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Are Birds Saurischian?



I am trying to get a handle on cladistics, really I am. And right now I 
am looking at the article "Why Cladistics?" in the June issue of Natural 
History. I can understand (through a glass darkly) the concept of "shared 
derived characteristics." Looking at the sample cladogram on page 34 of 
the magazine, it looks to me like Archaeopteryx is a saurischian 
dinosaur. If so, is it correct to say that all birds are saurischian 
dinosaurs? (I suppose what I'm asking is: If birds are dinosaurs, are 
they a third basic group, or are there still only saurischians and 
ornithischians, and birds belong completely in the former?) And if birds 
ARE saurischians, then only ornithischian dinosaurs became extinct, right?

Thanks for any comments that will help me see through the glass less darkly.

(Did Isaac Asimov write any essays on cladistics? He had a way of 
expressing complicated things simply.)

----- Amado Narvaez
      anarvaez@umd5.umd.edu