Dinosaurs of Great Britain and the role of the Geological Society of London in their discovery: Ornithischia

Completing our survey of British non-avian dinosaurs, we here review the ornithischians of Britain. Heterodontosaurids are present in the Lower Cretaceous Lulworth Formation of Dorset, and a few earlier possible records imply a long presence in the region of this clade. Britain’s thyreophoran record is rich and includes the earliest well-represented taxon, Scelidosaurus, as well as Middle Jurassic stegosaurs and ankylosaurs including a reasonably good Cretaceous record of polacanthids and nodosaurids. Cretaceous stegosaurs are known only from fragmentary remains, but the proposal that stegosaurs were present as early as the Rhaetian is rejected. Among British iguanodontian ornithopods, the possible dryosaurid Callovosaurus is the oldest global record whereas the proposed synonymy of Cumnoria with Camptosaurus requires confirmation. Iguanodon has become a taxonomic dumping ground for assorted iguanodontians and is in need of revision: most of the British species referred to this genus are almost certainly not closely allied to the neotype species I. bernissartensis and require new generic names. Fragmentary remains suggest the early presence of hadrosaurids in Britain. The only British record of Marginocephalia, the Wessex Formation skull roof named Yaverlandia bitholus, appears not to belong to this clade but seems to be from a maniraptoran theropod. Together with their saurischian relatives, ornithischian dinosaurs dominated terrestrial life during the Jurassic and Cretaceous periods. Characterized by an opisthopubic pelvis, a predentary bone and other distinctive characters, they include the familiar stegosaurs, ankylosaurs, hadrosaurs, pachycephalosaurs and ceratopsians. Their feeding mechanics, evolutionary interactions with plants, and evolution of armour and diverse cranial structures have proved highly fertile areas of research. The first ornithischian to be described was the ornithopod Iguanodon, based on isolated teeth from the Hastings Beds Group of East Sussex (Mantell 1825), and the second was the ankylosaur Hylaeosaurus from the same strata (Mantell 1833). The term ‘ornithischian’, referring to a pelvic girdle that superficially resembles that of birds, was published by Seeley (1888). Although ornithischians originated in the Triassic (Butler et al. 2007), their fossils from this time are rare and many alleged Triassic ornithischians have recently been reinterpreted (Irmis et al. 2006). The British ornithischian record has recently been extended to the Upper Triassic (Rhaetian), where two fragmentary bones have been referred to Stegosauria (Galton 2005). Convincing remains of thyreophorans are well documented from the Hettangian and Sinemurian of Dorset, where partial skeletons of Scelidosaurus, sometimes with skin preserved, have been known since the 1860s (Owen 1861). Isolated elements and rare partial skeletons of ornithischians have been reported from marine strata in the Bathonian, Callovian, Oxfordian and Kimmeridgian of central and eastern England. However, the most substantial remains come from the Lower Cretaceous Hastings Beds, Weald Clay and Wealden groups, where near-complete and partial skeletons of the ankylosaurs Hylaeosaurus and Polacanthus and the ornithopods Hypsilophodon, Valdosaurus, Mantellisaurus and Iguanodon have been recovered. The youngest non-avian British dinosaurs include ornithischian taxa from the Albian–Cenomanian Lower Chalk, Folkestone Beds, Gault Clay and Cambridge Greensand: among them are highly fragmentary ankylosaurs and hadrosaurids. In a previous paper (Naish & Martill 2007), we reviewed the fossil record, taxonomy and phylogenetic relationships of the saurischian dinosaurs from the British Mesozoic. Here, we review British ornithischian dinosaurs. As discussed previously (Naish & Martill 2007, p. 496), many names coined for British dinosaur taxa are no longer considered adequate in terms of establishing taxonomic validity (supplementary information is available online at http://www.geolsoc.org.uk/SUP18267), meaning that approximately 54% of named British dinosaur taxa are nomina dubia. In view of inconsistencies in the literature, we have used the original spellings for specific names, although this results in an apparent inconsistency within our own text. A number of British ornithischians have been incorrectly allocated to a genus (e.g. most species referred to Iguanodon) and, in most cases, it is the type species alone that should be regarded as being correctly associated with the generic name. All other designations to generic level should be assumed to be historical artefacts and, following convention, we use quotation marks.