The history of E. annectens predates both Edmontosaurus and annectens.
The first good specimen, the former holotype (specimen on which a taxonomic name is based) 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 for American paleontologist Edward Drinker Cope.
This specimen, found in Hell Creek Formation rocks, came from northeast of the Black Hills of South Dakota and originally had extensive skin impressions.
It was missing most of the pelvis and part of the torso due to a stream cutting through it.
The bill had impressions of a horny sheath with a tooth-like series of interlocking points on the upper and lower jaws.
When describing this specimen AMNH 5730, Cope assigned it to the species Diclonius mirabilis.
This species name was created by combining Diclonius, a hadrosaurid genus Cope had named earlier from teeth, with Trachodon mirabilis, an older name based on teeth and published by Joseph Leidy.
Cope believed that Leidy had failed to properly characterize the genus Trachodon and later abandoned its use, so he assigned the old species to his newer genus.
Leidy had come to recognize that his Trachodon was based on the remains of multiple kinds of dinosaurs, and though he had made some attempts to revise the genus, he had not yet made any formal declaration of his intentions.

Cope's description promoted hadrosaurids as amphibious, contributing to this long-time image.
His reasoning was that the teeth of the lower jaw were weakly connected to the bone and liable to break off if used to consume terrestrial food, and he described the beak as weak as well.
However, aside from misidentifying several of the skull bones, by chance the lower jaws were missing the walls supporting the teeth from the inside; the teeth were actually well-supported.
Cope intended to describe the skeleton as well as the skull, but his promised paper never appeared.
It was purchased for the American Museum of Natural History in 1899, where it acquired its present designation AMNH 5730.

Several years after Cope's description, his rival Othniel Charles Marsh published on a sizable lower jaw recovered by John Bell Hatcher in 1889 from Lance Formation rocks in Niobrara County, Wyoming.
Marsh named this partial jaw Trachodon longiceps.
It is cataloged as YPM 616.
As noted by Lull and Wright, this long slender partial jaw shares with Cope's specimen a prominent ridge running on its side.
However, it is much larger: Cope's specimen had a dentary, or tooth-bearing bone of the lower jaw, that is 92.0 centimetres (36.2 in) long, whereas Marsh's dentary is estimated at 110.0 centimetres (43.3 in) long.

A second mostly complete skeleton (AMNH 5886) was found in 1904 in Hell Creek Formation rocks at Crooked Creek in central Montana by Oscar Hunter, a rancher.
Upon finding the partially exposed specimen, he and a companion argued about whether or not the remains were recent or fossil.
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.
This specimen had a nearly complete vertebral column, permitting the restoration of Cope's specimen.
In 1908, these two specimens were mounted side-by-side in the American Museum of Natural History, under the name Trachodon mirabilis.
Cope's specimen is positioned on all fours with its head down, as if feeding, because it has the better skull, while Brown's specimen, with a less perfect skull, is posed bipedally with the head less accessible.
Henry Fairfield Osborn described the tableau as representing the two animals feeding along a marsh, the standing individual having been startled by the approach of a Tyrannosaurus.
Impressions of appropriate plant remains and shells based on associated fossils were included on the base of the group, including ginkgo leaves, Sequoia c
The history of E. annectens predates both Edmontosaurus and annectens. The first good specimen, the former holotype (specimen on which a taxonomic name is based) 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 for American paleontologist Edward Drinker Cope. This specimen, found in Hell Creek Formation rocks, came from northeast of the Black Hills of South Dakota and originally had extensive skin impressions. It was missing most of the pelvis and part of the torso due to a stream cutting through it. The bill had impressions of a horny sheath with a tooth-like series of interlocking points on the upper and lower jaws. When describing this specimen AMNH 5730, Cope assigned it to the species Diclonius mirabilis. This species name was created by combining Diclonius, a hadrosaurid genus Cope had named earlier from teeth, with Trachodon mirabilis, an older name based on teeth and published by Joseph Leidy. Cope believed that Leidy had failed to properly characterize the genus Trachodon and later abandoned its use, so he assigned the old species to his newer genus. Leidy had come to recognize that his Trachodon was based on the remains of multiple kinds of dinosaurs, and though he had made some attempts to revise the genus, he had not yet made any formal declaration of his intentions.
Cope's description promoted hadrosaurids as amphibious, contributing to this long-time image. His reasoning was that the teeth of the lower jaw were weakly connected to the bone and liable to break off if used to consume terrestrial food, and he described the beak as weak as well. However, aside from misidentifying several of the skull bones, by chance the lower jaws were missing the walls supporting the teeth from the inside; the teeth were actually well-supported. Cope intended to describe the skeleton as well as the skull, but his promised paper never appeared. It was purchased for the American Museum of Natural History in 1899, where it acquired its present designation AMNH 5730.
Several years after Cope's description, his rival Othniel Charles Marsh published on a sizable lower jaw recovered by John Bell Hatcher in 1889 from Lance Formation rocks in Niobrara County, Wyoming. Marsh named this partial jaw Trachodon longiceps. It is cataloged as YPM 616. As noted by Lull and Wright, this long slender partial jaw shares with Cope's specimen a prominent ridge running on its side. However, it is much larger: Cope's specimen had a dentary, or tooth-bearing bone of the lower jaw, that is 92.0 centimetres (36.2 in) long, whereas Marsh's dentary is estimated at 110.0 centimetres (43.3 in) long.
A second mostly complete skeleton (AMNH 5886) was found in 1904 in Hell Creek Formation rocks at Crooked Creek in central Montana by Oscar Hunter, a rancher. Upon finding the partially exposed specimen, he and a companion argued about whether or not the remains were recent or fossil. 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. This specimen had a nearly complete vertebral column, permitting the restoration of Cope's specimen. In 1908, these two specimens were mounted side-by-side in the American Museum of Natural History, under the name Trachodon mirabilis. Cope's specimen is positioned on all fours with its head down, as if feeding, because it has the better skull, while Brown's specimen, with a less perfect skull, is posed bipedally with the head less accessible. Henry Fairfield Osborn described the tableau as representing the two animals feeding along a marsh, the standing individual having been startled by the approach of a Tyrannosaurus. Impressions of appropriate plant remains and shells based on associated fossils were included on the base of the group, including ginkgo leaves, Sequoia c
Edmontosaurus annectens