Genasauria

Genasauria is a clade of extinct beaked, primarily herbivorous dinosaurs. Paleontologist Paul Sereno first named Genasauria in 1986. The name Genasauria is derived from the Latin word gena meaning ‘cheek’ and the Greek word saúra (σαύρα) meaning ‘lizard.’ It is hypothesized that Genasauria had diverged from Lesothosaurus by the Early Jurassic. Cranial features that characterize Genasauria include a medial offset of the maxillary dentition, a sprout-shaped mandibular symphysis, moderately sized coronoid process, and an edentulous (without teeth) anterior portion of the premaxilla. A distinguishing postcranial feature of Genasauria is a pubic peduncle of the ilium that is less robust than the ischial peduncle. Genasauria is commonly divided into Neornithischia and Thyreophora. Neornithischia is characterized by asymmetrical distributions of enamel covering the crowns of the cheek teeth, an open acetabulum, and a laterally protruding ischial peduncle of the ilium. Neornithischia includes ornithopods, pachycephalosaurs, and ceratopsians. Thyreophora is characterized by body armor and includes stegosaurs, ankylosaurs, Scelidosaurus, and Scutellosaurus. Paul Sereno's original, informal definition for the clade Genasauria was "Ankylosaurus, Triceratops, their most recent common ancestor and all descendants." In 2021, Genasauria was given a formal definition under the PhyloCode: "The smallest clade containing Ankylosaurus magniventris, Iguanodon bernissartensis, Stegosaurus stenops, and Triceratops horridus." Genasauria contains a medial offset of the maxillary dentition (buccal emargination), which is commonly referred to as the ‘ornithischian cheek.' Other characteristics of the ornithischian cheek include “a deep-set position of the tooth rows, away from the sides of the face, a spout-shaped front to the mandibles, and reduction in the size of the opening on the outside of the lower jaw (the external mandibular foramen)." The ornithischian cheek is largely inferred to be evidence for the possession of muscular cheeks that were used for complex chewing behavior and is a fundamentally Genasaurian characteristic.