Cranial ornamentation in the Late Cretaceous nodosaurid ankylosaur Hungarosaurus

Bony cranial ornamentation is developed by many groups of vertebrates, including ankylosaur dinosaurs. To date, the morphology and ontogenetic origin of ankylosaurian cranial ornamentation has primarily focused on a limited number of species from only one of the two major lineages, Ankylosauridae. For members of the sister group Nodosauridae, less is known. Here, we provide new details of the cranial anatomy of the nodosaurid Hungarosaurus from the Santonian of Europe. Based on a number of previously described and newly identified fragmentary skulls and skull elements, we recognize three different size classes of Hungarosaurus. We interpret these size classes as representing different stages of ontogeny. Cranial ornamentation is already well-developed in the earliest ontogenetic stage represented herein, suggesting that the presence of outgrowths may have played a role in intra- and interspecific recognition. We find no evidence that cranial ornamentation in Hungarosaurus involves the contribution of coossified osteoderms. Instead, available evidence indicates that cranial ornamentation forms as a result of the elaboration of individual elements. Although individual differences and sexual dimorphism cannot be excluded, the observed variation in Hungarosaurus cranial ornamentation appears to be associated with ontogeny.