|Originally posted by Dave Hone. Courtesy James Kirkland|
Kirkland mentions that the antorbital fenestrations in Ceratopsidae overall appear, disappear, and reappear in later species on a fairly baffling basis. Diabloceratops had such a fenestration "at the contact of the nasal, premaxilla, and maxilla forward of the antorbital fenestra that we call the accessory antorbital fenestra (AAF)." Some of the Protoceratopsidae possess this opening, such as Magnirostris, as did some of the Chasmosaurine dinosaurs like Chasmosaurus itself. The reason that the opening is apparent in some Ceratopsids and not others has not, to my knowledge, been explained as yet, but it seems to be quite an interesting phenomena to study alongside the reduction in horns within the Centrosaurine dinosaurs.