Edmontosaurus has had a very long and convoluted fossil history, with its species at many points overlapping with those of many questionable or now-invalid creatures such as Agathaumas, Anatosaurus, Anatotitan, Claosaurus, Diclonius, Hadrosaurus, Thespesius, and Trachodon. It has also been recently discovered to have had a crest-like mass of soft tissue on the top of its head.