Edmontosaurus (/ɛdˌmɒntəˈsɔːrəs/ ed-MON-tə-SOR-əs) (meaning "lizard from Edmonton"), with the second species often colloquially and historically known as Anatosaurus or Anatotitan (meaning "duck lizard" and "giant duck"), is a genus of hadrosaurid (duck-billed) dinosaur. It contains two known species: Edmontosaurus regalis and Edmontosaurus annectens. Fossils of E. regalis have been found in rocks of western North America that date from the late Campanian age of the Cretaceous period 73 million years ago, while those of E. annectens were found in the same geographic region from rocks dated to the end of the Maastrichtian age, 66 million years ago. Edmontosaurus was one of the last non-avian dinosaurs to ever exist, and lived alongside dinosaurs like Triceratops, Tyrannosaurus, Ankylosaurus, and Pachycephalosaurus shortly before the Cretaceous–Paleogene extinction event.

Edmontosaurus included two of the largest hadrosaurid species, with E. annectens measuring up to 12 metres (39 ft) in length and weighing around 5.6 metric tons (6.2 short tons) in average asymptotic body mass, although some individuals would have been much larger. Several well-preserved specimens are known that include numerous bones, as well as extensive skin impressions and possible gut contents. Edmontosaurus is classified as a genus of saurolophine (or hadrosaurine) hadrosaurid, a member of the group of hadrosaurids that lacked large, hollow crests and instead had smaller, solid crests or fleshy combs.

The first fossils named Edmontosaurus were discovered in southern Alberta (named after Edmonton, the capital city), in the Horseshoe Canyon Formation (formerly called the lower Edmonton Formation). The type species, E. regalis, was named by Lawrence Lambe in 1917, although several other species that are now classified in Edmontosaurus were named earlier. The best known of these is E. annectens, named by Othniel Charles Marsh in 1892. This species was originally as a species of Claosaurus, known for many years as a species of Trachodon, and later known as Anatosaurus annectens. Anatosaurus, Anatotitan, and probably Ugrunaaluk are now generally regarded as synonyms of Edmontosaurus.

Edmontosaurus was widely distributed across western North America, ranging from Colorado to the northern slopes of Alaska. The distribution of Edmontosaurus fossils suggests that it preferred coasts and coastal plains. It was a herbivore that could move on both two legs and four. Because it is known from several bone beds, Edmontosaurus is thought to have lived in groups and may have been migratory as well. The wealth of fossils has allowed researchers to study its paleobiology in detail, including its brain, how it may have fed, and its injuries and pathologies, such as evidence for tyrannosaur attacks on a few specimens.

Edmontosaurus has had a very long and complicated history in paleontology, having spent decades with various species classified in other genera. Its taxonomic history intertwines at various points with the genera Agathaumas, Anatosaurus, Anatotitan, Claosaurus, Hadrosaurus, Thespesius, and Trachodon, with references predating the 1980s typically using Anatosaurus, Claosaurus, Thespesius, or Trachodon for edmontosaur fossils (excluding those assigned to E. regalis) depending on the author and the date. Although Edmontosaurus was only named in 1917, its oldest well-supported species (E. annectens) was named in 1892 as a species of Claosaurus.

The first well-supported species of Edmontosaurus was named in 1892 as Claosaurus annectens by Othniel Charles Marsh. This species is based on USNM 2414, which is a partial skull-roof and skeleton, with a second skull and skeleton, YPM 2182, designated as the paratype. Both were collected in 1891 by John Bell Hatcher from the late Maastrichtian-age Upper Cretaceous Lance Formation of Niobrara County (then part of Converse County), Wyoming. This species has some historical footnotes attached, as it is among the first dinosaurs to receive a skeletal restoration and is the first hadrosaurid so restored. YPM 2182 and UNSM 2414 are, respectively, the first and second essentially complete mounted dinosaur skeletons in the United States. YPM 2182 was put on display in 1901 and USNM 2414 was put on display in 1904.

