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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).
Except in special cases, you can determine (for any three taxa) which two
share more derived character with each other.  The assumption is that,
barring additional evidence, those two which share more derived characters
share a more recent common ancestor (even if we have not discovered that
ancestor yet).  For example, we could determine that lions and tigers are
more closely related to each other than either are to bears, even if we
didn't have cat or bear fossils.

>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.

>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.

(And, of course, Protoceratops itself is much too young to be ancestral to

>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)!!

(And the common ancestor of birds & crocs is BY DEFINITION the common
ancestor of Archosauria itself!!)

                 Thomas R. Holtz, Jr.                  Phone: 703-648-5280      
                 Vertebrate Paleontologist         Fax:    703-648-5420
tholtz@geochange.er.usgs.gov ------------>       th81@umail.umd.edu
U.S. Geological Survey          ------------->       University of Maryland
Branch of Paleontology & Stratigraphy ---->       Department of Geology
MS 970 National Center
Reston, VA  22092               ------------->        College Park, MD  20742