On Pachycephalosaurs, Trade-Offs, and the Historical Genesis of Sociosexual Display Structures.

AbstractThe evolution of feeding performance traits is often inferred to reflect adaptation to dietary resources, but the role of constraint and trade-offs with other traits is rarely explored. Here, we focus on feeding performance in pachycephalosaurs, a group of Late Cretaceous dinosaurs known for their cranial dome. The bony dome is thought to be a sociosexual display structure, but we hypothesize that its presence constrained the space for jaw adductor musculature and reduced the angle of these muscles, thereby reducing feeding performance. We test this by reconstructing the cranial musculature of five phylogenetically and temporally diverse midsized herbivorous dinosaurs: two ornithischians (Heterodontosaurus tucki, Lesothosaurus diagnosticus), one ornithopod (Hypsilophodon foxii), one ceratopsian (Psittacosaurus lujiatunensis), and a pachycephalosaur (Stegoceras validum). Our model of Stegoceras validum-the first for any pachycephalosaur-suggests that its feeding performance was much closer to that of basal ornithischians and ornithopods than expected for a derived Late Cretaceous taxon and contrary to the derived feeding performance of Psittacosaurus lujiatunensis, another marginocephalian. We interpret that the dome constrained pachycephalosaurs to a more plesiomorphic dentition and low-fiber herbivore niche. This may constitute one of the first examples of a sociosexual display structure constraining the evolution of feeding traits. This research also highlights the unclear historical genesis of sociosexually selected structures, especially as to whether they are beneficial structures, costly structures, or "opulent" structures at their most incipient stages.