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RE: Fossil Discovery Threatens Theory of Birds' Evolution




> -----Original Message-----
> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> NJPharris@aol.com
> Sent: Sunday, June 25, 2000 4:02 AM
> To: dinosaur@usc.edu
> Subject: Re: Fossil Discovery Threatens Theory of Birds' Evolution
>
>
> In a message dated 6/25/00 12:05:38 AM Pacific Daylight Time,
> dbensen@gotnet.net writes:
>
> > >>The definition of "mammal" has changed in the last decade or
> so. How do
> > you know that a live Olyagopolis would look like a reptile?<<
> >  Isn't mammals' diagnostic feature (besides mummeries, I mean), their
> single
> > lower jawbone and the presence of ear bones?
>
> Not according to phylogenetic taxonomists.  The "diagnostic
> feature" for a
> mammal, if you're a PT practitioner, is that it is descended from
> (or is) the
> common ancestor of all living mammals.
>
> Personally, I'd prefer the apomorphy-based definition: Mammalia =
> {the first
> ancestor of modern mammals to possess a single lower jawbone + all of its
> descendents}; but, alas, it was not to be.

Tsk, tsk, tsk...

Please don't confuse "definition" with "diagnosis".  Definition, in PT, is a
statement of common ancestry.  Diagnosis remains a list of derived character
states optimized at that particular node.

I admit I would personally prefer a more inclusive definition of Mammalia
(perhaps all descendants of the MRCA of Morganucodontidae and Holotheria
(see below), which is Rowe's Mammaliaformes) and use of Hopson's Holotheria
for all descedants of the MRCA of monotremes, marsupials, and placentals.
However, priority of phylogenetic definition has its uses.

Also, although what Ken Kinman said is true to a degree (i.e., that most
biologists do not use the Rowe definition), it is important to remember that
the majority of biologists are working in a situation where it is
unambiguous if an animal is a mammal or not (i.e., they are neontologists).
HOWEVER, for those people working where the difference in definition matters
(workers on advanced non-mammalian therapsids and early mammals), there is
much much greater acceptance of these definitions.

                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