Because of the incomplete understanding of hadrosaurids at the time, following Marsh's death in 1897, Claosaurus annectens was variously classified as a species of Claosaurus, Thespesius or Trachodon. Opinions varied greatly, as textbooks and encyclopedias drew a distinction between the "Iguanodon-like" Claosaurus annectens and the "duck-billed" Hadrosaurus (based on remains now known as adult Edmontosaurus annectens), while Hatcher explicitly identified C. annectens as synonymous with the hadrosaurid represented by those same duck-billed skulls. Hatcher's revision, published in 1902, was sweeping, as he considered almost all hadrosaurid genera then known as synonyms of Trachodon. This included Cionodon, Diclonius, Hadrosaurus, Ornithotarsus, Pteropelyx, and Thespesius, as well as Claorhynchus and Polyonax, which are fragmentary genera now thought to be ceratopsians. Hatcher's work led to a brief consensus until post-1910, when new material from Canada and Montana showed a greater diversity of hadrosaurids than previously suspected. Charles W. Gilmore, in 1915, reassessed hadrosaurids and recommended that Thespesius be reintroduced for hadrosaurids from the Lance Formation and rock units of equivalent age and that Trachodon, based on inadequate material, should be restricted to a hadrosaurid from the older Judith River Formation and its equivalents. In regards to Claosaurus annectens, he recommended that it be considered the same as Thespesius occidentalis. His reinstatement of Thespesius for Lance-age hadrosaurids would have other consequences for the taxonomy of Edmontosaurus in the following decades.

During this time frame (1902–1915), two additional important specimens of C. annectens were recovered. The first, the "mummified" specimen AMNH 5060, was discovered in 1908 by Charles Hazelius Sternberg and his sons in Lance Formation rocks near Lusk, Wyoming. Sternberg was working for the British Museum of Natural History, but Henry Fairfield Osborn of the American Museum of Natural History was able to purchase the specimen for $2,000. The Sternbergs recovered a second similar specimen from the same area in 1910, which was not as well preserved. However, it was also found with skin impressions. They sold the specimen, SM 4036, to the Senckenberg Museum in Germany.

As a side note, Trachodon selwyni, described by Lawrence Lambe in 1902 for a lower jaw from what is now known as the Dinosaur Park Formation of Alberta, was erroneously described by Glut (1997) as having been assigned to Edmontosaurus regalis by Lull and Wright. It was not, instead being designated "of very doubtful validity." More recent reviews of hadrosaurids have concurred.

Edmontosaurus itself was coined in 1917 by Lawrence Lambe for two partial skeletons found in the Horseshoe Canyon Formation (formerly the lower Edmonton Formation) along the Red Deer River of southern Alberta. These rocks are older than the rocks in which Claosaurus annectens was found. The Edmonton Formation lends Edmontosaurus its name. The type species, E. regalis (meaning "regal", or, more loosely, "king-sized"), is based on NMC 2288, which consists of a skull, articulated vertebrae up to the sixth tail vertebra, ribs, partial hips, an upper arm bone, and most of a leg. It was discovered in 1912 by Levi Sternberg. The second specimen, paratype NMC 2289, consists of a skull and skeleton lacking the beak, most of the tail, and part of the feet. It was discovered in 1916 by George F. Sternberg. Lambe found that his new dinosaur compared best to Diclonius mirabilis (specimens now assigned to Edmontosaurus annectens) and drew attention to the size and robustness of Edmontosaurus. Initially, Lambe only described the skulls of the two skeletons, but returned to the genus in 1920 to describe the skeleton of NMC 2289. The postcrania of the type specimen remains undescribed, still in its plaster jackets to this day.

