What did the "Unossified zone" of the non-mammalian therapsid braincase house?

Most nonmammalian synapsids possess a mid-dorsal depression in the brain cavity known as the "unossified zone." It remains obscure which structures this zone contained, and, as candidates, the vermis of the cerebellum, the superior sagittal sinus, a junction of several blood vessels, the pineal gland or other midbrain structures were considered. Neutron tomography of a skull of Diictodon feliceps (Therapsida, Anomodontia) revealed some clear impressions of canals in this region of the brain cavity. Furthermore, the prootic sinus probably ran on the internal surface of the pila antotica and had a similar course in anomodonts as it has been proposed for cynodonts and Mesozoic mammals. Comparisons with the vascular systems of nonmammalian synapsids and mammals suggest that the unossified zone is best interpreted as a terminal chamber of the anterior segment of the medial head vein, which housed the junction of the superior sagittal sinus and the transverse sinuses. Consequently, the system of cranial vessels in Diictodon reveals a partial division of the medial head vein system into an anterior and a posterior segment at an early stage of synapsid evolution, which is consistent with the well-known common pattern of early ontogenetic development in amniotes. J. Morphol., 2017. © 2017 Wiley Periodicals, Inc.