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Re: Platypus (therapsid?)



Tom Holtz <tholtz@geochange.er.usgs.gov> writes: 

>I have not seen any (paleo- or neontological) systematics work which
>excludes monotremes from Mammalia.  
>(And, the group of marsupials + placentals is almost always called Theria).

Although not directly related to the monotreme discussion, these quotes from
J.A. Lillegraven in the textbook _Mesozoic Mammals: the First Two Thirds of
Mammalian History_, may be of interest:

Begin quotes:
  "All living therian mammals are viviparous, but marsupial developement
shows a series of features such as thin eggshell and vestigial egg-tooth
that strongly indicates a previously oviparous habit....."
  "All [living] marsupials are also born with a reptile-like articular-quadrate
jaw-joint."
  "Although the phylogenetic source of the Monotremata is quite uncertain,
the group has, with little doubt, been genetically separated from the Theria
since the Late Triassic.  Most classification schemes of the Mammalia
include the monotremes, but MacIntyre (1972) supports a minority opinion in
arguing that the living prototherians (and all of the extinct and extremely
heterogeneous nontherian groups) should be considered reptiles." 
End quotes:

  So it seems that, reproductively, at least, marsupials are only a "shade"
different from monotremes.  It is only a matter of degree, not of style.
I don't have access to MacIntyre's paper, so I don't know what evidence he
gives to support his position.  His paper is a little dated by now, and he
seems to be the only one with that point of view.