Hylaeosaurus (hi-LAY-ee-oh-SORE-us) 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.

Description and environment

Description and environment Hylaeosaurus lived about 135 million years ago, in the Valanginian to Berriasian ages of the near the beginning Cretaceous. Gideon Mantell originally estimated that the Hylaeosaurus was about 25 feet (7.5 meters) long, or about half the size of the other two original dinosaurs, the Iguanodon and the Megalosaurus. New estimates range from 3 to 6 meters (10 to 20 feet) in length.

It is a fairly typical unbreakable dinosaur, with three long spines on its shoulder, two at the hips, and three rows of armor running down its back. It may also have had a row of plates down its tail. It has a long head, extra like the head of a Nodosaurus than an Ankylosaurus; and a beak, which it probably used to crop low-lying vegetation.


Hylaeosaurus Dinosaur Hylaeosaurus armatus was first named by Gideon Mantell in 1833, and is at present considered the only species in the genera. It is known from only two partial skeletons, some horny (dermal) spines, armor, and a variety of other minor pieces. The best specimen (the original) is self-possessed of the front end of a skeleton minus most of the head, though only the parts on the face of the stone block are easily studied.

Polacanthoides ponderosus, Hylaeosaurus conybearei, and Hylaeosaurus oweni have all been measured distinct dinosaurs in the past, but are now measured to be alternate names for this species (junior synonyms), along with Hyaelosaurus. It has also been recommended that Polacanthus is same species, but there are a number of differences in their bone structure (osteology).

Hylaeosaurus is traditionally measured to be a primitive nodosaurid, in the Polcanthinae subfamily, like Gastonia and Polcanthus. In the 1990s, the polcanthines were reclassified as prehistoric ankylosaurids, because they were mistakenly believed to have small tail-clubs; they are almost certainly primitive ankylosaurids, but as a whole the polcanthines are poorly known. The group peaked in the Barremian age in North America and Europe, and then vanished soon after, replaced by more advanced ankylosaurians.


The first Hylaeosaurus fossils were exposed in Sussex. Additional remains have been discovered on the Isle of Wight (also part of Great Britain), and in Ardennes, France, though the leftovers from France may actually belong to a Polacanthus. Mantell published a lithograph of his find in The Geology of the South-east of England in 1833; and another sketch in the fourth edition of The Wonders of Geology, in 1840.

Gideon Mantell at first claimed the name Hylaeosaurus meant "forest lizard", after the Tilgate Forest in which it was discovered. Later, he claimed that it meant "Wealden lizard" ("wealden" being another word for forest), in orientation to the Wealden Group, the name for the early Cretaceous geological formation in which the dinosaur was first establish.

In any case, the species epithet armatus, or "armored", was selected by Mantell because there"Appears every reason to end that either its back was armed with a formidable row of spines, constituting a dermal fringe, or that its tail obsessed the same appendage".