But to assess your "crown jewel" if "transitional" forms:
Synapsids allegedly evolved from evolved from within the Protorothyridae, a family in the order Captorhinida in the subclass Anapsida. (Carroll, 199-201.) According to the fossil record, however, synapsids and anapsids appear simultaneously. (Carroll, Robert L. 1988. Vertebrate Paleontology and Evolution. W. H. Freeman. New York.)
Of course, one can always argue that anapsids actually preceded synapsids and that their contemporaneous appearance in the fossil record is due to the vagaries of fossilization, but it should be acknowledged that in doing so one has moved from data to speculation. One could just as easily claim that synapsids preceded anapsids.
Synapsids aren't evidence of a missing link. At best, they represent one of the few potential forms that could be plugged into an evolutionary tree. The problem is that most of the evolutionary trees don't even exist in theory, let alone backed by allowed "transitional" fossils.