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Dinosaurs Overview. The ornithopods ranged in size from the small fabrosaurids, 1 to 2 metres long, to the huge duck-billed types
that reached lengths of 10 to 12 metres or more. Ornithopods appear to have flourished the longest of the
Dinosaur varieties, thriving from the Late Triassic to the latest part of the Cretaceous. They inhabited all land
areas except Antarctica.
Ornithopod families included the early and somewhat primitive fabrosaurids, mostly from Eurasia and southern Africa, and heterodontosaurs, also largely from southern Africa. In the latter group was the oldest of the ornithischians, Pisanosaurus, a single very fragmentary specimen from the Late Triassic of Argentina. Better known are the slightly larger hypsilophodonts and much larger iguanodonts, mostly from North America and Europe. Representative of these groups are Hypsilophodon, about three metres in length, and the famed Iguanodon, about nine metres long. Another group, the hadrosaurs, sometimes called trachodonts, were the large duck-billed ornithopods of the Late Cretaceous, especially of North America and Eurasia. An abundant and diverse family, they included more than two dozen known genera. (The Late Triassic or Early Jurassic Scutellosaurus had some similarities to the ornithopods, but its affinities are still uncertain.) More advanced small to medium-size ornithopods, with only a suggestion of specialized dentition. Hypsilophodon and Thescelosaurus are examples. Late Jurassic to Late Cretaceous. Family Iguanodontidae Medium to large ornithopods, with the first stages of specialized grinding dentition. Iguanodon and Camptosaurus are the best known. Late Jurassic to Late Cretaceous. Family Hadrosauridae The duck-billed ornithopods, with highly specialized grinding dentition; medium to large size. Edmontosaurus, Corythosaurus , and Lambeosaurus are well known. Late Cretaceous. Infraorder Pachycephalosauria The dome-headed ornithischians; closely related to the ornithopods; usually with a massively thick bony skull roof; bipedal. Stegoceras and Pachycephalosaurus are the best examples. Late Cretaceous. Infraorder Ceratopsia The horned dinosaurs. Early to Late Cretaceous. Family Psittacosauridae Ancestral and most primitive of the ceratopsians; represented by the hornless and bipedal Psittacosaurus. Early Cretaceous
Family Protoceratopsidae Primitive quadrupedal ceratopsians, with short frills and very modest horns. Protoceratops and Leptoceratops are the best examples. Late Cretaceous. Family Ceratopsidae Advanced quadrupedal ceratopsians, with prominent horns and frills. Examples are Monoclonius, Torosaurus, and Triceratops. Late Cretaceous. Suborder Thyreophora The plated and armoured dinosaurs. Late Triassic or Early Jurassic to Late Cretaceous. Infraorder Stegosauria Family Scelidosauridae Primitive stegosaurs, with less well-developed back plates. Scelidosaurus is the most primitive form; Scutellosaurus perhaps the most advanced. Late Triassic or Early Jurassic. Family Stegosauridae Advanced stegosaurs, usually with well-developed back plates and spines. Stegosaurus and Kentrosaurus are the best known. Middle Jurassic to Early Cretaceous. Infraorder Ankylosauria Family Nodosauridae Primitive ankylosaurs, usually with less completely developed armour. Nodosaurus , Hylaeosaurus , and Sauropelta are well-known kinds. Early to Late Cretaceous. Family Ankylosauridae Advanced ankylosaurs such as Euoplocephalus and Ankylosaurus . Late Cretaceous. No universally accepted classification of Dinosaur exists. Fossil remains are often difficult to interpret, especially when only a few fragmentary specimens of a type have been found. Moreover, classifications may be constructed to serve different purposes that require different categories or organization. Occasionally, for example, the Sauropodomorpha have been divided into more or fewer lower-rank categories (e.g., families, subfamilies); but the twofold division into the infraorders Sauropoda and Prosauropoda has stood the test of time and has been followed here. Likewise, previous classifications divided the suborder Theropoda into two infraorders, the Carnosauria and the Coelurosauria. The former included all the larger animals and the latter all the smaller kinds. That arrangement did recognize certain distinctive anatomic features such as large heads and short necks in the Carnosauria and small heads and long necks in the Coelurosauria. But great numbers of theropod discoveries around the world in the past several decades have blurred those anatomic distinctions and reduced the importance of size as a diagnostic criterion. Accordingly, infraordinal categories are not always used in current classifications of the Theropoda; sometimes only family groupings are listed. In the classification adopted here, the theropods are divided into two infraorders, the Ceratosauria and the Tetanurae. The tetanuran theropods are further divided into certain subcategories - Coelurosauria, Ornithomimosauria, Maniraptora, Segnosauria, and Carnosauria - that are at a higher level than the families of this infraorder. It must be noted, however, that evolutionary affinities among all the theropod types are still being analyzed, and experts have not reached full agreement on a formal classification. Within the order Ornithischia, two distinct subdivisions are generally given equal rank, currently as the suborders Cerapoda and Thyreophora. A final example is the recently discovered Scutellosaurus, which has been assigned by some to the Fabrosauridae (Ornithopoda) and by others to the Stegosauria. Scutellosaurus might well represent an evolutionary link between the ornithopods and the later stegosaurs or ankylosaurs. Since its affinities are still unclear, it has here been tentatively placed with the Stegosauria.
- Encyclopedia Britannica
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