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.
