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.’ Genasauria is the most inclusive clade within the order Ornithischia. According to Sereno (1986), Genasauria represents all ornithischians except for the most primitive ornithischian, Lesothosaurus. Sereno’s formal definition is, “Ankylosaurus, Triceratops, their most recent common ancestor and all descendants.” 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.
Pachypoda Haeckel, 1895
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. Galton (1973) also suggests that the ornithischian cheek was found between the maxillary and dentary ridges to prevent the loss of food through the jaws. It may have consisted of connective tissue and skin, rather than muscle fibers, which meant that the tongue was used to move food that had accumulated between the teeth and the cheek, back to the tongue side of the cheek so that it could be further broken down by the teeth. The ornithischian cheek is absent or only weakly developed in Lesothosaurus, which supports its placement as a sister group to Genasauria. In Genasauria, the mandibular symphysis is shaped like a spout and forms at an acute angle. The mandibular symphysis is the point of fusion between the two lateral dentary bones. The mandible of Genasauria is also characterized by the possession of a coronoid process that is longer than 50 percent of the depth of the midlength of the dentary. The coronoid process is a thin anterior projection of bone from the dentary, which serves as a site for the attachment of muscles that aid in chewing behavior.
Post-cranial characteristics include reduced relative size of the pubic peduncle of the ilium and a fourth trochanter that is shifted distally on the shaft of the femur. The pubic peduncle of the ilium is an anterior extension of the ilium, which joins with the pubis. In Genasauria, the relative size of the public peduncle, compared to the size of the ilium, is reduced. The fourth trochanter is a process (extension) of the femur that serves as an attachment point for tail muscles, mainly for attachment of the Musculus caudofemoralis longus.
The members of Genasauria were primarily herbivores with some exceptions, like the omnivorous heterodontosaurids. Genasaurians most often had their head at the level of one meter, which suggests they were feeding primarily on “ground-level plants such as ferns, cycads, and other herbaceous gymnosperms."
Thyreophora are defined as representing all taxa more closely related to Ankylosaurus than to Triceratops and are characterized by extensive dorsal body armor scutes. The group spanned about 100 million years, beginning in the early Jurassic though the late Cretaceous. During their time on earth, they gave rise to over 50 different species. They contain the groups Ankylosauria and Stegosauria, as well as, a number of basal forms such as Scelidosaurus, Emausaurus, and Scutellosaurus. Fossils of Thyreophora have been primarily found in the northern hemisphere. Thyreophora can be distinguished from Neornithischia based on: transversely broad process of the jugal and parallel rows of keeled scutes on the dorsal surface of the body.
Neornithischia is a clade containing Ornithopoda and Marginocephalia, which is a node-based clade that contains Ceratopsia and Pachycephalosauria. Neornithischia was previously labeled as Cerapoda. However, this name has been more recently given a less inclusive definition. Neorniththischia evolved during the Jurassic period and persisted until the late Cretaceous period. Their fossils have only been found in the northern hemisphere. Neornithischia can be distinguished from the Thyreophora by the following derived characteristics: significant diastem between premaxillary and maxillary teeth, five or fewer maxillary teeth, and finger-like anterior trochancter.
This version of taxonomic classification is from The Dinosauria.
There is debate as to the placement of Lesothosaurus as a sister group to Genasauria as or as a basal member of Genasauria. Sereno (1986) argues that Lesothosaurus does not contain the defining Genasaurian synapomorphies of 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, and a pubic peduncle of the ilium that is less robust than the ischial peduncle. Butler (2011) argues that the synapomorphies that should exclude Lesothosaurus from Genasauria have been described in Lesothosaurus specimens. Butler writes “The position of Lesothosaurus within Neornithischia is supported by three unequivocal characters: reduction of the forelimb to less than 40% of the hind-limb length, presence of a dorsal groove on the ischium, and a strongly reduced, splint-like metatarsal one.” The following two cladograms illustrate the two opinions.
The following is a cladogram based on the paper by Sereno (1986) that originally defined Genasauria.
The following is a more recent cladogram based on an analysis by Butler et al. (2011).
