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From: winalski@psw.enet.dec.com (Paul S. Winalski)
Newsgroups: sci.bio
Subject: Re: Platypus bones (was Re: What's a monotreme?)
Keywords: monotremes
Message-ID: <1991Apr26.183747.21006@hollie.rdg.dec.com>
Date: 26 Apr 91 18:37:47 GMT
References: <1991Apr22.111159.29888@kuhub.cc.ukans.edu> <4896@kitty.UUCP> <1991Apr25.182824.18628@hollie.rdg.dec.com> <420@smds.UUCP>
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Reply-To: winalski@psw.enet.dec.com (Paul S. Winalski)
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In article <420@smds.UUCP>, rh@smds.UUCP (Richard Harter) writes:

|>The cases are quite different.  The dividing line between the threrapsid
|>reptiles and the mammals is the jaw bone.  I.e. fossils in that lineage
|>are classified as one or the other based on whether certain bones are
|>fused or not.

The actual dividing line between extant reptiles and mammals is lactation.
On the evidence of the extant species, the monotremes deserve classification
as mammals.  Of course, we cannot determine from the fossil record whether
theraspids suckled their young.  We have to fall back on bone structure
analysis. It seems to me that we have three classifications here:  separate
bones in the jaw (theraspids), fused bones in the jaw (marsupial and placental
mammals), and bones separate at hatching but fused in the adult (monotremes).
On the basis of this evidence of the extant species, I would be inclined to
group the monotremes with the mammals and to treat the lack of fusion of
the jaw in the juvenile as a neontological artifact in the same category
as the gill slits in human embryos.

--PSW