Hadrosaurids (from Ancient Greek ἁδρός (hadrós) 'stout, thick' and σαύρα (saúra) 'lizard'), or hadrosaurs, are members of the ornithischian family Hadrosauridae.

Unlike more primitive iguanodonts, the teeth of hadrosaurids are stacked into complex structures known as dental batteries, which acted as effective grinding surfaces.

[2][3] Ferdinand Vandeveer Hayden, during expeditions near the Judith River in 1854 through 1856, discovered the very first dinosaur fossils recognized from North America.

Around the same time in Philadelphia, on the other side of the continent, geologist William Parker Foulke was informed of numerous large bones accidentally uncovered by farmer John E. Hopkins some twenty years earlier.

[4] Among his 1858 work Leidy briefly suggested that the animal was likely amphibious in nature; this school of thought about hadrosaurs would come to be dominant for over a century to come.

[5] Further discoveries such as "Hadrosaurus minor" and "Ornithotarsus immanis" would come from the East, and Edward Drinker Cope led an expedition to the Judith River Formation where Trachodon was found.

[4][5] Research would continue in the Judith River area for years to come, but the formation never yielded much more than fragmentary remains, and Cope's species as well as Trachodon itself would in time be seen as of doubtful validity.

One such specimen was the very complete AMNH 5060 (belonging to Edmontosaurus annectens), recovered in 1908 by the fossil collector Charles Hazelius Sternberg and his three sons in Converse County, Wyoming.

This unrealized endeavor would come to be the inspiration for Richard Swann Lull and Nelda Wright to work on a similar project decades later.

They agreed with prior authors on the semi-aquatic nature of hadrosaurs, but re-evaluated Cope's idea of weak jaws and found quite the opposite.

This new approach was backed using evidence of the environment and climate they lived in, co-existing flora and fauna, physical anatomy, and preserved stomach contents from mummies.

Based on evaluation of all this data, Ostrom found the idea that hadrosaurs were adapted for aquatic life incredibly lacking, and instead proposed they were capable terrestrial animals that browsed on plants such as conifers.

[14] During the late early Maastrichtian, several lineages of Lambeosaurinae from Asia migrated into the European Ibero-Armorican Island (what is now France and Spain), including Arenysaurini and Tsintaosaurini.

[18] The family is now formally defined in the PhyloCode as "the smallest clade containing Hadrosaurus foulkii, Lambeosaurus lambei, and Saurolophus osborni".

Angulomastacator Hypacrosaurus altispinus Olorotitan Arenysaurus Blasisaurus The most recognizable aspect of hadrosaur anatomy is the flattened and laterally stretched rostral bones, which gives the distinct duck-bill look.

[18] In some genera, including Edmontosaurus, the whole front of the skull was flat and broadened out to form a beak, which was ideal for clipping leaves and twigs from the forests of Asia, Europe and North America.

While studying the chewing methods of hadrosaurids in 2009, the paleontologists Vincent Williams, Paul Barrett, and Mark Purnell found that hadrosaurs likely grazed on horsetails and vegetation close to the ground, rather than browsing higher-growing leaves and twigs.

[28] The most recent such finding before the publication of the Purnell study was conducted in 2008, when a team led by University of Colorado at Boulder graduate student Justin S. Tweet found a homogeneous accumulation of millimeter-scale leaf fragments in the gut region of a well-preserved partially grown Brachylophosaurus.

[30] In response to such findings, Purnell said that preserved stomach contents are questionable because they do not necessarily represent the usual diet of the animal.

[32] Coprolites (fossilized droppings) of some Late Cretaceous hadrosaurs show that the animals sometimes deliberately ate rotting wood.

[36][37][38] The brains of hadrosaurid dinosaurs have been studied as far back at the late 19th century, when Othniel Charles Marsh made an endocast of a specimen then referred to Claosaurus annectens; only basic remarks were possible but it was noted that the organ was proportionally small.

[39] John Ostrom would give a more informed analysis and review in 1961, pulling on data from Edmontosaurus regalis, E. annectens, and Gryposaurus notabilis (then considered a synonym of Kritosaurus).

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.

[37] As with previous studies, EQ values were investigated, although a wider number range was given to account for uncertainty in brain and body mass.

This is comparable to their size in saurolophine hadrosaurs, but far larger than in any ornithischians outside of Hadrosauriformes, and all large saurischian dinosaurs; maniraptors Conchoraptor and Archaeopteryx, an early bird, 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 (with 33%), though still distinctly larger than previously estimated numbers for more primitive iguanodonts like Lurdusaurus and Iguanodon (both at 19%).

They suggested that behavior, diet, soil condition, and competition between dinosaur species all potentially influenced where hadrosaurs nested.

The most common remains of young hadrosaurs in the Dinosaur Park Formation are dentaries, bones from limbs and feet, as well as vertebral centra.

[41] However, the joints often show "predepositional erosion or concave articular surfaces",[41] which was probably due to the cartilaginous cap covering the ends of the bones.

[2] Comparisons between the scleral rings of several hadrosaur genera (Corythosaurus, Prosaurolophus, and Saurolophus) and modern birds and reptiles suggest that they may have been cathemeral, active throughout the day at short intervals.