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Re: Therizanosaurs + Cladistics



>>As I understand it, a clade is supposed be represented by a particular
>>species and everything which descended from it. The problem is that it is
>>very hard to prove that one species evolved from another degree of certainty.
>
>Almost, but not quite.  Cladistics starts from the premise that things
>evolved from other things (as opposed to, for example, special creation).

Well, now, we wouldn't want to upset the creationists would we?

>>For example, protoceratopsians continue to appear in the fossil record after
>>the appearance of true ceratopsians.
>
>I expect you mean "Ceratopsidae" (i.e., the horned ceratopsians), since
>protoceratopsians are members of Ceratopsia.

Well yes. These names that are almost the same up to the last few letters
fonfust the hell out of me.

>>How can we tell where the ceratopsians
>>broke off the protoceratopsian line? So far as I know, we can't (at least
>>not without a lot more "in-between" species). All we can say with any
>>certainty is that they had a common anscestor. It could be that
>>Protoceratops itself is that common ancestor, but if that is the case the
>>Clade definition is still valid.
>
>If you review either Sereno 1986 or Dodson & Currie 1990, you will see two
>alternate hypotheses.  In Sereno, "protoceratopsians" are a paraphyletic
>outgroup to Ceratopsidae (i.e., some protoceratopsians [Protoceratops, for
>example] are closer to ceratopsids than are other protoceratopsians
>[Leptoceratops, for example]).  In Dodson & Currie, Protoceratopsidae does
>not include the ancestors of Ceratopsidae, but represent a second radiation
>of ceratopsians.

That is sort of what I was trying to say. Because Protoceratopsians are
"more primative" than ceratopsidae one might be tempted to claim that they
evolved from them. One then might decide to pick an early member of
protoceratopsidae (perhaps with a feature common to all ceratopsians) and
say "That one!" and put it in the box as the root of ceratopsia. However,
while I think it likely that ceratopsidae evolved from protoceratopsidae,
and certainly that the common ancestor looked very like a protoceratopsian,
I don't beleive we have enough evidence so say so with any certainty.

>>Now, my gripe with cladistics is definitions such as "All archosaurs closer
>>to birds than to crocodiles". This leaves some doubt over whether they had a
>>common ancestor (other than the common ancestor of the archosaurs itself).
>
>Unless you believe in special creation, ANY two taxa have a common ancestor
>(at some level)!!

Yes, of course they have a common ancestor, I just think this sort of
definition doesn't imply a proper cladistic relationship. Now I think about
it, its not a gripe with cladistics, but the way individuals use them.
(Oh-oh I'm bound to stepping on someone's toes now - sorry)

James Shields  -  jshields@iol.ie  -  http://www.iol.ie/~jshields
-------------------------------------------------------------------------------
And when the ark was finished Noah said unto Elvis, "What do you reckin?"
And Elvis checked out his own cabin and shook his head saying "poky".
And so did they knock several walls through and install a jaccuzzi.
And when it was all done Noah scratched his beard and said, "We don't have
room for all the animals now."
And Elvis perused the livestock list and in his wisdom said, "Lose the
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        -Robert Rankin, The Suburban Book of the Dead