Averostra, or "bird snouts", is a clade that includes most theropod dinosaurs that have a promaxillary fenestra (fenestra promaxillaris), an extra opening in the front outer side of the maxilla, the bone that makes up the upper jaw. Two groups of averostrans, the Ceratosauria and the Orionides, survived into the Cretaceous period. When the Cretaceous–Paleogene extinction event occurred, ceratosaurians and two groups of orionideans within the clade Coelurosauria, the Tyrannosauroidea and Maniraptoriformes, were still extant. Only one subgroup of maniraptoriformes, Aves, survived the extinction event and persisted to the present day.Avetheropoda
Avetheropoda, or "bird theropods", is a clade that includes carnosaurians and coelurosaurs to the exclusion of other dinosaurs.Cerapoda
Cerapoda ("ceratopsians and ornithopods") is a clade of the dinosaur order Ornithischia.Claorhynchus
Claorhynchus (meaning "broken beak", as it is based on broken bones from the snout region) is a dubious genus of cerapodan dinosaur with a confusing history behind it. It has been considered to be both a hadrosaurid and a ceratopsid, sometimes the same as Triceratops, with two different assignments as to discovery formation and location, and what bones make up its type remains.Dinosauriformes
Dinosauriformes is a clade of archosaurian reptiles that include the dinosaurs and their most immediate relatives. All dinosauriformes are distinguished by several features, such as shortened forelimbs and a partially to fully perforated acetabulum, the hole in the hip socket traditionally used to define dinosaurs. The oldest known member is Asilisaurus, dating to about 245 million years ago in the Anisian age of the middle Triassic period.Eocursor
Eocursor (meaning "dawn runner") was a primitive genus of dinosaur. It was an ornithischian which lived in what is now South Africa. Remains of this animal have been found in the Lower Elliot Formation and might be the most complete known from a Triassic ornithischian, shedding new light on the origin of this group.The exact age of this taxon is uncertain. It was originally interprereted as living during the Late Triassic (Norian age), around 210 million years ago; however, Olsen, Kent & Whiteside (2010) stated that there is no independent geochronological support for its assumed age, and the available data makes it impossible to conclusively determine whether Eocursor is of Triassic or Early Jurassic (potentially as young as Sinemurian) age. Eocursor was subsequently interpreted as a taxon of Early Jurassic age by Mcphee et al. (2017).Fossils of Eocursor were originally collected in 1993, but were not formally described until fourteen years later. The type species, Eocursor parvus, was described in 2007 by Richard J. Butler, Roger M. H. Smith, and David B. Norman. Eocursor was one of the earliest known ornithischians, and sheds some light on early dinosaur relationships because early dinosaurs are known from mostly incomplete skeletons. Eocursor is known from partial skeletal elements, including skull fragments, spinal elements, pelvis, long leg bones, and unusually large grasping hands.Hexinlusaurus
Hexinlusaurus is a genus of basal ornithischian dinosaur from the Middle Jurassic of China. The holotype (ZDM T6001, Zigong Dinosaur Museum, Dashanpu, People's Republic of China), consists of an almost complete, articulated skull and some postcranial material, collected from a terrestrial sandstone within the Lower Shaximiao Formation (?Bajocian) at the famous dinosaur-bearing quarries at Dashanpu. A paratype (ZDM T6002) consists of a partial skull and postcranial remains. Previously, it had been described as a species of Yandusaurus, Y. multidens (He and Cai, 1983), but was reclassified as a new taxon by Paul M. Barrett, Richard J. Butler and Fabien Knoll in 2005, who diagnosed this anatomically conservative species as follows: "A small ornithischian dinosaur distinguished from all other basal ornithischians by a single autapomorphy, the presence of a marked concavity that extends over the lateral surface of the postorbital." The etymology of the genus name honors Professor He Xin-Lu (from the Chengdu University of Technology) who originally named the specimen as Y. multidens + the Greek sauros (=lizard). Hexinlusaurus was a small, fleet-footed herbivore.
Other dinosaurs known from Dashanpu include the sauropod Shunosaurus, the theropod Gasosaurus, and the stegosaur Huayangosaurus.
