Hylaeosaurus ( ; Greek: / "belonging to the forest" and / "lizard") is a herbivorous ankylosaurian dinosaur that lived about 136 million years ago, in the late Valanginian stage of the early Cretaceous period of England. It was found in the Grinstead Clay Formation. Hylaeosaurus was one of the first dinosaurs to be discovered, in 1832 by Gideon Mantell. In 1842 it was one of the three dinosaurs Richard Owen based the Dinosauria on, the others being Iguanodon and Megalosaurus. Four species were named in the genus, but only the type species Hylaeosaurus armatus is today considered valid. Only limited remains have been found of Hylaeosaurus and much of its anatomy is unknown. It might have been a basal nodosaurid, although a recent cladistic analysis recovers it as a basal ankylosaurid. Hylaeosaurus was about five metres long. It was an armoured dinosaur that carried at least three long spines on its shoulder. History of discovery The first Hylaeosaurus fossils were discovered in the Grinstead Clay Formation, West Sussex. On 20 July 1832, fossil collector Gideon Mantell wrote to Professor Benjamin Silliman that when a gunpowder explosion had demolished a quarry rock face in Tilgate Forest, several of the boulders freed showed the bones of a saurian. A local fossil dealer had assembled the about fifty pieces, described by him as a "great consarn of bites and boanes". Having doubts about the value of the fragments, Mantell had nevertheless purchased the pieces and soon discovered they could be united into a single skeleton, partially articulated. Mantell was delighted with the find because previous specimens of Megalosaurus and Iguanodon had consisted of single bone elements. The discovery in fact represented the most complete non-avian dinosaur skeleton known at the time. He was strongly inclined to describe the find as belonging to the latter genus, but during a visit by William Clift, the curator of the Royal College of Surgeons of England museum, and his assistant John Edward Gray, he began to doubt the identification. Clift was the first to point out that several plates and spikes were probably part of a body armour, attached to the back or sides of the rump. In November 1832 Mantell decided to create a new generic name: Hylaeosaurus. It is derived from the Greek ὑλαῖος, hylaios, "of the wood". Mantell originally claimed the name Hylaeosaurus meant "forest lizard", after the Tilgate Forest in which it was