Phenotypic variation is the basic material upon which selection acts, and as such quantifying this variation is an important aspect of evolutionary biology. Specifically, research on a diverse array of living animals has documented higher rates of morphological variation in sociosexual display structures, than those under natural selection.
Many dinosaur species, particularly ornithischians, exhibit ‘exaggerated’ skeletal structures that lack obvious mechanical functions and have been hypothesized to have function is sexual and social display. These are often manifested as outgrowths/hypertrophy of the skull roof, including the solid and hollow crests of Hadrosauridae, the horns and frills of Ceratopsia, and the thickened domes of Pachycephalosauria. For dinosaur palaeobiology, however, intraspecific variation is often regarded as merely an obstacle to robust taxonomy.
Here morphological variation was quantified (using coefficient of variation) for the well-sampled species of horned dinosaur Centrosaurus apertus, as well as complementary dataset of the horned dinosaurs Anchiceratops, Chasmosaurus, Protoceratops, and the duck-billed dinosaurs Lambeosaurus, and Corythosaurus.
Levels of variation for the putative ornamentation structures (e.g. frills, horns, crests) are significantly higher (~2-3 times) than those for the remainder of the skull. These results are consistent with, and often statistically indistinguishable from, the known sexual displays of a comprehensive dataset of extant amniotes analogues (including mammals, birds, and squamates).
When combined with previous research suggesting these same structures in ornithischians are ontogenetically delayed, positively allometric, rapidly evolving, and highly species-specific, these data provide further support to the hypothesis that sociosexual selection was the evolutionary driver of these ‘exaggerated’ structures.