Mammalia sister of Reptilia?

[Ken Kinman]

Tom DiBenedetto wrote:
one can say "Mammalia IS the sister group of Reptilia".
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My response:
     Whether you use IS or ARE in this case is a minor distraction.  But for
those who are not familiar with Tom's  cladistic definition of Mammalia and
Reptilia (which have been manipulated to force them into a sister group
relationship), perhaps they would like to know how some cladists define
these taxa.
    The above cladistic Reptilia does include the birds, but it does not
include the mammal-like reptiles (pelycosaurs and therapsids).  These
mammal-like reptiles are transferred by many (but not all) cladists into a
cladistic Mammalia.
    Therefore the old definition of Mammalia based on the three inner ear
bones (which are easily studied in both living and fossil forms) is no
longer valid to such cladists.  Their early cladistic "mammals" didn't have
hair or those three inner ear bones or even nurse their young with
milk---indeed they make even the monotremes and multituberculates look
extremely modern.
     If this isn't bad enough, the mesosaurs might end up throwing a wrench
into the cladists' tidy dichotomy of a shrunken Reptilia and an expanded
Mammalia.  So their Reptilia might have to be further trimmed of these
reptilian (but not Reptilian with a capital R) mesosaurs as well.
     But if you suggest that birds be removed from Reptilia, be prepared for
a fire and brimstone sermon on the evils of paraphyly (Thou shalt not commit
paraphyly).
     Therefore according to some cladists, "Mammalia ARE (or IS) sister
group of Reptilia", but only if birds are included in the latter and the
pelycosaurs and therapsids are included in the former.   Well, at least for
the time being, until mesosaurs mess things up.
     Thanks, but no thanks.  I'll take a traditional, stable Reptilia any
day (as long as it has appropriate markers for its "or their?" mammal and
bird descendants).
                         -------Ken Kinman

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