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RE: Ornithischian cheeks (was Re: Dinosaur Planet, parts 3 & 4)



> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> Dino Guy Ralph
>
> "Ken Carpenter" <Kcarpenter@dmns.org> wrote:
>
> > One must also be careful in the assumptions made about birds (i.e.,
> picking the stand-in). Witmer argued by phyletic bracketing that birds and
> crocs lack muscular cheeks, therefore ornithischians lack them as
> ll.   --SNIP--
>
> In addition to the excellent points Ken raised, I ask the following:
>
> Are soft tissue features of ornithischian dinosaurs actually
> constrained by
> phylogenetic bracketing that examines extant crocodilians and birds when
> ornithischians are not themselves ancestral to birds and the hypothetical
> ornithischian features (cheeks of some kind in this case) are derived
> characters that may not have been present in the most recent
> common ancestor
> of ornithischians and birds (presumably the most basal dinosaur)?

There seems to be a major misunderstanding here.

Phylogenetic bracketing does not say "anything that isn't found in the most
recent common ancestor of crocs and birds CANNOT be found in any descendant
of that ancestor"!!

It states:
1) Traits shared in common by two extant sister taxa (derived states or
primitive states) are inferred to be present in any extinct taxon nested
within this clade *all other things being equal*

and

2) Inference that a trait not present in both extant sister taxa are
ambiguous *all other things being equal*.

and

3) The way to make things unequal are independant lines of evidence.

So, with regards to cheeks, the original Witmer papers did not say "cheeks
cannot have been present in any ornithischian"; only that "one must have
strong physical evidence to support such a claim."  Subsequent to that,
Witmer and colleagues have presented their interpetation that the
traditional evidence for ornithischian cheeks is weak.  Honestly, I don't
necessarily buy their interpretation (yet to be printed, as far as I know),
and I agree with Ken on a number of his points there.

So, yes: one cannot assume that all things crocodilian would be found in
non-avian dinosaurs, just as you cannot assume that all things avian would
be found in non-avian dinosaurs.  Nevertheless, you CAN assume that anything
found in BOTH crocs and birds WOULD be present in non-avian dinosaurs,
unless you have direct evidence to contradict it.

So, as most crocodilians and most birds share the (in this case derived)
trait of hanging around the nest and with the babies for a couple of weeks
at least, the inference is that non-avian dinosaurs, aetosaurs,
rauisuchians, etc. did as well.  That doesn't mean that at the end of those
few weeks the baby sauropods, baby tyrannosaurs, baby aetosaurs, baby
rauisuchians, etc., followed the parents around for the rest of their lives!
That is a very different issue.

                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): 301-405-0796