True E. annectens remains are known only from latest Maastrichtian-age rocks of the Hell Creek and Lance Formations from South Dakota, Montana, and Wyoming, USA and the Frenchman Formation of Saskatchewan, Canada.

The Lancian time interval was the last interval before the Cretaceous–Paleogene extinction event that eliminated non-avian dinosaurs.
Edmontosaurus was one of the more common dinosaurs of the interval.
Robert Bakker reports that it made up one-seventh of the large dinosaur sample, with most of the rest (five-sixths) made up of the horned dinosaur Triceratops.
The coastal plain Triceratops–Edmontosaurus association, dominated by Triceratops, extended from Colorado to Saskatchewan.
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.

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 that supported a variety of plants ranging from angiosperm trees, to conifers such as bald cypress, to ferns and ginkgos.
The coastline was hundreds of kilometers or miles to the east.
Stream-dwelling turtles and tree-dwelling multituberculate mammals were diverse, and monitor lizards as large as the modern Komodo dragon hunted on the ground.
Triceratops was the most abundant large dinosaur, and Thescelosaurus the most abundant small herbivorous dinosaur.
Edmontosaur remains have been collected here from stream channel sands, and include fossils from individuals as young as a meter- or yard-long infant.
The edmontosaur fossils probably represent accumulations from groups on the move.

The Lance Formation, as typified by exposures approximately 100 kilometers (62 mi) north of Fort Laramie in eastern Wyoming, has been interpreted as a bayou setting similar to the Louisiana coastal plain.
It was closer to a large delta than the Hell Creek Formation depositional setting to the north and received much more sediment.
Tropical araucarian conifers and palm trees dotted the hardwood forests, differentiating the flora from the northern coastal plain.
The climate was humid and subtropical, with conifers, palmettos, and ferns in the swamps, and conifers, ash, live oak, and shrubs in the forests.
Freshwater fish, salamanders, turtles, diverse lizards, snakes, shorebirds, and small mammals lived alongside the dinosaurs.
Small dinosaurs are not known in as great of abundance here as in the Hell Creek rocks, but Thescelosaurus once again seems to have been relatively common.
Triceratops is known from many skulls, which tend to be somewhat smaller than those of more northern individuals.
The Lance Formation is the setting of two edmontosaur "mummies".
True E. annectens remains are known only from latest Maastrichtian-age rocks of the Hell Creek and Lance Formations from South Dakota, Montana, and Wyoming, USA and the Frenchman Formation of Saskatchewan, Canada.
The Lancian time interval was the last interval before the Cretaceous–Paleogene extinction event that eliminated non-avian dinosaurs. Edmontosaurus was one of the more common dinosaurs of the interval. Robert Bakker reports that it made up one-seventh of the large dinosaur sample, with most of the rest (five-sixths) made up of the horned dinosaur Triceratops. The coastal plain Triceratops–Edmontosaurus association, dominated by Triceratops, extended from Colorado to Saskatchewan. 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.
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 that supported a variety of plants ranging from angiosperm trees, to conifers such as bald cypress, to ferns and ginkgos. The coastline was hundreds of kilometers or miles to the east. Stream-dwelling turtles and tree-dwelling multituberculate mammals were diverse, and monitor lizards as large as the modern Komodo dragon hunted on the ground. Triceratops was the most abundant large dinosaur, and Thescelosaurus the most abundant small herbivorous dinosaur. Edmontosaur remains have been collected here from stream channel sands, and include fossils from individuals as young as a meter- or yard-long infant. The edmontosaur fossils probably represent accumulations from groups on the move.
The Lance Formation, as typified by exposures approximately 100 kilometers (62 mi) north of Fort Laramie in eastern Wyoming, has been interpreted as a bayou setting similar to the Louisiana coastal plain. It was closer to a large delta than the Hell Creek Formation depositional setting to the north and received much more sediment. Tropical araucarian conifers and palm trees dotted the hardwood forests, differentiating the flora from the northern coastal plain. The climate was humid and subtropical, with conifers, palmettos, and ferns in the swamps, and conifers, ash, live oak, and shrubs in the forests. Freshwater fish, salamanders, turtles, diverse lizards, snakes, shorebirds, and small mammals lived alongside the dinosaurs. Small dinosaurs are not known in as great of abundance here as in the Hell Creek rocks, but Thescelosaurus once again seems to have been relatively common. Triceratops is known from many skulls, which tend to be somewhat smaller than those of more northern individuals. The Lance Formation is the setting of two edmontosaur "mummies".
Edmontosaurus annectens