[2] First appearing around 156 million years ago, in the Jurassic, Ankylopollexia became an extremely successful and widespread clade during the Cretaceous, and were found around the world.

[4] The largest known ankylopollexian, dating to the late Campanian age (around 70 million years ago), belonged to the hadrosaurid family, and is named Shantungosaurus.

[13] Camptosaurus dispar Owenodon hoggii Uteodon aphanoecetes Cumnoria prestwichii Cedrorestes crichtoni Osmakasaurus depressus Hippodraco scutodens Theiophytalia kerri Iguanacolossus fortis Planicoxa venenica Dakotadon lakotaensis Lurdusaurus arenatus Lanzhousaurus magnidens

Kukufeldia tilgatensis Barilium dawsoni Fukuisaurus tetoriensis Proa valdearinnoensis Iguanodon bernissartensis Hypselospinus fittoni Mantellisaurus atherfieldensis NHMUK R3741 (cf.

Mantellisaurus) Ouranosaurus nigeriensis Altirhinus kurzanovi Jinzhousaurus yangi Ratchasimasaurus suranareae Penelopognathus weishampeli Equijubus normani Xuwulong yueluni Gongpoquansaurus mazongshanensis Jintasaurus meniscus Probactrosaurus gobiensis Eolambia caroljonesa Hadrosauromorpha The neurobiology of ankylopollexians has been studied as far back as 1871, when a well preserved cranium (specimen NHMUK R2501[14]) discovered in September 1869 from the Wealden Group on the Isle of Wight and tentatively referred to the genus Iguanodon was described by John Hulke.

[19][20] Ostrom supported the view that the brains of hadrosaurs and other dinosaurs would've likely only filled a portion of the cranial cavity, therefore hindering the ability to learn from endocasts, but noted they were still useful.

The latter two genera, which lived later than Camptosaurus, had somewhat higher EQs than the Jurassic taxon, which, being at the lower end, was more comparable to the ceratopsian genus Protoceratops.

Reasonings suggested for their comparably high intelligence were the need for acute senses in the lack of defensive weapons, and more complex intraspecific behaviours as indicated by their acoustic and visual display structures.

[21] In a first for any terrestrial fossil vertebrate, Brasier et al. (2017) reported mineralized soft tissues from the brain of an iguanodontian dinosaur, from the Valanginian age (around 133 million years ago) Upper Tunbridge Wells Formation at Bexhill, Sussex.

Fragmentary ornithopod remains were associated with the fossil, and though assigning the specimen to any one taxon with certainty wasn't possible, Barilium or Hypselospinus were put forward as likely candidates.

The specimen compared well to endocasts of similar taxa, such as one from a Mantellisaurus on display at the Oxford University Museum of Natural History.

In a 2009 study by palaeontologist David C. Evans and colleagues, the brains of various lambeosaurine hadrosaur genera were scanned and compared to each other, related taxa, and previous predictions.

The size of the cerebral hemispheres was, for the first time, remarked upon, being far larger than in other ornithischians and all large saurischian dinosaurs; maniraptorans Conchoraptor and Archaeopteryx had very similar proportions.

Additionally, the relative cerebral volume was only 30% in Amurosaurus, significantly lower than in Hypacrosaurus, closer to that of theropods like Tyrannosaurus, though still distinctly larger than previously estimated numbers for more primitive iguanodonts.