Hylaeosaurus (/haɪˌliːoʊˈsɔːrəs/ hy-LEE-o-SOR-əs; Greek: hylaios/ὑλαῖος "belonging to the forest" and sauros/σαυρος "lizard") is a herbivorous ankylosaurian dinosaur that lived about 136 million years ago, in the late Valanginian stage of the early Cretaceous period of England.

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.

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.

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.

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.

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".

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.

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 holotype consists of the rear of the skull and perhaps lower jaws, ten vertebrae, both scapulae, both coracoids and several spikes and armour plates.

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.

[10] Additional remains have been referred to Hylaeosaurus, from the Isle of Wight, (the Ardennes of) France,[11] Bückeberg Formation, Germany,[12] Spain[13] and Romania.

In 1928 Franz Nopcsa made specimen BMNH 2584, a left scapula referred by Mantell to H. armatus,[20] part of the type material of Polacanthoides ponderosus.

Maidment gave two autapomorphies, unique derived traits: the scapula did not fuse with the coracoid, even when the animal was of a considerable size; there were three long spines on its shoulder.

Hylaeosaurus is often styled as a fairly typical nodosaur, with rows of armour plating on the back and tail combined with a relatively long head, equipped with a beak used to crop low-lying vegetation.

The acromial process is shelf-shaped instead of thumb-like or folded, from a point positioned at a third from the top edge projecting obliquely to below and sideways instead of strictly laterally.

[22] This is still a usual classification, Hylaeosaurus being recovered as a basal nodosaurid in most exact cladistic analyses, sometimes more precisely as a member of the Polacanthinae, and thus being related to Gastonia and Polacanthus.

A 2012 study finding Hylaeosaurus to be a basal nodosaurid but not a polacanthine is shown in this cladogram:[27] Antarctopelta Mymoorapelta Hylaeosaurus Anoplosaurus Tatankacephalus Horshamosaurus Gargoyleosaurus Hoplitosaurus Gastonia Peloroplites Polacanthus Struthiosaurus Zhejiangosaurus Hungarosaurus Animantarx Niobrarasaurus Nodosaurus Pawpawsaurus Sauropelta Silvisaurus Stegopelta Texasetes Edmontonia Panoplosaurus It was defined as a non-ankylosaurine ankylosaurid in a phylogenetic analysis by Zheng et al.

[28][29] Below is a simplified cladogram from that study:[28] Kunbarrasaurus Hylaeosaurus Liaoningosaurus Chuanqilong Cedarpelta Shamosaurus Gobisaurus Jinyunpelta Crichtonpelta Pinacosaurus spp.

Saichania Zaraapelta Tarchia Tsagantegia Talarurus Nodocephalosaurus Ziapelta Euoplocephalus Ankylosaurus Anodontosaurus Scolosaurus Zuul Dyoplosaurus In one of two phylogenetic analyses by Xing et al. (2024) Hylaeosaurus is resolved as a non-euankylosaur ankylosaur.