Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!wuarchive!uunet!tdatirv!sarima
From: sarima@tdatirv.UUCP (Stanley Friesen)
Newsgroups: sci.bio
Subject: Re: Platypus bones (was Re: What's a monotreme?)
Keywords: monotremes
Message-ID: <220@tdatirv.UUCP>
Date: 27 Apr 91 17:29:44 GMT
References: <1991Apr22.111159.29888@kuhub.cc.ukans.edu> <4896@kitty.UUCP> <1991Apr25.182824.18628@hollie.rdg.dec.com> <420@smds.UUCP>
Reply-To: sarima@tdatirv.UUCP (Stanley Friesen)
Organization: Teradata Corp., Irvine
Lines: 18

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.

Hmm, as I think on this, I seem to remember that it is not a matter of
fusion so much as reorganization.   The reptilian jaw has several bones
that the mammalian jaw does not.  In mammals these bones are known to have
moved into the middle ear and become ear ossicles.  In conjunction with this
a new jaw joint developed between two bones that do not even touch in
reptiles.

In fossils, where lactation is unobservable, it is the existance of this
secondary jaw joint that is used as the working definition of mammal.
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uunet!tdatirv!sarima				(Stanley Friesen)