British Wildlife Wiki

Hylaeosaurus (hy-LEE-o-SAWR-rəs; Greek: hylē/υλη "forest" and saurus/σαυρος "lizard") is the most obscure of the three animals used by Sir Richard Owen to first define the new group Dinosauria, in 1842. The original specimen, recovered by Gideon Mantell from the Tilgate Forest in the south of England 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.

Description and environment[]

Hylaeosaurus lived about 136 million years ago, in the late Valanginian age of the early Cretaceous period. Gideon Mantell originally estimated that the Hylaeosaurus was about 25 feet (7.6 m) long, or about half the size of the other two original dinosaurs, the Iguanodon and the Megalosaurus. Modern estimates range up to 6 metres (20 ft) in length.

It was a fairly typical nodosaur, 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 had a long head, more like the head of a Nodosaurus than an Ankylosaurus, and a beak, which it probably used to crop low-lying vegetation.

History of discovery[]

The first Hylaeosaurus fossils were discovered in Sussex. Additional remains have been discovered on the Isle of Wight (also part of Great Britain), and in Ardennes, France, though the remains 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 drawing in the fourth edition of The Wonders of Geology, in 1840.

Gideon Mantell originally 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 reference to the Wealden Group, the name for the early Cretaceous geological formation in which the dinosaur was first found.

In any case, the species epithet armatus, or "armored", was chosen by Mantell because there

"appears every reason to conclude that either its back was armed with a formidable row of spines, constituting a dermal fringe, or that its tail possessed the same appendage".


Hylaeosaurus armatus was first named by Gideon Mantell in 1833, and is currently considered the only species in the genus. It is known from only two partial skeletons, some horny (dermal) spines, armor, and various other minor pieces. The best specimen (the original) is composed 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 considered distinct dinosaurs in the past, but are now considered to be alternate names for this species (junior synonyms), along with Hylaeosaurus. It has also been suggested that Polacanthus is same species, but there are a number of differences in their bone structure (osteology).

Hylaeosaurus is traditionally considered to be a primitive nodosaurid, in the Polocanthinae subfamily, like Gastonia and Polocanthus. In the 1990s, the polocanthines were reclassified as primitive ankylosaurids, because they were mistakenly believed to have small tail-clubs; they are probably primitive ankylosaurids, but as a whole the polacanthines are poorly known. The group peaked in the Barremian age in North America and Europe, and then vanished shortly after, replaced by more advanced ankylosaurians.