It has a long taxonomic history, and specimens have at times been classified as Diclonius, Trachodon, Hadrosaurus, Claosaurus, Thespesius, Anatosaurus, and Anatotitan before all being grouped together in Edmontosaurus.

[8][9] References predating the 1980s typically use Anatosaurus, Claosaurus, Diclonius, Thespesius, or Trachodon for E. annectens fossils, depending on the author and date.

The first quality specimen, the former holotype of Anatosaurus copei (Anatotitan), was a complete skull and most of a skeleton collected in 1882 by Dr. J. L. Wortman and R. S. Hill[11] for American paleontologist Edward Drinker Cope.

This specimen, found in Hell Creek Formation rocks,[12] came from northeast of the Black Hills of South Dakota, and originally had extensive skin impressions.

[18] Several years after Cope's description, his arch-rival, Othniel Charles Marsh, published a paper on a sizable lower jaw recovered by John Bell Hatcher in 1889 from the Lance Formation rocks in Niobrara County, Wyoming.

As noted by Lull and Wright, this long, slender partial jaw shares with Cope's specimen a prominent ridge running on its side.

Hunter demonstrated that they were brittle and thus stone by kicking the tops off the vertebrae, an act later lamented by the eventual collector Barnum Brown.

Another cowboy, Alfred Sensiba, bought the specimen from Hunter for a pistol and later sold it to Brown, who excavated it for the American Museum of Natural History in 1906.

Henry Fairfield Osborn described the tableau as representing the two animals feeding alongside a marsh, the standing individual having been startled by the approach of a Tyrannosaurus.

The first, the "Trachodon mummy," AMNH 5060, was discovered in 1908 by Charles Hazelius Sternberg and his sons in the Lance Formation rocks near Lusk, Wyoming.

[26] In 1926, Charles Mortram Sternberg named Thespesius saskatchewanensis for NMC 8509, a skull and partial skeleton from the Wood Mountain plateau of southern Saskatchewan.

Opinions varied greatly, with textbooks and encyclopedias drawing a distinction between the "Iguanodon-like" Claosaurus annectens and the "duck-billed" Hadrosaurus (based on Cope's Diclonius mirabilis); conversely, Hatcher explicitly identified C. annectens as synonymous with the hadrosaurid represented by those same duck-billed skulls,[9] the two differentiated only by individual variation or distortion from pressure.

This included Cionodon, Diclonius, Hadrosaurus, Ornithotarsus, Pteropelyx, and Thespesius, as well as Claorhynchus and Polyonax,[30] fragmentary genera now thought to be ceratopsians.

Hatcher's work led to a brief consensus until about 1910, when new material from Canada and Montana showed a greater diversity of hadrosaurids than previously suspected.

In their monograph on hadrosaurian dinosaurs of North America, they opted to settle the questions revolving around the AMNH duckbills, Marsh's Claosaurus annectens, and several other species, by creating a new generic name.

[34][35][36] 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.

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.

[44][45] In a 2011 study by Nicolás Campione and David Evans, the authors conducted the first-ever morphometric analysis of the various specimens assigned to Edmontosaurus.

[29] To be fair to Cope, a dozen vertebrae, the hips, and thigh bones had been carried away by a stream cutting through the skeleton, and the tip of the tail was incomplete.

The lower jaw was long, straight, and lacking the downward curve seen in other hadrosaurids, as well as possessing a heavy ridge running its length.

E. annectens was also historically classified in an independent genus, Anatosaurus, following the influential 1942 revision of Hadrosauridae by Richard Swann Lull and Nelda Wright, until it was reclassified as a species of Edmontosaurus by Michael K.

[51] A preserved rhamphotheca present in specimen LACM 23502, housed in the Los Angeles County Museum, also indicates the beak of Edmontosaurus was more hook-shaped and extensive than many illustrations in scientific and public media have previously depicted.

Campione and Evans interpreted these results as strongly suggesting that the shape of Edmontosaurus skulls changed dramatically as they grew and matured.

[27] In a 2022 study, Wosik and Evans proposed that E. annectens reached maturity in nine years, based on their analysis for various specimens from different localities.

[55] Typical dinosaur faunas of the Lancian formations where Edmontosaurus annectens has been found also included: the hypsilophodont Thescelosaurus; the rare ceratopsid Torosaurus; the pachycephalosaurid Pachycephalosaurus; the ankylosaurid Ankylosaurus; and the theropods Ornithomimus, Pectinodon, Acheroraptor, Dakotaraptor, and Tyrannosaurus.

[56][57] The Hell Creek Formation, as typified by exposures in the Fort Peck area of Montana, has been interpreted as a flat, forested floodplain, with a relatively dry subtropical climate supporting a variety of plants that ranged from angiosperm trees to conifers, such as bald cypress, as well as ferns and ginkgos.

Stream-dwelling turtles and tree-dwelling multituberculate mammals were diverse, and monitor lizards as large as the modern Komodo dragon hunted on the ground.

[58] The Lance Formation, as typified by exposures approximately 62 miles (100 km) north of Fort Laramie in eastern Wyoming, has been interpreted as a bayou setting similar to the Louisiana coastal plain.

Tropical araucarian conifers and palm trees dotted the hardwood forests, differentiating the flora from the northern coastal plain.

[60] Freshwater fish, salamanders, turtles, lizards, snakes, shorebirds, and small mammals lived alongside the dinosaurs.