Product Code Database
Ornithopoda () is a clade of , called ornithopods (). They represent one of the most successful groups of herbivore dinosaurs during the Cretaceous. The most primitive members of the group were bipedal and relatively small-sized, while advanced members of the subgroup Iguanodontia became quadrupedal and developed large body size. 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 such as the domestic cow. They reached their apex of diversity and ecological dominance in the Hadrosauridae (colloquially known as 'duck-bills'), before they were wiped out by the Cretaceous–Paleogene extinction event along with all other non-aves . Members are known worldwide.
The early ornithopods were only about 1 metre (3 feet) long, but probably very fast. They had a stiff tail, like the theropoda, to help them balance as they ran on their hind legs. Later ornithopods became more adapted to grazing on all fours; their vertebral column 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- bipedal ornithischians, but a phylogenetic reappraisal has shown such species to be paraphyletic. As such, the hypsilophodont family is currently represented only by Hypsilophodon.
Later ornithopods became larger, but never rivalled the incredible size of the long-necked, long-tailed . The very largest, such as Shantungosaurus, were as heavy as medium-sized sauropods (up to 23 /25 ), but never grew much beyond 15 metres (50 feet).
Based on evidence derived from examination of dental microwear, early ornithopods were mainly browsers or Frugivore, whereas larger, more derived iguanodontians were bulk feeders that fed on tougher, less nutritious vegetation and were able to subsist on it due to their long gut passage time.
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 family Iguanodontidae. However, phylogenetic studies show that the traditional "iguanodontids" are a paraphyletic grade leading up to the hadrosaurid (duck-billed dinosaurs). Groups like Iguanodontoidea are sometimes still used as unranked in the scientific literature, though many traditional "iguanodontids" are now included in the more inclusive group Hadrosauroidea. Iguanodontia was originally phylogenetically defined, by Paul Sereno, in 1998, as the most inclusive group containing Parasaurolophus walkeri but not Hypsilophodon foxii. Later, in 2005, he amended the definition to include Thescelosaurus neglectus as a secondary external specifier, alongside Hypsilophodon, accounting for the paraphyletic nature of Hypsilophodontidae. A 2017 study which named and described Burianosaurus noted that the type species Iguanodon bernissartensis must be part of the definition, and that the 2005 definition would, in their analysis, include a far larger group than intended (including Marginocephalia). They proposed an entirely new, node-based definition: the last common ancestor of Iguanodon bernissartensis, Dryosaurus altus, Rhabdodon priscus, and Tenontosaurus tilletti.
In 2021, Iguanodontia was given a formal definition under the PhyloCode: "The smallest clade containing Dryosaurus altus, Iguanodon bernissartensis, Rhabdodon priscus, and Tenontosaurus tilletti, provided that it does not include Hypsilophodon 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.
The slightly less inclusive clade Dryomorpha was named by Paul Sereno in 1986 and given a formal definition in the PhyloCode as "the smallest clade containing Dryosaurus altus and Iguanodon bernissartensis". This group includes basal members such as Hesperonyx, members of the family Dryosauridae, and the derived clade Ankylopollexia.
|
|
