements.  If we only had the 
derived representatives to base our analyses on (e.g., _Erlikosaurus_, 
_Velociraptor_, _Oviraptor_, _Troodon_) the resulting phylogenies would be 
much murkier and less stable (as indeed they were, prior to the discovery of 
the Jehol fauna).



Tim

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