David marjanovic 14 April 2002 wrote:
1) You mean if they laid amniote eggs? Should
Amniota exchange its node based definition for a currently inapplicable
apomorphy-based one?
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 "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.
The term "Amniota" itself, perhaps lead little
astray, referring to a condition that is not immediately detectable in
fossils. As you wrote " to lay amniote eggs is an inapplicable node based
definition", but all the recent discussions on diadectomorphs concern if they
lay amniote eggs or not (Lee & Spencer 1997) (Laurin & Reisz
1999).
Perhaps the clade
containing advanced terrestrial tetrapods should be defined and named by osteological characters, the only ones
that is possible to detect in fossils.
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.
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. 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
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. Diadectomorphs, among
the other things, show polimorphy in the skull, tseajaia has the parietal that
reach the squamose as the sauropsid captorhinomorphs, diadectes and limnoscelis,
instead, have the postorbital and the supratemporal in contact as the synapsid
eothyris, diadectes has one coronoid as early sauropsids, limnoscelis two as the
early synapsids, to complicate the things there is labirhyntine infolding in
limnoscelis teeth and the problematic "otic notch" of diadectes and
tseajaia.
Steve Brusatte in "Pelycosaurs: sprawling or not ?
16 April 2002 wrote:
There is also the possibility that some pelycosaurs
could have evolved a more erect gait independent of the lineage that lead
directly to mammals.
It seems to me that dimetrodon and Sphenacodontidae
in general, although have more elongated limbs, show no evidence of more erect
gait than sauropsids. The proximal head of the femur is not inflected medially
but is directly terminal on the shaft (Reisz 1986). There is a broad surface of
the humerus in correspondence to a screw shaped glenoid surface of the
scapulacoracoid, the two ends of the humerus retain the primitive twist (Romer
1956). Probably the neural spines limited the lateral ondulation of the body,
that in any case, remains important in the locomotion of othe other pelycosaurs.
The connection between stiff vertebral column and erect gait is not evident as
shown by the terrestrial turtles, moreover, most therapsids, in
which more erect gait is well demonstrated, show a vertebral column not
less flexible than the one of pelycosaurs in spite of the less useful of the
reptilian habit of lateral ondulation (Hotton III 1991)
Alberto Arisi
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