Evolutionary convergence in a small cursorial styracosternan ornithopod dinosaur from western Europe

Abstract

ABSTRACT A metatarsal IV from a hitherto unknown endemic small-bodied styracosternan is described from uppermost Maastrichtian strata of NE Spain, part of the Ibero-Armorican island of the Late Cretaceous European Archipelago. This element is unique among the fourth metatarsals of all other ornithopods in combining the greatly elongated proportions of basally branching members of the clade with the prominent medial flange seen in other styracosternans. This specimen becomes the holotype of a new genus and species. Histological data indicate that this individual was a late subadult at the time of death, but not far from reaching somatic maturity. By analogy with the elongate metatarsals of lightly built, small-bodied cursorial ornithopods, it is likely that this animal was also capable of rapid locomotion. This is consistent with the differential distribution of intense bone remodeling in metatarsal IV, likely resulting from biomechanical stress produced during rapid hindlimb propelling. Notably, this styracosternan represents an exception to the mediportality and large body size that characterize all other members of the clade. The elongation of metatarsal IV, inferred cursoriality, and small body size constitute a case of evolutionary convergence with the distantly related non-iguanodontian ornithopods and dryosaurid and elasmarian iguanodontians. Unlike other regions of the world where, during the latest stages of the Cretaceous, the small-bodied cursorial herbivore ecological niche was represented by non-iguanodontians and non-hadrosauriforms, in the Ibero-Armorican island this niche was likely occupied by a styracosternan.