| Ornithopods | |
|---|---|
 | |
| Seven archetypal ornithopods (top left to bottom right):Ouranosaurus,Tenontosaurus (background) withConvolosaurus (foreground),Muttaburrasaurus,Edmontosaurus annectens,Dryosaurus altus,Corythosaurus casuarius | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | †Ornithischia |
| Clade: | †Cerapoda |
| Clade: | †Ornithopoda Marsh, 1881 |
| Subgroups | |
| Synonyms[1] | |
Ornithopoda (/ˌɔːrnəˈθɒpədə/)[2] is aclade ofornithischiandinosaurs, calledornithopods (/ˈɔːrnəθəˌpɒdz,ɔːrˈnɪθ-/).[3][4] They represent one of the most successful groups ofherbivorous dinosaurs during theCretaceous. The most primitive members of the group were bipedal and relatively small-sized, while advanced members of the subgroupIguanodontia became quadrupedal and developed large body size. Their major evolutionary advantage was the progressive development of achewing apparatus that became the most sophisticated ever developed by a non-avian dinosaur, rivaling that of modernmammals such as the domesticcow. They reached their apex of diversity and ecological dominance in thehadrosaurids (colloquially known as 'duck-bills'), before they were wiped out by theCretaceous–Paleogene extinction event along with all other non-aviandinosaurs. Members are known worldwide.
In 1870,Thomas Henry Huxley listedIguanodontidae (coined byEdward Drinker Cope a year earlier[5]) as one of his three families of dinosaurs (alongsideMegalosauridae andScelidosauridae), including within it the generaIguanodon,Hypsilophodon, andHadrosaurus, in addition toCetiosaurus and tentativelyStenopelix.[6] The term Ornithopoda was erected byOthniel Charles Marsh in 1881 as part of his then still ongoing investigation of the classification of Dinosauria. It was considered one of the four definiteorders of dinosaurs, the others beingTheropoda,Sauropoda, andStegosauria (Hallopoda was considered a possible fifth). He subdivided the order into three families:Camptonotidae,Iguanodontidae, andHadrosauridae; the former was a new name, whereas the latter two were carried over from the nomenclatures of Huxley andEdward Drinker Cope respectively. Within Camptonotidae he included the EuropeanHypsilophodon and three American taxa he named himself,Camptonotus,Laosaurus, andNanosaurus.[7]Camptonotus was in 1885 renamed toCamptosaurus, as the original name was pre-occupied bya cricket; the associated family followed suit, becoming Camptosauridae.[8] In Iguanodontidae, only found in Europe, he includedIguanodon andVectisaurus. In Hadrosauridae, he includedHadrosaurus,Cionodon, and tentativelyAgathaumas.[7]
Ornithopoda means "bird feet", from theGreekornithos,ornis ("bird") andpous,podos ("feet"); this is in reference to members’ characteristic birdlike feet.[9] They were also characterized as lacking in body armour, not developing a hornybeak, having an elongatedpubis (that eventually extended past theilium), and having a missing hole in the lowerjaw (aMandibular fenestra). A variety of ornithopods, and relatedornithischians, had thincartilaginous 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 withHypsilophodon,Nanosaurus,Parksosaurus,Talenkauen,Thescelosaurus,[10] andMacrogryphosaurus to date.[11]
The early ornithopods were only about 1 metre (3 feet) long, but probably very fast. They had a stiff tail, like thetheropods, to help them balance as they ran on their hind legs. Later ornithopods became more adapted to grazing on all fours; theirspines curved, and came to resemble the spines of modern ground-feeders, such as thebison. 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 theHypsilophodontidae is problematic. The group previously consisted of all non-iguanodontian bipedal ornithischians, but a phylogenetic reappraisal has shown such species to beparaphyletic. As such, the hypsilophodont family is currently represented only byHypsilophodon.[12]
Later ornithopods became larger, but never rivalled the incredible size of the long-necked, long-tailedsauropods. The very largest, such asShantungosaurus, were as heavy as medium-sized sauropods (up to 23metric tons/25short tons), but never grew much beyond 15 metres (50 feet).[citation needed]
Historically, most indeterminate ornithischianbipeds were lumped in as ornithopods. Most have since been reclassified.[citation needed]
Ornithopoda is usually given the rank of Suborder, within the order Ornithischia. While ranked taxonomy has largely fallen out of favour among dinosaur paleontologists, some researchers have continued to employ such a classification, though sources have differed on what its rank should be. Benton (2004) placed it as an infraorder within the suborderCerapoda (originally named as an unrankedclade), while others, such as Ibiricuet al. 2010, have retained it at its traditional ranking of suborder.[13]
Iguanodontia is often listed as an infraorder within a suborder Ornithopoda, though Benton (2004) lists Ornithopoda as an infraorder and does not rank Iguanodontia. Traditionally, iguanodontians were grouped into the superfamily Iguanodontoidea and familyIguanodontidae. However,phylogenetic studies show that the traditional "iguanodontids" are aparaphyletic grade leading up to thehadrosaurs (duck-billed dinosaurs). Groups like Iguanodontoidea are sometimes still used as unrankedclades in the scientific literature, though many traditional "iguanodontids" are now included in the more inclusive groupHadrosauroidea.[citation needed] Iguanodontia was originally phylogenetically defined, byPaul Sereno, in 1998, as the most inclusive group containingParasaurolophus walkeri but notHypsilophodon foxii. Later, in 2005, he amended the definition to includeThescelosaurus neglectus as a secondary external specifier, alongsideHypsilophodon, accounting for the paraphyletic nature ofHypsilophodontidae.[14] A 2017 study which named and describedBurianosaurus noted that the type speciesIguanodon bernissartensis must be part of the definition, and that the 2005 definition would, in their analysis, include a far larger group than intended (includingMarginocephalia). They proposed an entirely new,node-based definition: the last common ancestor ofIguanodon bernissartensis,Dryosaurus altus,Rhabdodon priscus, andTenontosaurus tilletti.[15]
In 2021, Iguanodontia was given a formal definition under thePhyloCode: "The smallest clade containingDryosaurus altus,Iguanodon bernissartensis,Rhabdodon priscus, andTenontosaurus tilletti, provided that it does not includeHypsilophodon foxii." Under this revised definition, Iguanodontia is limited to its traditionally included species, and if it were found to include hypsilophodonts, which were not traditionally considered iguanodontians, it would become an invalid grouping.[1]
The slightly less inclusive cladeDryomorpha was named by Paul Sereno in 1986 and given a formal definition in thePhyloCode as "the smallest clade containingDryosaurus altus andIguanodon bernissartensis".[1] This group includes basal members such asHesperonyx, members of the familyDryosauridae, and the derived cladeAnkylopollexia.[16]
In 2021, Ornithopoda was given a formal definition under thePhyloCode: "The largest clade containingIguanodon bernissartensis but notPachycephalosaurus wyomingensis andTriceratops horridus."[1]Thecladogram below follows a 2024 analysis of Fonsecaet al.[16]
Huxley, Thomas H. (1870). "On the Classification of the Dinosauria, with observations on the Dinosauria of the Trias".Quarterly Journal of the Geological Society of London. Vol. 26. pp. 32–51.doi:10.1144/gsl.jgs.1870.026.01-02.09 – viaWikisource.
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