In 1870, Thomas Henry Huxley listed Iguanodontidae (coined by Edward Drinker 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.

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.

[9] 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).

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.

[citation needed] 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.

They proposed an entirely new, node-based definition: the last common ancestor of Iguanodon bernissartensis, Dryosaurus altus, Rhabdodon priscus, and Tenontosaurus tilletti.

[15] 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."

"[1] The cladogram below follows a 2024 analysis of Fonseca et al.[16] Thescelosauridae Kulindadromeus Marginocephalia Gideonmantellia Hypsilophodon Tenontosauridae Rhabdodontoidea Anabisetia Diluvicursor Gasparinisaura Notohypsilophodon Elasmaria Iyuku Dryosauridae