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, but it may have possibly lived into the early Maastrichtian.

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.

In 1874, Cope named (but did not describe) Agathaumas milo for a sacral vertebra and shin fragments from the late Maastrichtian-age Upper Cretaceous Laramie Formation of Colorado.

[7] Two more species that would come to be included with Edmontosaurus regalis were named from Canadian remains in the 1920s, but both would initially be assigned to the dubious genus Thespesius.

[4] Its forelimbs, 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'd named Trachodon marginatus,[25] but he changed his mind.

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.

[4] 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.

Their results showed that within Edmontosaurus regalis, many features previously used to classify additional species were directly correlated with skull size.

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

The Campanian species Thespesius edmontoni, previously considered a synonym of E. annectens because of its small size and skull shape, is more likely a subadult specimen of E.

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

[36] Because of its wide distribution, which covers a distance from Alaska to Colorado and includes polar settings that would have had little light during a significant part of the year, Edmontosaurus regalis has been considered possibly migratory.

A 2008 review of dinosaur migration studies by Phil R. Bell and Eric Snively proposed that E. regalis was capable of an annual 2,600 kilometres (1,600 mi) round-trip journey, provided it had the requisite metabolism and fat deposition rates.

The possible migratory nature of E. regalis contrasts with many other dinosaurs, such as theropods, sauropods, and ankylosaurians, which Bell and Snively found were more likely to have overwintered.

[37][38] In contrast to Bell and Snively, Anusuya Chinsamy and colleagues concluded from a study of bone microstructure that polar edmontosaurs overwintered.

[40] The Horseshoe Canyon Formation is interpreted as having a significant marine influence, due to the encroaching Western Interior Seaway.

This shallow sea stretched from the Gulf of Mexico to the Arctic Ocean, covering the midsection of North America through much of the Cretaceous.

[41] E. regalis shared this setting with fellow hadrosaurids Hypacrosaurus and Saurolophus, the parksosaurid Parksosaurus, the ceratopsids Montanoceratops, Anchiceratops, Arrhinoceratops, and Pachyrhinosaurus, the pachycephalosaurid Stegoceras, the ankylosaurid Euoplocephalus, the nodosaurid Edmontonia, the ornithomimids Ornithomimus and Struthiomimus, a variety of poorly known small theropods that included troodontids and dromaeosaurids, and the tyrannosaurid Albertosaurus.

[43] The typical edmontosaur habitat of this formation has been described as the back regions of bald cypress swamps and peat bogs on delta coasts.