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amniotes philogeny

I would ask to the members some clarifications about the early amniotes philogeny:
the distintion among the early amniotes is usually made on the basis of temporal fenestration on the two side of the skull, so the synapsids have one fenestra per side bordered by postorbital, squamose, jugal (sometimes quadratojugal also), the diapsids have two fenestrae per side, the lower of them is similar or identical to the synapsid one, the anapsids have not fenestration, but in some of them like millerettids and lanthanosuchids (the last seems related to turtles) there is a fenestra, this time also, identical or similar to the synapsids one; the ancient synapsids are generally considered more primitive than any known diapsid or anapsid.
If the ancient synapsids are the more primitive, among the early known amniotes, in have a temporal fenestration that is also present in the more evolute diapsids and anapsids,
what prevent synapsids to be themselves directly ancestral to them (diapsids and anapsids)?
Could be the anapsid condition like the one present in paleothyris or captorhinus be obtained secondarily?
Kemp (1980) suggested that the temporal fenestration of synapsids evolved directly from the remnant of crossopterygian hinge line present in amphibians quasi-reptile similar to limnoscelis, is it possible that the synapsid
fenestration was achieved at the same time of the amniote condition?
The low temporal fenestration present in early synapsids diapsids and anapsids is bordered by the same bones, is it real probable that a three time convergent evolution use the same bones?
Thank you for your kind attention.
Alberto Arisi - Italy