Pachycephalosaurs are a being suggested to just represent youngsters of others.of ornithischian dinosaurs characterized, among others, by a unique morphology of their skull roofs which are thickened and dome-shaped. They were (or possibly ) closely related to the ceratopsians, the group that contains the popular Triceratops, with which they form the clade Marginocephalia. The interrelationships of Pachycephalosauria, however, have long been problematic, with some species
A new study by Ryan K. Schott and David C. Evans sheds light on some of the issues regarding pachycephalosaur anatomy and evolution. The study is based on the thesis of the first author that was defended in 2011. The aim of the study was especially to look at the fossil remains originally named Stegoceras brevis.
The history of Stegoceras brevis is very complicated and exemplifies almost all of the potential problems that paleontologists have to face. It was named in 1902 by Lawrence Lambe as a species closely related to a pachycephalosaur named Stegoceras validus. The material of S. brevis consists of several skull fragments belonging to a few individuals. It was discovered in strata deposited some 76.6 to 74.8 million years ago in what is now Alberta, Canada.
During most of the 20th century, the material belonging to S. brevis was considered either to represent juvenile females of S. validus or, as Lambe initially proposed, a distinct species of Stegoceras. The things turned in a new direction in 2000 when Robert M. Sullivan found the material of S. brevis to be more closely related to Prenocephale prenes from the Campanian of Mongolia, rather then to Stegoceras validum (as the name validus was changed in 1987). By the turn of the millenia, it became rather clear that S. brevis is distinct from all other pachycephalosaurs but the hypotheses of pachycephalosaur interrelationships were too differing to find a conclusive placement for the species brevis within the evolutionary context.
The new comprehensive study by Schott and Evans is a step forward in the understanding of this as yet enigmatic pachycephalosaur. The authors looked at its material, consisting of skulls belonging to young as well as older individuals, and analyzed it from morphological and histological perspective. And both, morphology as well as histology were concluded to support a unique nature of “S.” brevis, leading the authors to propose a new name for it – Foraminacephale.
Schott and Evans found that the skull of Foraminacephale brevis was thickening differently from those of Stegoceras validum. As the authors note, “[i]n St. validum, the frontoparietal [the bone structure originated by the fusion of the frontal and parietal to form a dome in pachycephalosaurs – DM] grows from a fully flat-headed to a domed state […]. In the smallest F. brevis complete frontoparietal dome ([the specimen known under the number – DM] TMP 1991.036.0265), the frontal is essentially flat, but the parietal is already partially domed, or at least ‘down-turned’, suggesting that doming begins in the parietal region of the dome, and proceeds towards the frontals later in ontogeny [as the animal was growing – DM], instead of concurrently, as in St. validum.”
The new study suggests that Stegoceras validum and Foraminacephale brevis represent members of two separate branches of the pachycephalosaurid tree. However, subsequent studies are still needed to support this hypothesis.
Featured image by Ryan Steiskal. CC BY 2.5.