'helmeted lizard') is a genus of hadrosaurid "duck-billed" dinosaur from the Late Cretaceous period, about 77–75.7 million years ago, in what is now western North America.

The holotype skeleton is only missing the last section of the tail and part of the front legs, but was preserved with impressions of polygonal scales.

Inside the cavity were remains of conifer needles, seeds, twigs, and fruits, suggesting that Corythosaurus probably fed on all of these.

The first specimen, AMNH 5240, was discovered in 1911 by Barnum Brown in Red Deer River of Alberta and secured by him in the Fall of 1912.

[4] The left or underside of the skeleton was preserved in carbonaceous clay, making it difficult to expose the skin to the elements.

[5] Both scapulae and coracoids are preserved in position, but the rest of the front legs are gone (except for phalanges and pieces of the humeri, ulnae, and radii).

[5] Impressions of the integument were preserved covering over a large part of the skeleton’s outlining and shows the form of the body.

[8] The two best preserved specimens of Corythosaurus, found by Charles H. Sternberg in 1912, were lost on December 6, 1916, while being carried by the SS Mount Temple to the United Kingdom during World War I.

They were being sent to Arthur Smith Woodward, a paleontologist of the British Museum of Natural History in England, when the ship transporting them was sunk by the German merchant raider SMS Möwe in the middle of the ocean.

[12][13][14] C. intermedius lived at a slightly later time in the Campanian than C. casuarius and the two species are not identical, which supported the separation of them in a 2009 study.

[15] The invalid species, C. excavatus (specimen UALVP 13), was based on only a skull found in 1920 and wouldn't be reunited with the rest of its remains until 2012.

As with other lambeosaurines, the animal bore a tall, elaborate, bony crest atop its skull that contained the elongate narial passages.

[23] The narial passages extended into the crest, first into separate pockets in the sides, then into a single central chamber, and onward into the respiratory system.

[17] They are formed by a combination of the praemaxillae, nasals, prefrontals, and frontals, as in Saurolophus, but instead of projecting backwards as a spine, they rise up to make the highest point above the orbit.

Brown assumed that they extended from the beaks' tip to the highest spot along the crest and that, unlike those in other genera, the nasals meet in the center and are not separated in front by an ascending premaxillary process.

In the revised version, these extra features include a comparatively short skull with a high helmet-like crest formed by nasals, prefrontals, and frontals; the nasals not being separated in front by premaxillaries; a narrow beak; expanded section in front of the elongated nares; a small narial opening; a vertebral formula of 15 cervicals, 19 dorsals, 8 sacrals, and 61+ caudals; possession of dorsal spines of a medium height; high anterior caudal spines; long chevrons; long scapulae that possess a blade of medium width; a radius considerably longer than the humerus; comparatively short metacarpals, an anteriorly decurved ilium; a long ischium with a foot-like terminal expansion; a pubis with an anterior blade that is short and broadly expanded at the end; a femur that is longer than the tibia; the phalanges of pes are short; that the integument over the sides and tail composed of polygonal tuberculate scales without pattern, but graded in size in different parts of the body; and a belly with longitudinal rows of large conical limpet-like scales separated by uniformly large polygonal tubercles.

[32] Later, Brown revised the phylogeny of Corythosaurus, finding that it was closely related and possibly ancestral to Hypacrosaurus.

[34] Benson et al. (2012) found that Corythosaurus was closely related to Velafrons, Nipponosaurus, and Hypacrosaurus, with them forming a group of fan-crested lambeosaurines.

[17][23] Scientists speculate that Corythosaurus could make loud, low pitched cries "like a wind or brass instrument",[23] such as a trombone.

[37] Corythosaurus casuarius is one of a few lambeosaurines, along with Lambeosaurus lambei, Hypacrosaurus stebingeri, and H. altispinus, to have had surviving fossilized juveniles assigned to it.

Earlier, four genera and thirteen species were recognized from the formation's area when paleontologists used differences in size and crest shape to differentiate taxa.

Young and adult Corythosaurus are similar to Lambeosaurus and Hypacrosaurus, but dissimilar to Parasaurolophus in that the sutures of the skull are sinuous, not smooth and straight.

Based on the climate of the Late Cretaceous, they guessed that Corythosaurus would have been a selective feeder, eating only the juiciest fruits and youngest leaves.

Inside the cavity were remains of conifer needles, seeds, twigs, and fruits, meaning that Corythosaurus probably fed on all of these, implying that it was a browser.

[15] The holotype specimen was clearly a carcass that had floated up on a beach, as Unio shells, water-worn bones, and a baenid turtle were preserved all around it.

Also from the section of the formation are the theropods Daspletosaurus and Saurornitholestes, the hadrosaurids Brachylophosaurus, Gryposaurus, and Parasaurolophus, the ankylosaurid Scolosaurus, and the ceratopsians Coronosaurus and Chasmosaurus.

[45] Corythosaurus lived alongside numerous other giant herbivores, such as the hadrosaurids Gryposaurus and Parasaurolophus, the ceratopsids Centrosaurus and Chasmosaurus, and the ankylosaurids Scolosaurus, Edmontonia,[15] and Dyoplosaurus[15] in the earliest stages of the formation, Dyoplosaurus, Panoplosaurus, [15] and Euoplocephalus in the middle age, and Euoplocephalus alone in later stages of the formation.

Studies of the jaw anatomy and mechanics of these dinosaurs suggests they probably all occupied slightly different ecological niches in order to avoid direct competition for food in such a crowded eco-space.

[15] Thomas M. Lehman has observed that Corythosaurus hasn't been discovered outside of southern Alberta, even though it is one of the most abundant Judithian dinosaurs in the region.

[45] This restricted distribution strongly contrasts with modern mammalian faunas whose large herbivores' ranges "typical[ly] ... span much of a continent.