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GRADE VS CLADE
Jon Wagner had brought up the subject of grade vs clade a couple of weeks ago
and now that I have some time, I think I will comment on it a little bit.
For the most part, I agree with Jon's statements that the term "grade" is
phylogenetically useless for the most part. There are a few instances where
the term can be usefull in a general way.
Lemme get to some examples using ornithopods :-) According to my unpublished,
preliminary, hand-claded phylogeny of Ornithopoda, there are in fact no
hypsilophodontids besides Hypsilophodon itself. There are however, two very
fragmentary ornithopods that I call "of Hypsilophodont grade": Yandusaurus
hongheensis and Atlascopcosaurus leadsi.
They are of hypsilophodont grade because they have teeth that are similar to
Hypsilophodon, but show no synapomorphies that either link them to
Hypsilophodon or Iguanodontia. In other word, they fall into an unresolved
polytomy, but are too fragmentary to have their relationships known more
This is different from the more common useage of "grade." Using again, the
same unpublished, preliminary, hand claded phylogeny of Ornithopoda, many
people would refer to the term "hypsilophodont grade" as being a paraphyletic
"stem" that is equivilant to all ornithopods that aren't iguanodontians or
heterodontosaurids or marginocephailians.
Darling, can I just tell you something? You can never have enough gloves,
hats. and shoes.