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Re: Theropod phylogenies





"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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