Artwork of a pair of Edmontosaurus

Artwork of a male (left) and female duck-billed dinosaur, Edmontosaurus. Edmontosaurus was a hadrosaurid - in the same group as such animals as Corythosaurus and Parasaurolophus - that lived during the late Cretaceous period in what is now Canada. Originally known as Anatosaurus and then Anatotitan, experts now realise that these are all representations of a single genus, called Edmontosaurus. At up to 12 metres long, and possibly as long as 15 metres, this is the largest known hadrosaur. Unlike its cousing Corythosaurus and Parasaurolophus, Edmontosaurus lacked a bony crest on its skull. However, some specimens have been found with a soft-tissue crest on the head. It could be, as shown here, that this was an example of sexual dimorphism, where males and females have differences other than their sexual organs., by MARK GARLICK/SCIENCE PHOTO LIBRARY