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RE: tyrannosaurid ancestry, raptors and other fun things...



> -----Original Message-----
> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> Buckaroobwana@aol.com
> Sent: Saturday, November 13, 1999 11:01 PM
> To: dinosaur@usc.edu
> Subject: tyrannosaurid ancestry, raptors and other fun things...
>
>
> Greetings,
> Can anybody tell me how closely related tyrannosaurids are to
> dromeosaurids.
> I know that the current theory is that the tyrannosaurs evolved
> from smaller
> carnivorous dinosaurs, but which ones?

This questions seems in part an "ancestor hunt".  Pleased be advised that it
is unlikely to discovery the exact particular ancestors for any given group
of animals and plants in the fossil record, since so much of the Tree of
Life is missing.  However, one can (and does!) look for the closest
relatives to a group that is preserved.

There are three main models of tyrannosaurid origins:
A) Tyrannosaurids are basal coelurosaurs, outside the
ornithomimosaur-maniraptoran radiation Maniraptoriformes.  This result was
found by Makovicky & Sues, the AMNH team (at the Ostrom Symposium), and
others: I *think* this is Greg Paul's current position as well.

B) Tyrannosaurids are maniraptoriforms, and closer to ornithomimosaurs than
to birds (i.e., are arctometatarsalians).  This is my result, has been found
by a few other authors (i.e., Perez-Moreno et al. 1993), and was first
suggested by Matthew & Brown and Huene in the 1920s (not that they used the
same terms we do now).

C) Tyrannosaurids are maniraptoriforms, and are closer to birds than to
ornithomimosaurs (i.e., are maniraptorans).  Sereno is the main proponent of
this idea, with tyrannosaurs as the basalmost members of Maniraptora (which
he has renamed Tyrannoraptora, moving Maniraptora to a different node...).

So, tyrannosaurids would not be descended from any particular named "family"
of coelurosaurs, but would arise out of the same ancestral stock as
compsognathids, ornithomimosaurs, oviraptorosaurs, dromaeosaurs, birds, and
the rest.  In part we suffer the problem of there being no tyrannosaurid
analogue to _Archaeopteryx_: by the time we get good tyrannosaurid fossils
in the record, they are well and truly derived tyrannosaurids.

Note that the three different models make some different predictions about
tyrannosaurid ancestry (i.e., in model B the ancestors of tyrannosaurids
would probably have an arctometatarsus and incisiform teeth prior to the
reduced forelimb, whereas in A and C the order of acquisition of these
features is uncertain).

> Is there any evidence to
> support Greg
> Pauls contention that tyrannosaurs arose from advanced allosaurs from the
> early Cretaceous?

Not as such.  Very few characters are found in tyrannosaurids and
allosaurids which are not also found in some or all of the small
coelurosaurs.

> Has the debate regarding the placement of
> Triceratops front
> legs been resolved? It would be really interesting to see if Triceratops
> could gallop.

No, it hasn't been fully resolved.  Yes, it would be interesting!!

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742
http://www.geol.umd.edu/~tholtz/tholtz.htm
http://www.geol.umd.edu/~jmerck/eltsite
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-314-7843