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Re: definition of dinosaur



In a message dated 96-02-22 10:57:09 EST, nking.ucs@smtp.usi.edu (King, Norm)
writes:

>I believe I read somewhere (I think it was here) that we should define 
>dinosaurs as all the descendants of the most recent common ancestor of 
>_Megalosaurus_ and _Iguanodon_.
>
>I can provide that definition to my students (but I will have to follow 
>up with an explanation as to how they can recognize a dinosaur when they 
>see one).  I predict that these questions will be asked about the 
>cladistic definition:
>
>Why are those two genera singled out?  Isn't it true that after all these 
>years we still do not have even one complete composite skeleton of 
>_Megalosaurus_?  Why not choose _Allosaurus_, instead?

That suggestion was mine. _Megalosaurus_ and _Iguanodon_ were the first two
dinosaur genera to be scientifically described, and by coincidence they
happen to represent the two clades of Dinosauria (in the taxonomy proposed by
Michael Cooper [1985] and, probably independently, by Bob Bakker [1986], and
supported by my own investigations, such as they are): Phytodinosauria
Bakker, 1986 (= Ornithischiformes Cooper, 1985) and Aves _sensu lato_,
including all theropod-like archosaurs from _Longisquama_ to extant birds.
All we need at this level of definition is to have _Iguanodon_ be a
certifiable phytodinosaur (it is, more specifically, an ornithopod) and
_Megalosaurus_ to be a certifiable avian (it is, more specifically, a
theropod).

Define Phytodinosauria to be all dinosaurs closer to _Iguanodon_ than to
_Megalosaurus_, and Aves to be all dinosaurs closer to _Megalosaurus_ than to
_Iguanodon_, and you're done. This classification, not entirely by
coincidence, separates the plant-eating dinosaurs from the meat-eating
dinosaurs (not counting, of course, those avians that evolved herbivory
secondarily much later in the evolution of Aves).

I'll have more details in _Mesozoic Meanderings_ #2, which I'm still
rewriting. Lots of text, lots of cladograms, lots of characters discussed.
(Too much work.)