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From: rh@smds.UUCP (Richard Harter)
Newsgroups: sci.bio
Subject: On the classification of Platypus
Summary: History tells us the answer
Keywords: monotremes
Message-ID: <421@smds.UUCP>
Date: 27 Apr 91 07:07:59 GMT
References: <1991Apr22.111159.29888@kuhub.cc.ukans.edu> <4896@kitty.UUCP> <1991Apr26.183747.21006@hollie.rdg.dec.com>
Organization: SMDS Inc., Concord, MA
Lines: 59

In article <1991Apr26.183747.21006@hollie.rdg.dec.com>, winalski@psw.enet.dec.com (Paul S. Winalski) writes:

	I wrote:

> |>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.

	And Paul S. Winalski replied

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

Therapsid, not theraspid.

Gould's new book has a relevant chapter on naming of species.  Tho it is
not strictly relevant since we are dealing with classification rather than
naming, the issue is prior practice.  Platypus was initially classified
as a mammal on the basis of hair.  [Perhaps some of our informed readers
can tell us if there is any extant species of mammal that is completely
hairless or any species of reptile that has hair.]  Lactation in Platypus
wasn't discovered until much later; indeed lactation in Platypus is what
you might expect in an intermediate form between reptiles and mammals.
The working out of the evolutionary sequence from therapsids to mammals
was worked out much later.

By prior practice Platypus is classified as a mammal.  Since taxonomists
are a conservative bunch we may expect that the classification will remain
in effect; there is no compelling reason for reclassification.

If one considers the historical situation there was, at one time,
animals which I will call proto-mammals.  That is, they had some
mammalian characteristics, but not all.  Platypus seems to be in a line
of descent that retains more characteristics of proto-mammals.  A real
question that I don't know the answer to is whether the monotreme
classification is legitimate, i.e. whether the descent lines are like

	therapsid -> monotreme
	          -> marsupial
	          -> placental

or whether, instead, there are separate descent lines for the extant
monotremes.  I rather expect no one knows.
-- 
Richard Harter, Software Maintenance and Development Systems, Inc.
Net address: jjmhome!smds!rh Phone: 508-369-7398 
US Mail: SMDS Inc., PO Box 555, Concord MA 01742
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