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Re: Platypus a living fossil?

>>(Bill Adlam writes):
>>>I don't feel Ornithorhynchus qualifies as a living fossil:
>>The platypus is only a living fossil in so much as some believe
>>that it and the Spiny Anteater are the last surviving therapsids
>>(Does anyone have an opinion on this?  I always regarded them as
>>mammals myself).
>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.

>James Shields  -  jshields@iol.ie  -  http://www.iol.ie/~jshields

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.

The supporters of the "monotremes are therapsids" view argue, as I
understand it, that the loss of the quadrate-articular articulation is so
nearly achieved in Diarthrognathus that its final disappearance (or
disconnection from the jaw apparatus as it did not actually disappear) may
have happened more than once, and therefore monotremes may be convergent to
true mammals in that regard.  Of course another way to look at it is that
our arbitrary choice of the jaw articulation as the deciding defining factor
for mammalia is in part an artifact of the fact that we only have skeletal
evidence for most of the borderline taxa.  If more of these things 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).
Ronald I. Orenstein                           Phone: (905) 820-7886 (home)
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