A redescription of the skull of Euoplocephalus tutus (Archosauria: Ornithischia): a foundation for comparative and systematic studies of ankylosaurian dinosaurs

Abstract

Euoplocephalus tutusLambe (1902) from the Late Cretaceous of North America, was the first ankylosaurian dinosaur known from significant cranial material. Previous descriptions of this and other members of the Ankylosauria have been constrained by a paucity of material and an extremely apomorphic skull architecture, including the pervasive development of an embossing osseous ornamentation, and the absence of traditional morphological landmarks. A relative abundance of more recently collected and prepared cranial material attributable to Euoplocephalus enables a reappraisal of this taxon (including the type specimen), and permits it to be employed as a morphological representative of the clade. In recognition of previous difficulties encountered due to peculiarities of ankylosaurian anatomy, a fresh descriptive approach is necessitated. Herein, the skull is subdivided into five mutually exclusive topographic regions, within which individual elements are assigned with the assistance of outgroup comparison. Euoplocephalus is characterized by a distinctive pattern of cranial sculpturing across the preorbital area, relatively small, variably fluted teeth lacking a cingulum, a modified palpebral and a shallow nasal vestibule. Among ankylosaurine ankylosaurs, Euoplocephalus is unique in having medially inflected maxillary tooth rows. Osteological evaluation of the type skull of Anodontosaurus lambeiSternberg, 1929 supports its placement into synonymy with Euoplocephalus.