[Taxacom] phylogeny (Monotremata)
kennethkinman at webtv.net
Tue Aug 9 21:29:23 CDT 2011
I certainly agree that the UCMP statement was poorly worded.
However, one must also be aware that UCMP is probably defining "mammals"
as Mammalia "sensu stricto", which is the crown group clade which
includes only living clades, and that automatically makes the clade
including monotremes "basal" by such a PhyloCode-type definition. Such
a restricted "crown group" definition can (not surprisingly) cause
problems. Therefore, neither this definition of Mammalia ("mammal"),
nor even the older (somewhat broader) Mammalia that I have always
prefered (renamed clade "Mammaliaformes" by PhyloCode-type splitters),
is probably not terribly gradistic (if at all). Such gradism seems to
occur earlier in the phylogeny (earlier theriodont evolution).
In any case, instead of removing the phrase "for this reason", I
would try to anticipate PhyloCodist objections, and instead recommend
just changing it to "for this and other reasons" (and they hopefully
would then also feel the obligation to discuss such other reasons as
Furthermore, I would not be surprised if there were egg-layers in
the stem group between the common ancestor of all living mammals and the
common ancestor of living therians (or even the earliest members of the
metatherian or eutherian lines that it gave rise to). Therefore,
singling out the lack of egg laying (which is obviously difficult to
document from fossils) as "the" defining characteristic of the Theria
(much less the immediate stem-group leading up to it) is indeed
problematic. So even if they avoided the gradistic nature of
Mammaliaformes (or more inclusive clades) by using a restricted crown
group Mammalia, the egg-laying characteristic alone (without other,
supporting characteristics, hopefully those of bones that are far more
fossilizable) is indeed problematic. Therefore, I agree with Stephen
that it is misleading, and that the wording needs to be changed.
Stephen Thorpe wrote:
Yes, you are undoubtedly correct about the details of mammal evolution
making the case more complex. Nevertheless, I was thinking more about
the general simple schema of saying something like: group G can be
split into those with character C (e.g. giving birth to live young), and
those with alternative C' (e.g. laying eggs), and for this reason those
with C' are the sister group to those with C. To a beginner in
phylogeny, I imagine that this could give a seriously misleading
impression! Cheers, Stephen
From: Alex Borisenko <aborisen at uoguelph.ca>
I am afraid that part of the problem is that 'mammalisation' of
theriodonts was a largely parallel process that happened with complete
disregard to the principles of monophyly, e.g., see Tatarinov (1976).
Most key mammalian traits appeared gradually and independently in
different theriodont lineages of which monotremes and therian mammals
are the only ones alive. The key difference from 'amphibians' and
'reptiles' is that mammals are less 'basal' to all other vertebrates.
Just like the former two groupings, 'mammals' is a highly operational
'common sense' term and should not be abolished just because it is not a
perfect fit to the present day cladistic-driven nomenclatural paradigm.
If you look at mammals as a gradistic grouping, the phrase below has
only minor logical flaws. I would remove the statement 'for this reason'
and would say "all other *living*" mammals. My two cents...
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