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The current definition of Amniota as the clade
composed by the most recent common ancestor of extant amniotes and all its
descendants, seems to me not very informative. Can this common ancestor be
recognized? probably not, being free from evolutionary novelties exclusive to
itself. If the common ancestor cannot be recognized, how can one determines
its immediate descendants?
The other way round. :-) Diagnoses are not
abolished. Said ancestor and its immediate descendants will share some features
that will allow us to recognize them as amniotes. Diagnoses can change as they
always did (remember the times when feathers were diagnostic of Aves...);
definitions can't (if they are registered under the PhyloCode which isn't
implemented yet...).
The "terrestriality" of the herbivorous diadectes
is out of doubt, but nevertheless it is condemned to a cotylosaurian
purgatory, being not synapsid and not sauropsid, the only two lineages that
the current definition of Amniota can accept [apart from their last
ancestor].
Where is the problem? What "purgatory"? It
has found a comfortable home:
--+--Solenodonsaurus
`--+--Diadectomorpha
`--+--Theropsida = Synapsida
`--Sauropsida
Would you consider Tritylodontidae in a
purgatory?
The term "Amniota" itself, perhaps lead little
astray, referring to a condition that is not immediately detectable in
fossils.
yep
As you wrote " to lay amniote eggs is an
inapplicable node based definition",
(I didn't. However, to lay amniote
eggs can be part of a diagnosis or an apomorphy-based definition, but not a
node-based definition.)
but all the recent discussions on diadectomorphs
concern if they lay amniote eggs or not (Lee & Spencer 1997)
(Laurin & Reisz 1999).
But not whether this should affect the
definition of Amniota.
Perhaps the clade
containing advanced terrestrial tetrapods should be [...] named by osteological characters, the only ones that is
possible to detect in fossils.
I agree in principle, but Amniota has IMHO
been used that way too long for changing it.
2) "Fish" has not been dismissed "Pisces" has
been.
You mean, if I have well understood, that the
popular term "fish" can be saved also in divulging scientific books, but its
latin translation cannot be used in technical studies.
yep
It would be interesting to see a latin
translation of the book of J.A. Long, after all latin is still the official
language of Vatican state.
:-)
(Italian is official there,
too.)
It seems to me that the dichotomy between
scientific language and popular language should be avoided when it is not
strictly necessary, to not increase confusion at educational
level
Well, in this case, I do think it's
strictly necessary. Textbooks have for a long time used Osteichthyes,
Chondrichthyes, Placodermi and Agnatha, but not Pisces anymore (though largely
because of ranks). Not using "fish" at all is BTW better than using it in a
different way from the popular languages... which of course differ from each
other. In English there are jellyfish, shellfish and crayfish, for example,
which aren't called fish in German, but in German Sepia is the ink
fish; not to mention the common use of "whale fish" in German (of which many
people really think they are fish) or the silver fish (Zygentoma).
T.Michael Keesey 15 April 2002
wrote:
If Diadectomorpha does belong to Amniota, by
virtue of being descended from the last common ancestor of Reptilia and
Mammalia, then it depends on whether it shares more recent ancestry with
Mammalia or with Reptilia. (If the former, they are synapsids, if the latter
they are sauropsids).
Perhaps there is the third possibility that they
are amniotes without belonging to synapsids or
sauropsids.
Only if the definition of Amniota is
changed.
[...] diadectes has one coronoid as early
sauropsids, limnoscelis two as the early synapsids,
If Diadectes really has only one
coronoid, then this is an autapomorphy, a feature totally peculiar to itself...
Sauropsida retained 2 coronoids "up to" Dromaeosauridae. (Don't know enough
about the rest of the characters you listed.)
to complicate the things there is labirhyntine
infolding in limnoscelis teeth and the problematic "otic notch" of diadectes
and tseajaia.
Plesiomorphies -- just old, retained
features.
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