The first
Hylaeosaurus fossils were discovered in the
Grinstead Clay,
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 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. On 30 November Mantell sent the piece to the
Geological Society of London. Shortly afterwards he himself went to London and on 5 December during a meeting of the Society, in which he for the first time personally met
Richard Owen, reported on the find to large acclaim. However, he was also informed that a paper he had already prepared, was a third too long. On advice of his friend
Charles Lyell, Mantell decided instead of rewriting the paper, to publish an entire book on his fossil finds and dedicate a chapter to
Hylaeosaurus. Within three weeks Mantell composed the volume from earlier notes. On 17 December
Henry De la Beche warned him that the changed conventions in nomenclature implied that only he who provided a full species name was recognised as the author: to
Hylaeosaurus a
specific name needed to be added. and another drawing in the fourth edition of
The Wonders of Geology, in 1840.
Hylaeosaurus is the most obscure of the three animals used by Sir Richard Owen to first define the new group
Dinosauria, in 1842, the other genera being
Megalosaurus and
Iguanodon. Not only has
Hylaeosaurus received less public attention, despite being included in the life-sized models by
Benjamin Waterhouse Hawkins placed in the
Crystal Palace Park, it also never functioned as a "wastebasket taxon". Owen in 1840 developed a new hypothesis about the spikes; noting they were asymmetrical he correctly rejected the notion they formed a row on the back but incorrectly assumed they were
gastralia or belly-ribs. The original specimen, recovered by Gideon Mantell from the
Tilgate Forest, was in 1838 acquired by the
Natural History Museum of
London. It has the inventory number
NHMUK PV OR 3775 (earlier BMNH 3775). It was found in a layer of the
Tunbridge Wells Sand Formation dating from the
Valanginian, about 137 million years old. This
holotype is the best specimen and is composed of the front end of a skeleton minus most of the head and the forelimbs, though only the parts on the face of the stone block are easily studied. The block measures about 135 by 75 centimetres. The holotype consists of the rear of the skull and perhaps lower jaws, ten vertebrae, both scapulae, both coracoids and several spikes and armour plates. The skeleton is viewed from below. For a long time no further preparation had taken place, beyond the assembly and chiselling out by Mantell himself, but in the early twenty-first century the museum began to further free the bones by both chemical and mechanical means. This has proven difficult because the acids used tended to dissolve the glue and gypsum with which the fossils had been repaired, causing the blocks to fall apart. The limited information gained by the preparations by Mark Graham since 2003, was published in 2020, together with a revised description. Several finds from the mainland of Britain have been referred to
Hylaeosaurus armatus. However, in 2011
Paul Barrett and
Susannah Maidment concluded that only the holotype could with certainty be associated with the species, in view of the presence of
Polacanthus in the same layers. Additional remains have been referred to
Hylaeosaurus, from the
Isle of Wight, (the
Ardennes of)
France,
Bückeberg Formation,
Germany,
Spain and
Romania. The remains from France may actually belong to
Polacanthus and the other references are today also considered dubious.
Later species Hylaeosaurus armatus Mantell 1833 is currently considered the only valid
species in the
genus. However, three others have been named. In 1844, Mantell named
Hylaeosaurus oweni based on the same specimen as
H. armatus, wanting to honour Richard Owen. This has been sunk as a
junior objective synonym of
H. armatus.
Polacanthus Owen 1865 was by
Walter Coombs in 1971 renamed into
Hylaeosaurus foxii. These last two names have found no acceptance;
H. foxii remained an invalid
nomen ex dissertatione. It has also been suggested that
Polacanthus is simply the same species as
Hylaeosaurus armatus and thus a junior synonym, but there are a number of differences in their
osteology. Sometimes bones from the
Hylaeosaurus material have later been made separate species. In 1928
Franz Nopcsa made specimen NHMUK PV OR 2584, a left scapula referred by Mantell to
H. armatus, part of the type material of
Polacanthoides ponderosus. Though in 1978 synonymised with
Hylaeosaurus,
Polacanthoides is today considered a
nomen dubium, an indeterminate member of the
Thyreophora. ==Description==