Parasaurolophus (/ˌpærəsɔːˈrɒləfəs, -ˌsɔːrəˈloʊfəs/; meaning "beside crested lizard" in reference to Saurolophus)[2] is a genus of hadrosaurid "duck-billed" dinosaur that lived in what is now western North America and possibly Asia during the Late Cretaceous period, about 76.9–73.5 million years ago.

Parasaurolophus was a hadrosaurid, part of a diverse family of large Late Cretaceous ornithopods that are known for their range of bizarre head adornments, which were likely used for communication and increased hearing.

[4] It is based on ROM 768, a skull and partial skeleton missing most of the tail and the back legs below the knees, which was found by a field party from the University of Toronto in 1920 near Sand Creek along the Red Deer River in Alberta.

William Parks named the specimen P. walkeri in honor of Sir Byron Edmund Walker, the chairman of the Board of Trustees of the Royal Ontario Museum.

[6] In some faunal lists, there is a mention of possible P. walkeri material in the Hell Creek Formation of Montana, a rock unit of the late Maastrichtian age.

In 1921, Charles H. Sternberg recovered a partial skull (PMU.R1250) from what is now known as the slightly younger Kirtland Formation in San Juan County, New Mexico.

Using computed tomography of this skull, Robert Sullivan and Thomas Williamson gave the genus a monographic treatment in 1999 that covered aspects of its anatomy and taxonomy, as well as the functions of its crest.

[8][7] Its closest known relative appears to be Charonosaurus, a lambeosaurine with a similar skull (but no complete crest yet) from the Amur region of northeastern China.

[12] The following cladogram is after the 2007 redescription of Lambeosaurus magnicristatus (Evans and Reisz, 2007):[7] Hadrosaurinae Aralosaurus Tsintaosaurus Jaxartosaurus Amurosaurus Charonosaurus P. cyrtocristatus P. tubicen P. walkeri Nipponosaurus Lambeosaurus lambei L. magnicristatus Corythosaurus Olorotitan Hypacrosaurus altispinus H. stebingeri As a hadrosaurid, Parasaurolophus was a large bipedal and quadrupedal herbivore, eating plants with a sophisticated skull that permitted a grinding motion analogous to chewing.

[17] As noted by Robert Bakker, lambeosaurines have narrower beaks than hadrosaurines, implying that Parasaurolophus and its relatives could feed more selectively than their broad-beaked, crestless counterparts.

At 25% of the total adult size, the juvenile show that crest growth of Parasaurolophus began sooner than in related genera, such as Corythosaurus.

[31][32] It is now believed that it may have had several functions: visual display for identifying species and sex, sound amplification for communication, and thermoregulation.

Williamson noted that in any case, juvenile Parasaurolophus probably had small, rounded crests like P. cyrtocristatus, that probably grew faster as individuals approached sexual maturity.

[7] Many early suggestions focused on adaptations for an aquatic lifestyle, following the hypothesis that hadrosaurids were amphibious, a common line of thought until the 1960s.

William Parks, in 1922, suggested that the crest was joined to the vertebrae above the shoulders by ligaments or muscles, and helped with moving and supporting the head.

[38] Othenio Abel proposed it was used as a weapon in combat among members of the same species,[39] and Andrew Milner suggested that it could be used as a foliage deflector, like the helmet crest (called a 'casque') of the cassowary.

[43][44] The next people to publish a related idea were Teresa Maryańska and Osmólska, who realized that like modern lizards, dinosaurs could have possessed salt glands, and cooled off by osmo-regulation.

[33] Parasaurolophus is often hypothesized to have used its crest as a resonating chamber to produce low frequency sounds to alert other members of a group or its species.

He noted that the crest's internal structures are similar to those of a swan and theorized that an animal could use its elongated nasal passages to create noise.

[19] Computer modeling of a well-preserved specimen of P. tubicen, with more complex air passages than those of P. walkeri, has allowed the reconstruction of the possible sound its crest produced.

Subsequently, reconstructions of Parasaurolophus with a substantial frill of skin between the crest and neck appeared in influential paleoart including murals by Charles R. Knight and in the Walt Disney animated film, Fantasia.

[47] Parasaurolophus walkeri, from the Dinosaur Park Formation, was a member of a diverse and well-documented fauna of prehistoric animals, including well-known dinosaurs such as the horned Centrosaurus, Chasmosaurus, and Styracosaurus; ornithomimids Struthiomimus; fellow duckbills Gryposaurus and Corythosaurus; tyrannosaurids Gorgosaurus and Daspletosaurus; and armored Edmontonia, Euoplocephalus and Dyoplosaurus.

[6] The Dinosaur Park Formation is interpreted as a low-relief setting of rivers and floodplains that became more swampy and influenced by marine conditions over time as the Western Interior Seaway transgressed westward.

Specifically, its contemporaries were the ceratopsian Pentaceratops sternbergii;[8] the pachycephalosaur Stegoceras novomexicanum;[49] and some unidentified fossils belonging to Tyrannosauridae, ?Ornithomimus, ?Troodontidae, ?Saurornitholestes langstoni, ?Struthiomimus, Ornithopoda, ?Chasmosaurus, ?Corythosaurus, Hadrosaurinae, Hadrosauridae, and Ceratopsidae.

[50] Ornithischians from the formation are represented by the hadrosaurids Anasazisaurus horneri, Naashoibitosaurus ostromi, Kritosaurus navajovius, and P. tubicen; the ankylosaurids Ahshislepelta minor and Nodocephalosaurus kirtlandensis; the ceratopsians Pentaceratops sternbergii[8] and Titanoceratops ouranos;[52] and the pachycephalosaurs Stegoceras novomexicanum[49] and Sphaerotholus goodwini.

[51] Turtles are fairly plentiful, and are known from Denazinemys nodosa, Basilemys nobilis, Neurankylus baueri, Plastomenus robustus and Thescelus hemispherica.

[50] Argon-argon radiometric dating indicates that the Kaiparowits Formation was deposited between 76.6 and 74.5 million years ago, during the Campanian age of the Late Cretaceous period.

The plateau where dinosaurs lived was an ancient floodplain dominated by large channels and abundant wetland peat swamps, ponds and lakes, and was bordered by highlands.

[58] Parasaurolophus shared its paleoenvironment with other dinosaurs, such as dromaeosaurid theropods, the troodontid Talos sampsoni, ornithomimids like Ornithomimus velox, tyrannosaurids like Teratophoneus, armored ankylosaurids, the duckbilled hadrosaur Gryposaurus monumentensis, the ceratopsians Utahceratops gettyi, Nasutoceratops titusi and Kosmoceratops richardsoni and the oviraptorosaurian Hagryphus giganteus.

[59] Paleofauna present in the Kaiparowits Formation included chondrichthyans (sharks and rays), frogs, salamanders, turtles, lizards and crocodilians like the apex predator Deinosuchus.