This confusing situation was temporarily resolved in 1942 by Richard Swann Lull and Nelda Wright.
In their monograph on hadrosaurian dinosaurs of North America, they opted to settle the questions revolving around the American Museum duckbills, Marsh's Claosaurus annectens, and several other species by creating a new generic name.
They created the new genus Anatosaurus ("duck lizard", because of its wide, duck-like beak; Latin anas = duck + Greek sauros = lizard) and made Marsh's species the type species, calling it Anatosaurus annectens.
 To this genus, they also assigned Marsh's Trachodon longiceps, a pair of species that had been assigned to Thespesius under Gilmore's "Lance Formation hadrosaurid" conception (T. edmontoni from Gilmore in 1924 and T. saskatchewanensis), and Cope's Diclonius mirabilis.
Lull and Wright decided to remove the American Museum specimens from Diclonius (or Trachodon) because they found no convincing reason to assign the specimens to either.
Because this left the skeletons without a species name, Lull and Wright gave them their own species, Anatosaurus copei, in honor of Cope.
Cope's original specimen (AMNH 5730) was made the holotype of the species, with Brown's (AMNH 5886) as the plesiotype.
Anatosaurus would come to be called the "classic duck-billed dinosaur."

This state of affairs persisted for several decades, until Michael K. Brett-Surman reexamined the pertinent material for his graduate studies in the 1970s and 1980s.
He concluded that the type species of Anatosaurus, A. annectens, was actually a species of Edmontosaurus, and that A. copei was different enough to warrant its own genus.
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.
His replacement name, Anatotitan (Latin anas ("duck") and the Greek Titan, meaning large), was known and published as such in the popular literature by 1990.
Formal publication of the name Anatotitan copei took place the same year, in an article co-written by Brett-Surman with Ralph Chapman (although the name is sometimes credited as Brett-Surman vide Chapman and Brett-Surman because it came out of Brett-Surman's work).
Because the type species of Anatosaurus (A. annectens) was sunk into Edmontosaurus, the name Anatosaurus is abandoned as a junior synonym of Edmontosaurus.

Of the remaining species of Anatosaurus, A. saskatchewanensis and A. edmontoni were assigned to Edmontosaurus as well,  and A. longiceps went to Anatotitan, as either a second species or as a synonym of A. copei.
A. longiceps may be a synonym of E. annectens, though it has also been treated as a nomen dubium by some.

The conception of Edmontosaurus that emerged included three valid species: the type E. regalis, E. annectens (including Anatosaurus edmontoni, emended to edmontonensis), and E. saskatchewanensis.
The debate about the proper taxonomy of the A. copei specimens continues to the present: 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.
In 2007 another "mummy" was announced; nicknamed "Dakota", it was discovered in 1999 by Tyler Lyson, and came from the Hell Creek Formation of North Dakota.

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.
They concluded that only two species are valid: E. regalis, from the late Campanian, and E. annectens, from the late Maastrichtian.
Their study provided further evidence that Anatotitan copei is a synonym of E. annectens; specifically, that the long, low skull of A. copei is the result of ontogenetic change and represents mature E. annectens individuals.
E. saskatechwanensis represents young E. annectens, and Anatosaurus edmontoni specimens belong to E. re
This confusing situation was temporarily resolved in 1942 by Richard Swann Lull and Nelda Wright. In their monograph on hadrosaurian dinosaurs of North America, they opted to settle the questions revolving around the American Museum duckbills, Marsh's Claosaurus annectens, and several other species by creating a new generic name. They created the new genus Anatosaurus ("duck lizard", because of its wide, duck-like beak; Latin anas = duck + Greek sauros = lizard) and made Marsh's species the type species, calling it Anatosaurus annectens.  To this genus, they also assigned Marsh's Trachodon longiceps, a pair of species that had been assigned to Thespesius under Gilmore's "Lance Formation hadrosaurid" conception (T. edmontoni from Gilmore in 1924 and T. saskatchewanensis), and Cope's Diclonius mirabilis. Lull and Wright decided to remove the American Museum specimens from Diclonius (or Trachodon) because they found no convincing reason to assign the specimens to either. Because this left the skeletons without a species name, Lull and Wright gave them their own species, Anatosaurus copei, in honor of Cope. Cope's original specimen (AMNH 5730) was made the holotype of the species, with Brown's (AMNH 5886) as the plesiotype. Anatosaurus would come to be called the "classic duck-billed dinosaur."
This state of affairs persisted for several decades, until Michael K. Brett-Surman reexamined the pertinent material for his graduate studies in the 1970s and 1980s. He concluded that the type species of Anatosaurus, A. annectens, was actually a species of Edmontosaurus, and that A. copei was different enough to warrant its own genus. 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. His replacement name, Anatotitan (Latin anas ("duck") and the Greek Titan, meaning large), was known and published as such in the popular literature by 1990. Formal publication of the name Anatotitan copei took place the same year, in an article co-written by Brett-Surman with Ralph Chapman (although the name is sometimes credited as Brett-Surman vide Chapman and Brett-Surman because it came out of Brett-Surman's work). Because the type species of Anatosaurus (A. annectens) was sunk into Edmontosaurus, the name Anatosaurus is abandoned as a junior synonym of Edmontosaurus.
Of the remaining species of Anatosaurus, A. saskatchewanensis and A. edmontoni were assigned to Edmontosaurus as well,  and A. longiceps went to Anatotitan, as either a second species or as a synonym of A. copei. A. longiceps may be a synonym of E. annectens, though it has also been treated as a nomen dubium by some.
The conception of Edmontosaurus that emerged included three valid species: the type E. regalis, E. annectens (including Anatosaurus edmontoni, emended to edmontonensis), and E. saskatchewanensis. The debate about the proper taxonomy of the A. copei specimens continues to the present: 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. In 2007 another "mummy" was announced; nicknamed "Dakota", it was discovered in 1999 by Tyler Lyson, and came from the Hell Creek Formation of North Dakota.
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. They concluded that only two species are valid: E. regalis, from the late Campanian, and E. annectens, from the late Maastrichtian. Their study provided further evidence that Anatotitan copei is a synonym of E. annectens; specifically, that the long, low skull of A. copei is the result of ontogenetic change and represents mature E. annectens individuals. E. saskatechwanensis represents young E. annectens, and Anatosaurus edmontoni specimens belong to E. re
Edmontosaurus annectens