Two more species that would come to be included with Edmontosaurus were named from Canadian remains in the 1920s, but both would initially be assigned to Thespesius. Gilmore named the first, Thespesius edmontoni, in 1924. T. edmontoni also came from the Horseshoe Canyon Formation. It was based on NMC 8399, another nearly complete skeleton lacking most of the tail. NMC 8399 was discovered on the Red Deer River in 1912 by a Sternberg party. Its arms, ossified tendons, and skin impressions were briefly described in 1913 and 1914 by Lambe, who at first thought it was an example of a species he had named Trachodon marginatus, but then changed his mind. The specimen became the first dinosaur skeleton to be mounted for exhibition in a Canadian museum. Gilmore found that his new species compared closely to what he called Thespesius annectens, but left the two apart because of details of the arms and hands. He also noted that his species had more vertebrae than Marsh's in the back and neck, but proposed that Marsh was mistaken in assuming that the annectens specimens were complete in those regions.

In 1926, Charles Mortram Sternberg named Thespesius saskatchewanensis for NMC 8509, which is a skull and partial skeleton from the Wood Mountain plateau of southern Saskatchewan. He had collected this specimen in 1921 from rocks that were assigned to the Lance Formation, now the Frenchman Formation. NMC 8509 included an almost complete skull, numerous vertebrae, partial shoulder and hip girdles, and partial legs, representing the first substantial dinosaur specimen recovered from Saskatchewan. Sternberg opted to assign it to Thespesius because that was the only hadrosaurid genus known from the Lance Formation at the time. At the time, T. saskatchewanensis was unusual because of its small size, estimated at 7 to 7.3 metres (23 to 24 ft) in length.

In 1942, Lull and Wright attempted to resolve the complicated taxonomy of crestless hadrosaurids by naming a new genus, Anatosaurus, to take in several species that did not fit well under their previous genera. Anatosaurus, meaning "duck lizard", because of its wide, duck-like beak (Latin anas = duck + Greek sauros = lizard), had as its type species Marsh's old Claosaurus annectens. Also assigned to this genus were Thespesius edmontoni, T. saskatchewanensis, a large lower jaw that Marsh had named Trachodon longiceps in 1890, and a new species named Anatosaurus copei for two skeletons on display at the American Museum of Natural History that had long been known as Diclonius mirabilis (or variations thereof). Thus, the various species became Anatosaurus annectens, A. copei, A. edmontoni, A. longiceps, and A. saskatchewanensis. Anatosaurus would come to be called the "classic duck-billed dinosaur."

This state of affairs persisted for several decades until Michael K. Brett-Surman reexamined the pertinent material for his graduate studies in the 1970s and 1980s. He concluded that the type species of Anatosaurus, A. annectens, was actually a species of Edmontosaurus and that A. copei was different enough to warrant its own genus. Although theses and dissertations are not regarded as official publications by the International Commission on Zoological Nomenclature, which regulates the naming of organisms, his conclusions were known to other paleontologists and were adopted by several popular works of the time. Brett-Surman and Ralph Chapman designated a new genus for A. copei (Anatotitan) in 1990. Of the remaining species, A. saskatchewanensis and A. edmontoni were assigned to Edmontosaurus as well and A. longiceps went to Anatotitan as either a second species or as a synonym of A. copei. Because the type species of Anatosaurus (A. annectens) was sunk into Edmontosaurus, the name Anatosaurus is abandoned as a junior synonym of Edmontosaurus.

The conception of Edmontosaurus that emerged included three valid species: the type species E. regalis, E. annectens (including Anatosaurus edmontoni, amended to edmontonensis), and E. saskatchewanensis. The debate about the proper taxonomy of the A. copei specimens continues to the present day. Returning to Hatcher's argument of 1902, Jack Horner, David B. Weishampel, and Catherine Forster regarded Anatotitan copei as representing specimens of Edmontosaurus annectens with crushed skulls. In 2007, another "mummy" was announced. Nicknamed "Dakota", it was discovered in 1999 by Tyler Lyson and came from the Hell Creek Formation of North Dakota.

