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The Two Principal Dinosaur Clades Defined



In a message dated 95-12-08 13:26:25 EST, Jeff Poling wrote (and I've taken
the liberty of posting this little portion of your e-mail to the whole list,
just to clear up--perhaps--what may be a slight misconception):

>>Make them "_small_ arboreal dinosaurs" and you've got BCF. The issue is
>>arboreality, not the name bestowed upon the still largely hypothetical
>>dino-birds.
>
>   Not necessarily.  THere's a question of descent here.  If I recall
>correctly, you believe that (all )dinosaurs evolved from small arboreal
>proto-birds.  "Mainstream" belief has it the other way around.

Cladistically, the proto-birds, dino-birds, or small arboreal
dinosaurs--whatever you'd like to call them--would _all_ be dinosaurs--as are
birds themselves--being members of the clade Dinosauria: descended from the
last common ancestor of _Megalosaurus_ and _Iguanodon_. The way I redefine
Aves, however--as the clade of all dinosaurs closer to _Megalosaurus_ than to
_Iguanodon_--would also make avians out of proto-birds, dino-birds, small,
arboreal dinosaurs, and all known theropods, including birds. And just to
make the picture _almost_ complete, the clade Phytodinosauria is defined as
all dinosaurs closer to _Iguanodon_ than to _Megalosaurus_.

This leaves the actual most recent common ancestor of Dinosauria _out_ of
either Phytodinosauria or Aves. When and if it ever turns up, we could (1)
create a little paraphyletic group just for it, which would preserve Aves and
Phytodinosauria as clades; (2) append it to either Aves or Phytodinosauria,
which would paraphyletize the clade to which it was appended; or (3) leave it
outside the classification of Dinosauria, which I guess is what cladists
might do to try to avoid the demon Paraphyly (although this is really the
same thing as alternative 1, only the larger paraphyletic group is not
explicitly created and the common ancestor itself becomes the paraphyletic
group). There are no other alternatives.

By the way, now you see the general problem the common ancestor causes for
the cladists' insistence on completely monophyletic groups. Something _has_
to be paraphyletic, no matter how you slice it.