Hylaeosaurus (forest lizard) is the most obscure of the three animals used by Richard Owen to define Dinosauria, in 1842. The original specimen, recovered by Gideon Mantell in 1832, now resides in the Natural History Museum of London, where it is still encased in the limestone block in which it was found. Despite never having been prepared, it is still the best specimen that exists of this genus of primitive, armored ankylosaurian dinosaur. Hylaeosaurus was an armored, quadrupedal, plant-eating dinosaur from the early Cretaceous period, about 135-119 million years ago. It looked a lot like its more advanced relative Ankylosaurus, but didn't have the bony nodes at the end of its tail or horns on its head.