Horns and Family Evolution

Originally posted by Dave Hone. Courtesy James Kirkland

This image is loaded with information about half of the Marginocephalian clade. The Protoceratopsidae are clearly separated out until it branches over to Zuniceratops and moves into the true Ceratopsidae; I think the paleontology community as a whole would have very little argument in agreeing with the majority of that much of the image. Chasmosaurines are not stressed here and are thus relegated to an outlier branch with little to no description while Zuniceratops' other descendants are laid out in a manner which trends in two ways from oldest to youngest as well as from the general trend of number of horns; decreasing as the line progressed over time. It is interesting to note that the general trend is described as "losing horns" but it seems as though not all of the animals represented are truly losing horns. In fact, Styracosaurus, right in the middle, seems to have far more horns, on the frill, than any other Centrosaurine; though it does in fact only possess a nasal horn on the remainder of the skull. Diabloceratops had four horns that are clearly seen, a nasal horn is sometimes depicted on the dinosaur, while the furthest cousin down the line, Pachyrhinosaurus, is thought by many to have no horns at all; this is sometimes countered by the idea that the bony boss on the nasal bone is actually the base of a keratin based horn like in rhinoceroses which has yet to be found preserved in the fossil record. Looking at the skulls it almost feels like Centrosaurus and Einiosaurus should be more closely related also, given the shapes of their horns. Switching the placement of Styracosaurus and Centrosaurus would also, visually, follow the trend of lose of horns more accurately it appears.

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.