Abstract

Iguanodontian dinosaurs are known from Middle Jurassic to Late Cretaceous deposits worldwide and are historically important because they include the first fossils to be identified as giant extinct reptiles that later became known as dinosaurs. Due to historical taxonomic practices and the fragmentary nature of many specimens, discoveries from the 19th century were referred to Iguanodon with little appraisal, resulting in the genus becoming something of a ‘wastebasket taxon’. Reinterpretations of holotype specimens are an important step in attempting to understand the evolutionary history of iguanodontian dinosaurs. Here, we redescribe the holotype of Mantellisaurus atherfieldensis, one of the most complete British dinosaur specimens, from the Barremian/Aptian of the Isle of Wight, UK, and assess its phylogenetic position. We find that Mantellisaurus atherfieldensis is a valid taxon distinct from the genus Iguanodon based on three autapomorphies of the premaxilla, maxilla, and scapula, and resolves as an early diverging hadrosauroid styracosternan, representing an important step in the evolutionary history of the ornithopods.