In a 2011 study by Nicolás Campione and David Evans, the authors conducted the first ever morphometric analysis to compare the various specimens assigned to Edmontosaurus. They concluded that only two species are valid: E. regalis, from the late Campanian, and E. annectens, from the late Maastrichtian. Their study provided further evidence that Anatotitan copei is a synonym of E. annectens. Specifically, the long, low skull of A. copei is the result of ontogenetic change and represents mature E. annectens individuals.

Edmontosaurus is currently regarded as having two valid species: the type species E. regalis and E. annectens. E. regalis is known only from the Horseshoe Canyon Formation of Alberta, dating from the late Campanian age of the late Cretaceous period. At least a dozen individuals are known, including seven skulls with associated postcrania and five to seven other skulls. The species formerly known as Thespesius edmontoni or Anatosaurus edmontoni represents immature individuals of E. regalis.

E. annectens is known from the Frenchman Formation of Saskatchewan, the Hell Creek Formation of Montana, and the Lance Formation of South Dakota and Wyoming. It is limited to late Maastrichtian rocks and is represented by at least twenty skulls, some with postcranial remains. One author, Kraig Derstler, has described E. annectens as "perhaps the most perfectly-known dinosaur to date [1994]." Anatosaurus copei and E. saskatchewanensis are now thought to be growth stages of E. annectens, with A. copei as adults and E. saskatchewanensis as juveniles. Trachodon longiceps may be a synonym of E. annectens as well. Anatosaurus edmontoni was mistakenly listed as a synonym of E. annectens in both reviews of Dinosauria, but this does not appear to be the case.

E. annectens differed from E. regalis by having a longer, lower, and less robust skull and the lack of a comb-like crest. Although Brett-Surman regarded E. regalis and E. annectens as potentially representing males and females of the same species, all E. regalis specimens come from older formations than E. annectens specimens. Edmontosaurine specimens from the Prince Creek Formation of Alaska formerly assigned to Edmontosaurus sp. were given their own genus and species name, Ugrunaaluk kuukpikensis, in 2015. However, the identification of Ugrunaaluk as a separate genus was questioned by a 2017 study from Hai Xing and colleagues, who regarded it as a nomen dubium that was indistinguishable from other Edmontosaurus. In 2020, Ryuji Takasaki and colleagues agreed that the Prince Creek remains should be classified as Edmontosaurus, though species designation is unclear because the specimens are juveniles. Another study found the Alaskan material to be referable to Edmontosaurus cf. regalis based on craniomandibular anatomy. Edmontosaurus was also reported from the Javelina Formation of Big Bend National Park, western Texas based on TMM 41442-1, but was later referred to Kritosaurus cf. navajovius by Wagner (2001), before being assigned to Kritosaurus sp. by Lehman et al. (2016).

Edmontosaurus has been described in detail from numerous specimens. Traditionally, E. regalis has been regarded as the largest species, though this was challenged by the hypothesis that the larger hadrosaurid Anatotitan copei is a synonym of Edmontosaurus annectens, as put forward by Jack Horner and colleagues in 2004, and supported in studies by Campione and Evans in 2011.

Edmontosaurus was among the largest hadrosaurids to ever exist. Like other hadrosaurids, it was a bulky animal with a long, laterally flattened tail and an expanded, duck-like beak. The arms were not as heavily built as the legs, but were long enough to be used for standing or for quadrupedal movement. Depending on the species, previous estimates suggested that a fully grown adult could have been 9–12 metres (30–39 ft) long and some of the larger specimens reached the range of 12–13 metres (39–43 ft) with a body mass on the order of 4 metric tons (4.4 short tons).

