Following Trends

©Nobu Tamura

Kirkland and DeBlieux, as mentioned a couple of weeks ago now, described Diabloceratops. In doing so they made some generalizations about the evolutionary trends of the Centrosaurines based on their skulls. The Centrosaurine family tree was already organized in the way in which they described the evolutionary trends and as such they were not truly reinventing the family tree anywhere near as much as they were describing some observable traits in the skulls of the animals that they knew at the time to be members of the Centrosaurine clade. The general trend, it appears, starting at Diabloceratops (and now with Xenoceratops' discovery with that Centrosaurine dinosaur) the trend described is a continuous trend of parietal frill adornment while we see a reduction in supraorbital (think eyebrows!) horn reduction, the replacement of those horns with a solid nasal horncore, and the subsequent reduction of that horn structure. As each horncore site is reduced into nonexistence the bone at the site of origin becomes denser and rougher, eventually replacing the horn with a rough dense structure of bone, which would be highly suited to dispersal of forces consistent with headbutting another animal "safely". Achelousaurus is a long way down the Centrosaurine family tree, situated snugly between Einiosaurus and Pachyrhinosaurus, and as such exhibits traits in the skull reminiscent of both the previous and following animals (whether the three are a direct ancestry line or not is still open to debate, but the evidence is very strong for a direct lineage based on the skull evolution).

Einiosaurus has a dense boss where the supraorbital horns were in previous Centrosaurines and the nasal horn is exceedingly large but very forward curving and shows a large reduction in physical height from base to tip. Einiosaurus was preceded by Styracosaurus in the family, which had the largest nasal horns of the group, and the reduction of the horn in the way that it occurs seems quite random until we look at the skulls of Achelousaurus. Achelousaurus possesses a very stunted horncore that, in many interpretations, is argued to be simply the edge of the boss on the nasal region, but in the youngest skull discovered (age-wise, not geologically) appears to be a forward, straight off of the nasal and above the beak of the skull, jutting miniscule nasal; following the reduction trend and followed by the extremely dense nasal boss of the Pachyrhinosaurs. Seeing the trends in sequence (perhaps I will play with this sometime when I have more time) would be fantastic. The morphology of the Centrosaurine line is quite interesting and demands more study than has so far been afforded to it.