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Re: Platypus a living fossil?

>>The distinction between mammals and therapsids is fairly clearly defined (It
>>has to do with the shape of the jaw). I'm pretty sure both Platypus and
>>Spiny Anteaters fall on the mammal side.
>Strictly speaking it is not the shape of the jaw but the nature of the jaw
>articulation.  Non-mammalian tetrapods have the articulation between the
>quadrate of the skull and the articular of the jaw.  In "true" mammals the
>quadrate and that articular have become the malleus and incus of the middle
>ear, and the mandible consists of a single bone, the dentary, that
>articulates with the squamosal of the  skull.  One therapsid,
>Diarthrognathus, actually has both articulations present.

Strictly STRICTLY speaking (okay, cladistically speaking), Mammalia and
other taxa are not *defined* by characters, only *diagnosed* by such.
Mammalia should (and has) been defined phylogenetically: the most recent
common ancestor of monotremes, marsupials, and placentals and all of that
ancestor's descendants.  As such, Ornithorhyncus is necessarily a mammal!

(Some people have quibbled with the above [Tim Rowe's] definition of
Mammalia, but even so, a phylogenetic definition is not based directly on
the presence or absence of a particular character.)

> If more of these things [nonmammalian synapsids]  were
>still around we would have a broad range of soft-part characters to work
>from, and may well have defined mammals differently as a result (for a
>cladist, I suppose the question is which node is taken as the basal one for
>Mammalia, and put that way I guess it's pretty arbitrary).

Well, as you say, these things are ultimately arbitrary.  Some near-mammals
of the Mesozoic do lose out under the above defintion, while others (esp.
Multituberculata) are ambiguous to being true mammals or an outgroup.

Also, I would add that if nonavian theropods were still around, these
critters would have long been recognized as being close to birds, and the
resistance to the dinosaurian nature of birds would have been much, much

                 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