Hypacrosaurus (meaning "near the highest lizard" [Greek υπο-, hypo- = less + ακρος, akros, high], because it was almost but not quite as large as Tyrannosaurus)[1][2] was a genus of duckbill dinosaur similar in appearance to Corythosaurus.

[6] Not long after, Richard Swann Lull and Nelda Wright identified an American Museum of Natural History skeleton (AMNH 5461) from the Two Medicine Formation of Montana as a specimen of Procheneosaurus.

Although he was mostly concerned with the earlier, Dinosaur Park Formation genera Corythosaurus and Lambeosaurus, he suggested that Cheneosaurus would turn out to be composed of juvenile individuals of the contemporaneous Hypacrosaurus altispinus.

[10] H. altispinus, the type species, is known from 5 to 10 articulated skulls with some associated skeletal remains, from juvenile to adult individuals found in the Horseshoe Canyon Formation.

[12] The new species Hypacrosaurus stebingeri was named for a variety of remains, including hatchlings with associated eggs and nests, found near the top of the late Campanian (Upper Cretaceous) Two Medicine Formation in Glacier County, Montana, and across the border in Alberta.

[16] These genera, particularly Corythosaurus and Hypacrosaurus, are regarded as the "helmeted" or "hooded" branch of the lambeosaurines, and the clade they form is sometimes informally designated Lambeosaurini.

[12] The following cladogram illustrating the relationships of Lambeosaurus and its close relatives was recovered in a 2022 phylogenetic analysis by Xing Hai and colleagues.

[17] Xuwulong Bactrosaurus Telmatosaurus Gryposaurus Edmontosaurus Canardia Aralosaurus Pararhabdodon Tsintaosaurus Jaxartosaurus Blasisaurus Arenysaurus Parasaurolophus cyrtocristatus "Charonosaurus" jiayinensis Parasaurolophus tubicen Parasaurolophus walkeri Olorotitan Velafrons Amurosaurus Lambeosaurus clavinitialis Lambeosaurus magnicristatus Lambeosaurus lambei Corythosaurus intermedius Corythosaurus casuarius Hypacrosaurus altispinus "Magnapaulia" laticaudus Hypacrosaurus stebingeri As a hadrosaurid, Hypacrosaurus would have been a bipedal/quadrupedal herbivore, eating a variety of plants.

[9] The hollow crest of Hypacrosaurus most likely had social functions, such as a visual signal allowing individuals to identify sex or species, and providing a resonating chamber for making noises.

Respiratory turbinates function to prevent water loss through evaporation and are found only in birds and mammals, modern endotherms (warm-blooded animals) who could lose a great deal of water while breathing because they breathe more often than comparably sized ectotherms (cold-blooded animals) to support their higher metabolism.

[18] Ruben and others in 1996 concluded that respiratory turbinates were probably not present in Nanotyrannus, Ornithomimus or Hypacrosaurus based on CT scanning, thus there was no evidence that those animals were warm-blooded.

[21] This is in contrast to the Ruben et al. (1996) finding that Hypacrosaurus was not warm-blooded, which was based on the absence of nasal turbinates (see Crest functions subsection, above).

The team performed histological analyses on skull and limb bones of nestling individuals of the specimen MOR 548, a large nesting ground in the Two Medicine Formation attributed to H. stebingeri, and the results showed calcified cartilage within a supraoccipital bone, and upon microscopic magnification, chondrocyte-like structures were found.

[26] The discovery of tooth marks in the fibula of a Hypacrosaurus specimen inflicted by a bite from the teeth of a tyrannosaurid indicated that this, and other hadrosaurids were either preyed upon or scavenged by large theropod dinosaurs during the Late Cretaceous period.

[citation needed] The large, monospecific assemblage of Hypacrosaurus stebingeri in Montana was interpreted as a group of dinosaurs that was killed by a volcanic ashfall.

[citation needed] H. altispinus shared the Horseshoe Canyon Formation with fellow hadrosaurids Edmontosaurus and Saurolophus, hypsilophodont Parksosaurus, ankylosaurid Anodontosaurus, nodosaurid Edmontonia, horned dinosaurs Montanoceratops, Anchiceratops, Arrhinoceratops, and Pachyrhinosaurus, pachycephalosaurid Stegoceras, ostrich-mimics Ornithomimus and Struthiomimus, a variety of poorly known small theropods including troodontids and dromaeosaurids, and the tyrannosaurid Albertosaurus.

[29] The Horseshoe Canyon Formation is interpreted as having a significant marine influence, due to an encroaching Western Interior Seaway, the shallow sea that covered the midsection of North America through much of the Cretaceous.