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synapsids are reptiles



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