Before being officially named Hexinlusaurus, this genus was briefly known under the informal name "Proyandusaurus". This name originally appeared in an abstract attributed to Fabien Knoll, which was apparently published without his consent.Isaberrysaura
Isaberrysaura is a genus of ornithischian dinosaur from the Early Jurassic Los Molles Formation of Patagonia, Argentina. The genus contains a single species, I. mollensis, described by Salgado et al. in 2017 from a single specimen. Although initially classified as a basal neornithischian, subsequent analysis has allied it with the Stegosauria; the morphology of its skull resembles those of other members of the group.Jeholosaurus
Jeholosaurus is a genus of ornithischian dinosaur from the Early Cretaceous Period. It is thought to have been a herbivorous small ornithopod.Jingshanosaurus
Jingshanosaurus (meaning "Jingshan lizard") is a genus of sauropodomorph dinosaurs from the early Jurassic period.Leaellynasaura
Leaellynasaura (meaning "Leaellyn's lizard") is a genus of small herbivorous ornithischian dinosaurs from the Albian stage of the Early Cretaceous (dated to between 118 and 110 million years ago), first discovered in Dinosaur Cove, Australia. The only known species is Leaellynasaura amicagraphica. It was described in 1989, and named after Leaellyn Rich, the daughter of the Australian palaeontologist couple Tom Rich and Patricia Vickers-Rich who discovered it. The specific name L. amicagraphica translates to "friend writing" and honours both the Friends of the Museum of Victoria and the National Geographic Society for their support of Australian paleontology.Lesothosaurus
Lesothosaurus was a genus of omnivorous ornithischian dinosaur from South Africa and Lesotho. It was named by paleontologist Peter Galton in 1978, the name meaning "lizard from Lesotho". The genus has only one valid species, Lesothosaurus diagnosticus.Manidens
Manidens is a genus of heterodontosaurid dinosaur from the Middle Jurassic of Patagonia. Fossils have been found in the Cañadón Asfalto Formation in Chubut Province, Argentina, dating to the Bajocian.Neornithischia
Neornithischia ("new ornithischians") is a clade of the dinosaur order Ornithischia. They are the sister group of the Thyreophora within the clade Genasauria. Neornithischians are united by having a thicker layer of asymmetrical enamel on the inside of their lower teeth. The teeth wore unevenly with chewing and developed sharp ridges that allowed neornithischians to break down tougher plant food than other dinosaurs. Neornithischians include a variety of basal forms historically known as "hypsilophodonts", including the Parksosauridae; in addition, there are derived forms classified in the groups Marginocephalia and Ornithopoda. The former includes clades Pachycephalosauria and Ceratopsia, while the latter typically includes Hypsilophodon and the more derived Iguanodontia.Neotheropoda
Neotheropoda (meaning "new theropods") is a clade that includes coelophysoids and more advanced theropod dinosaurs, and the only group of theropods who survived the Triassic–Jurassic extinction event. Yet all of the neotheropods became extinct during the early Jurassic period except for Averostra.Orionides
Orionides is a clade of tetanuran theropod dinosaurs from the Middle Jurassic to the Present. The clade includes most theropod dinosaurs, including birds.Ornithischia
Ornithischia () is an extinct clade of mainly herbivorous dinosaurs characterized by a pelvic structure similar to that of birds. The name Ornithischia, or "bird-hipped", reflects this similarity and is derived from the Greek stem ornith- (ὀρνιθ-), meaning "of a bird", and ischion (ἴσχιον), plural ischia, meaning "hip joint". However, birds are only distantly related to this group as birds are theropod dinosaurs.Ornithischians with well known anatomical adaptations include the ceratopsians or "horn-faced" dinosaurs (e.g. Triceratops), armored dinosaurs (Thyreophora) such as stegosaurs and ankylosaurs, pachycephalosaurids and the ornithopods. There is strong evidence that certain groups of ornithischians lived in herds, often segregated by age group, with juveniles forming their own flocks separate from adults. Some were at least partially covered in filamentous (hair- or feather- like) pelts, and there is much debate over whether these filaments found in specimens of Tianyulong, Psittacosaurus, and Kulindadromeus may have been primitive feathers.Serendipaceratops
Serendipaceratops (meaning "serendipitous horned face") is a dubious genus of herbivorous ornithischian dinosaur from the early Cretaceous Period of Australia.Thyreophora
Thyreophora ("shield bearers", often known simply as "armored dinosaurs") is a group of armored ornithischian dinosaurs that lived from the early Jurassic Period until the end of the Cretaceous.
Thyreophorans are characterized by the presence of body armor lined up in longitudinal rows along the body. Primitive forms had simple, low, keeled scutes or osteoderms, whereas more derived forms developed more elaborate structures including spikes and plates. Most thyreophorans were herbivorous and had relatively small brains for their body size.
Thyreophora includes various subgroups, including the suborders Ankylosauria and Stegosauria. In both the suborders, the forelimbs were much shorter than the hindlimbs, particularly in stegosaurs. The clade has been defined as the group consisting of all species more closely related to Ankylosaurus than to Triceratops. Thyreophora is the sister group of Cerapoda within Genasauria.