E. annectens was a saurolophine or "flat-headed" hadrosaurid; this group was historically known as Hadrosaurinae.
Species now considered to be synonyms of Edmontosaurus annectens were long recognized as closely related to both the genus and the species.
However, the skull of the sub-adult type specimen of E. annectens differs noticeably from fully mature remains, so many researchers had classified the two growth stages as different species or even genera.
On the other side of the issue, other authors, from John Bell Hatcher in 1902, to Jack Horner, David B. Weishampel, and Catherine Forster in 2004, and most recently Nicolás Campione and David Evans, have proposed that the large, flat-headed specimens most recently classified as Anatotitan copei belong to E. annectens.

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. Brett-Surman.
With the discovery that A. copei and E. annectens likely represent the same species, some paleontologists have proposed using Anatosaurus as a valid genus name for E. annectens.

The cladogram below follows Godefroit et al. (
2012) analysis.
E. annectens was a saurolophine or "flat-headed" hadrosaurid; this group was historically known as Hadrosaurinae. Species now considered to be synonyms of Edmontosaurus annectens were long recognized as closely related to both the genus and the species. However, the skull of the sub-adult type specimen of E. annectens differs noticeably from fully mature remains, so many researchers had classified the two growth stages as different species or even genera. On the other side of the issue, other authors, from John Bell Hatcher in 1902, to Jack Horner, David B. Weishampel, and Catherine Forster in 2004, and most recently Nicolás Campione and David Evans, have proposed that the large, flat-headed specimens most recently classified as Anatotitan copei belong to E. annectens.
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. Brett-Surman. With the discovery that A. copei and E. annectens likely represent the same species, some paleontologists have proposed using Anatosaurus as a valid genus name for E. annectens.
The cladogram below follows Godefroit et al. (2012) analysis.
Edmontosaurus annectens