Hylaeosaurus is the most difficult to understand of the three animals used by Sir Richard Owen to first define the new group Dinosauria, in 1842. The original specimen, improved by Gideon Mantell in 1832 from the Tilgate Forest in the south of England in 1832, now resides in the Natural History Museum of London, where it is still covered in the limestone block in which it was found. Despite never having been prepared, it is still the best specimen that exists of this prehistoric, armored ankylosaurian.