Hylaeosaurus ( hy-LEE-o-SOR-əs) is a herbivorous ankylosaurian dinosaur that lived about 136 million years ago, in the late Valanginian stage of the early Cretaceous period of England. It was found in the Grinstead Clay. Hylaeosaurus was one of the first dinosaurs to be discovered, in 1832 by Gideon Mantell. In 1842 it was one of the three dinosaurs Richard Owen based the Dinosauria on, the others being Iguanodon and Megalosaurus. Four species were named in the genus, but only the type species Hylaeosaurus armatus is today considered valid. Only limited remains have been found of Hylaeosaurus and much of its anatomy is unknown. It might have been a basal nodosaurid, although a recent cladistic analysis recovers it as a basal ankylosaurid. Hylaeosaurus was about five metres long. It was an armoured dinosaur that carried at least three long spines on its shoulder.
History of discovery
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. Mantell on 19 December chose armatus, Latin for "armed" or "armoured", in reference to the spikes and armour plates. As Mantell himself put it: "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". In May 1833 his The Geology of the South-East of England appeared, hereby validly naming the type species Hylaeosaurus armatus. Mantell published a lithograph of his find in The Geology of the South-East of England; 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.
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