Comparing therapsid skulls shows that basal forms, like the gorgonpsids (Fig. 1) had a postfrontal and postorbital. Derived forms (like Pachygenelus (Fig. 1) had neither. You can see the jugal rise in certain cynodonts, taking over where the postorbital retreated. You can also see either the fusion of the postfrontal and postorbital, or the disappearance of the postorbital (but workers like to label the remaining bone the postorbital even though it is largely a postfrontal). As this all goes into scoring, it’s important to that end.
Not much else today. Just wanted to share this and invite comments. Does anyone know the transitional taxon that might clarify this issue? Likely a basal cynodont, like Charassognathus (Fig. 1). Aelurognathus might have documented something on this subject, but the parts are missing from the fossil. I think we’re looking for a small gorgonopsid or therocephalian to show us, something like Regisaurus (Fig. 1), but more primitive.
All this and more from a PhD study by Gebauer (2007).
Updated the next day, February 16, 2014.
I just discovered a therocephalian originally named Anna and later renamed Annatherapsidus (Fig. 2) that had a reduced postfrontal and reduced postorbital along with wide temporal fenestra and a rather flat skull, both as in Procynosuchus (Fig.1). This appears to be a transitional taxon, the proximal outgroup to the Cynodontia. And this taxon appears to duplicate the pre-fused shape of the postfrontal/postorbital.
Gebauer EVI 2007. Phylogeny and Evolution of the Gorgonopsia with a Special Reference to the Skull and Skeleton of GPIT/RE/7113 (‘Aelurognathus?’ parringtoni). PhD Dissertation, Eberhard-Karls University at Tübingen. Online here.