Mantellisaurus is a genus of dinosaur formerly known as Iguanodon atherfieldensis. The new genus was erected by Gregory Paul in 2007. According to Paul, it is more lightly built than Iguanodon and more closely related to Ouranosaurus, making Iguanodon in its traditional sense paraphyletic. It is known from many complete and almost complete skeletons. The genus name honours Gideon Mantell, the discoverer of Iguanodon. Mantellisaurus lived during the Early Cretaceous in what is now England.
Compared to Iguanodon bernissartensis, Mantellisaurus was smaller, estimated at 0.75 tons in weight. Its forelimbs were proportionally shorter than those of I. bernissartensis. In Mantellisaurus the forelimbs were about half the length of the hindlimbs whereas they were about 70 percent the length of the hindlimbs in I. bernissartensis. Due to the short length of its forelimbs and the shortness of its body, Paul proposed that it was primarily bipedal, only going on all fours when standing still or moving slowly.
The type fossil was originally discovered by Reginald Walter Hooley in 1914 in the upper Vectis Formation of southern England and reported upon in 1917. He posthumously named it Iguanodon atherfieldensis in 1925. Atherfield is the name of a village on the southwest shore of the Isle of Wight where the fossil was found. Heterosaurus, Sphenospondylus, and Vectisaurus may or may not be synonyms of Mantellisaurus, because Paul (2007) did not address the status of each of these names. Each of these taxa were included as synonyms of I. atherfieldensis (as it was then known) in the 2004 review of Iguanodontia in The Dinosauria (2nd edition). Other possible synonyms include Dollodon and Proplanicoxa.