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Re: dinosaur synapomorphies?



In a message dated 8/27/01 11:44:25 AM EST, kinman@hotmail.com writes:

<< It amazes me that so many people now take the holophyly of Dinosauria for 
granted. >>

Holophyly of Dinosauria is now legislated by its definition, which is 
node-based: the common ancestor of modern birds and Triceratops and all its 
descendants. This is beyond taking it for granted. I'm not too thrilled by 
this definition and for historical reasons would have preferred this one: the 
common ancestor of Megalosaurus and Iguanodon and all its descendants. This 
is, as far as we know, the exact same clade (though it need not be, for 
example, in the unlikely event that birds are not descended from this common 
ancestor). According to the Triceratops-birds definition, all birds are 
dinosaurs, period. It's not even an issue any more.

Definitions of clades are stated in terms of relationships, not in terms of 
characters or features; today's synapomorphies are tomorrow's homoplasies, 
and character-based definitions are likewise subject to instability. What 
makes any animal a dinosaur is simply that it descended from the common 
ancestor of birds and Triceratops. What might allow us to determine whether a 
particular animal is a dinosaur are its anatomical characters and features, 
which support or refute its hypothesized descent from that common ancestor. 
If it shares enough features with animals that are already known to belong to 
this elite clade (such as Triceratops and modern birds, which are dinosaurs 
by definition), then it is likely that it is a dinosaur as well.

You should frame your critiques of dinosaur features keeping these ideas in 
mind.