E. annectens is often seen as smaller. Two mounted skeletons, USNM 2414 and YPM 2182, measure 8.00 metres (26.25 ft) long and 8.92 metres (29.3 ft) long, respectively. However, these are probably subadult individuals There is also at least one report of a much larger potential E. annectens specimen that is almost 12 metres (39 ft) long. Two specimens still under study in the collection of the Museum of the Rockies - a 7.5 m (25 ft) tail labelled as MOR 1142 and another labelled as MOR 1609 - indicate that Edmontosaurus annectens could have grown to much larger sizes and reach nearly 15 metres (49 ft) in length, similar to the closesly related Shantungosaurus which weighed 13 metric tons (14 short tons), but such large individuals were likely very rare.

A 2022 study on the osteohistology and growth of E. annectens suggested that previous estimates might have underestimated or overestimated the size of this dinosaur and proposed that a fully grown adult E. annectens would have measured up to 11–12 metres (36–39 ft) in length and approximately 5.6 metric tons (6.2 short tons) in average asymptotic body mass, while the largest individuals measured more than 6 metric tons (6.6 short tons) and even up to 6.6–7 metric tons (7.3–7.7 short tons) when based on the comparison between various specimens of different sizes from the Ruth Mason Dinosaur Quarry and other specimens from different localities. According to this analysis, E. regalis may have been heavier, but not enough samples exist to provide a valid estimate and examination on its osteohistology and growth, so the results for E. regalis aren't statistically significant.

The skull of a fully grown Edmontosaurus could be over a metre long. One skull of E. annectens (formerly Anatotitan) measures 3.87 feet (1.18 m) long. The skull was roughly triangular in profile, with no bony cranial crest. Viewed from above, the front and rear of the skull were expanded, with the broad front forming a duck-bill or spoon-bill shape. The beak was toothless, and both the upper and lower beaks were extended by keratinous material. Substantial remains of the keratinous upper beak are known from the "mummy" kept at the Senckenberg Museum. In this specimen, the preserved nonbony part of the beak extended for at least 8 centimetres (3.1 in) beyond the bone, projecting down vertically. The nasal openings of Edmontosaurus were elongate and housed in deep depressions surrounded by distinct bony rims above, behind, and below.

In at least one case (the Senckenberg specimen), rarely preserved sclerotic rings were preserved in the eye sockets. Another rarely seen bone, the stapes (the reptilian ear bone), has also been seen in a specimen of Edmontosaurus. It has been suggested that Edmontosaurus may have had binocular vision based on the 3D scan of a nearly complete skull of E. regalis (CMN 2289).

Teeth were present only in the maxillae (upper cheeks) and dentaries (main bone of the lower jaw). The teeth were continually replaced, taking about half a year to form. They were composed of six types of tissues, rivaling the complexity of mammal teeth. They grew in columns, with an observed maximum of six in each, and the number of columns varied based on the animal's size. Known column counts for the two species are: 51 to 53 columns per maxilla and 48 to 49 per dentary (teeth of the upper jaw being slightly narrower than those in the lower jaw) for E. regalis; and 52 columns per maxilla and 44 per dentary for E. annectens (an E. saskatchewanensis specimen).

The number of vertebrae differs between specimens. E. regalis had thirteen neck vertebrae, eighteen back vertebrae, nine hip vertebrae, and an unknown number of tail vertebrae. A specimen once identified as belonging to Anatosaurus edmontoni (now considered to be the same as E. regalis) is reported as having an additional back vertebra and 85 tail vertebrae, with an undisclosed amount of restoration. Other hadrosaurids are only reported as having 50 to 70 tail vertebrae, so this appears to have been an overestimate. The anterior back was curved toward the ground, with the neck flexed upward and the rest of the back and tail held horizontally. Most of the back and tail were lined by ossified tendons arranged in a latticework along the neural spines of the vertebrae. This condition has been described as making the back and at least part of the tail "ramrod" straight. The ossified tendons are interpreted as having strengthened the vertebral column against gravitational stress, incurred through being a large animal with a horizontal vertebral column otherwise supported mostly by the hind legs and hips.