Re: Synapsids are reptiles and coronoid

Mesosuchus Browni is the most plesiomorphic member of rhynchosaurs (scaphonyx & Co.), and is the only one Archosauromorph of which I was able to find description of the internal jaw.

I see.

Also mesosaurus, seems to have only one coronoid (Modesto), like millerettids (Gow), and protorothyridids (Carroll), and the rest of sauropsids.

Interesting.

It seems that the presence of two coronoids is a character present only in synapsids and dinosaurs.

Rather strange... the plesiomorphy for tetrapods is to have 3, if not 4. For example, Greerpeton, a stem tetrapod, has 3 respectively 4 coronoids (at the front end, between the symphysis and coronoid I, there's a bone called the adsymphysial or parasymphysial; it bears a row of teeth plus a big tusk). So does Whatcheeria IIRC. In salamanders all 3 coronoids appear one after the other through ontogeny. Therefore I'm pretty sure that at least the 2 coronoids of basal synapsids are the normal condition for amniotes. Indeed, the compound cladogram on p. 98 of Benton's Vertebrate Palaeontology, 2nd ed., ascribes "loss of first coronoid" to Seymouriamorpha, Diadectomorpha and Amniota. Another cladogram in the same book considers it diagnostic of Therapsida that coronoid II is lost.

I think similar cases are the absences of tabular and supratemporal, which have cropped up all over Amniota... remember the times when "clavicles rudimentary or absent" was considered diagnostic of Ornithodira? :-)