Ornithopods Temporal range: Middle Jurassic - Late Cretaceous,
Mounted skeleton of Parasaurolophus cyrtocristatus, Field Museum of Natural History
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	†Ornithischia
Clade:	†Cerapoda
Clade:	†Ornithopoda Marsh, 1881
Subgroups
†Burianosaurus †Changmiania? †Convolosaurus? †Gideonmantellia †Hypsilophodon? †Thescelosauridae? (paraphyletic?) †Elasmaria †Iguanodontia
Synonyms[1]
?Clypeodonta Norman, 2014 ?Euornithopoda Sereno, 1986 ?Hypsilophodontia Cooper, 1985

Ornithopoda (/ˌɔːrnəˈθɒpədə/)^[2] is a clade of ornithischian dinosaurs, called ornithopods (/ˈɔːrnəθəˌpɒdz, ɔːrˈnɪθ-/),^[3][4] that started out as small, bipedal running grazers and grew in size and numbers until they became one of the most successful groups of herbivores in the Cretaceous world, dominating the North American land. Their major evolutionary advantage was the progressive development of a chewing apparatus that became the most sophisticated ever developed by a non-avian dinosaur, rivaling that of modern mammals such as the domestic cow. They reached their apex of diversity and ecological dominance in the hadrosaurids (colloquially known as 'duck-bills'), before they were wiped out by the Cretaceous–Paleogene extinction event along with all other non-avian dinosaurs. Members are known from all seven continents, though they are generally rare in the Southern Hemisphere.

History of research

In 1870, Thomas Henry Huxley listed Iguanodontidae (coined by Cope a year earlier^[5]) as one of his three families of dinosaurs (alongside Megalosauridae and Scelidosauridae), including within it the genera Iguanodon, Hypsilophodon, and Hadrosaurus, in addition to Cetiosaurus and tentatively Stenopelix.^[6] The term Ornithopoda was erected by Othniel Charles Marsh in 1881 as part of his then still ongoing investigation of the classification of Dinosauria. It was considered one of the four definite orders of dinosaurs, the others being Theropoda, Sauropoda, and Stegosauria (Hallopoda was considered a possible fifth). He subdivided the order into three families: Camptonotidae, Iguanodontidae, and Hadrosauridae; the former was a new name, whereas the latter two were carried over from the nomenclatures of Huxley and Edward Drinker Cope respectively. Within Camptonotidae he included the European Hypsilophodon and three American taxa he named himself, Camptonotus, Laosaurus, and Nanosaurus.^[7] Camptonotus was in 1885 renamed to Camptosaurus, as the original name was pre-occupied by a cricket; the associated family followed suit, becoming Camptosauridae.^[8] In Iguanodontidae, only found in Europe, he included Iguanodon and Vectisaurus. In Hadrosauridae, he included Hadrosaurus, Cionodon, and tentatively Agathaumas.^[7]

Description

Three-toed feet of Iguanodon

Ornithopoda means "bird feet", from the Greek ornithos ("bird") and pous ("feet"); this is in reference to members’ characteristic three-toed feet, although many early forms retained four toes. They were also characterized as lacking in body armour, not developing a horny beak, having an elongated pubis (that eventually extended past the ilium), and having a missing hole in the lower jaw (a Mandibular fenestra). A variety of ornithopods, and related cerapods, had thin cartilaginous plates along the outside of the ribs; in some cases, these plates mineralized and were fossilized. The function of these intercostal plates is unknown. They have been found with Hypsilophodon, Othnielosaurus, Parksosaurus, Talenkauen, Thescelosaurus,^[9] and Macrogryphosaurus to date.^[10]

Size of a variety of numerous ornithopods

The early ornithopods were only about 1 metre (3 feet) long, but probably very fast. They had a stiff tail, like the theropods, to help them balance as they ran on their hind legs. Later ornithopods became more adapted to grazing on all fours; their spines curved, and came to resemble the spines of modern ground-feeders, such as the bison. As they became more adapted to eating while bent over, they became facultative quadrupeds; still running on two legs, and comfortable reaching up into trees, but spending most of their time walking or grazing on all fours. The taxonomy of dinosaurs previously ascribed to the Hypsilophodontidae is problematic. The group previously consisted of all non-iguanodontian bipedal ornithischians, but a phylogenetic reappraisal has shown such species to be paraphyletic. As such, the hypsilophodont family is currently represented only by Hypsilophodon.^[11]

Later ornithopods became larger, but never rivalled the incredible size of the long-necked, long-tailed sauropods that they partially supplanted. The very largest, such as Shantungosaurus, were as heavy as medium-sized sauropods (up to 23 metric tons/25 short tons), but never grew much beyond 15 metres (50 feet).

Classification

An artist's interpretation of Gideonmantellia, sometimes considered one of the most primitive ornithopods

Restoration of Muttaburrasaurus, an early iguanodont

Skeleton of Dysalotosaurus, a dryosaurid ornithopod from the Jurassic

Life restoration of Iguanacolossus, an early styracosternan

Reconstruction of Mantellisaurus, a primitive member of the Hadrosauriformes

Mounted skeleton of Edmontosaurus, a saurolophine hadrosaur, and one of the last ornithopods

Historically, most indeterminate ornithischian bipeds were lumped in as ornithopods. Most have since been reclassified.

References

External links

• GEOL 104 dinosaurs: a Natural history, ornithopoda: beaks, bills & crests, by Thomas R. Holtz Jr.