| Hylaeosaurus | |
|---|---|
 | |
| Holotype NHMUK PV OR 3775 | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | †Ornithischia |
| Clade: | †Thyreophora |
| Clade: | †Ankylosauria |
| Clade: | †Euankylosauria |
| Family: | †Nodosauridae |
| Genus: | †Hylaeosaurus Mantell,1833 |
| Species: | †H. armatus |
| Binomial name | |
| †Hylaeosaurus armatus Mantell, 1833 | |
| Synonyms | |
| |
Hylaeosaurus (/haɪˌliːoʊˈsɔːrəs/hy-LEE-o-SOR-əs) is a herbivorousankylosauriandinosaur that lived about 136 million years ago, in the lateValanginianstage of the earlyCretaceousperiod ofEngland. It was found in theGrinstead Clay.
Hylaeosaurus was one of the first dinosaurs to be discovered, in 1832 byGideon Mantell. In 1842 it was one of the three dinosaursRichard Owen based theDinosauria on, the others beingIguanodon andMegalosaurus. Fourspecies were named in thegenus, but only thetype speciesHylaeosaurus armatus is today considered valid. Only limited remains have been found ofHylaeosaurus and much of its anatomy is unknown. It might have been a basalnodosaurid, although a recent cladistic analysis recovers it as a basalankylosaurid.[1]
Hylaeosaurus was about five metres long. It was an armoured dinosaur that carried at least three long spines on its shoulder.
The firstHylaeosaurusfossils were discovered in theGrinstead Clay,West Sussex. On 20 July 1832, fossil collectorGideon Mantell wrote to ProfessorBenjamin Silliman that when a gunpowder explosion had demolished a quarry rock face inTilgate 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 ofMegalosaurus andIguanodon 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 byWilliam Clift, the curator of theRoyal College of Surgeons of England museum, and his assistantJohn 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.[2] 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 nameHylaeosaurus meant "forest lizard", after the Tilgate Forest in which it was discovered.[3] Later, he claimed that it meant "Wealden lizard" ("wealden" being another word forforest), in reference to theWealden Group, the name for the early Cretaceousgeological formation in which the dinosaur was first found.[4]
On 30 November Mantell sent the piece to theGeological 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 metRichard 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 friendCharles Lyell, Mantell decided instead of rewriting the paper, to publish an entire book on his fossil finds and dedicate a chapter toHylaeosaurus. Within three weeks Mantell composed the volume from earlier notes. On 17 DecemberHenry 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: toHylaeosaurus aspecific name needed to be added.[2] Mantell on 19 December chosearmatus,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 hisThe Geology of the South-East of England appeared, hereby validly naming thetype speciesHylaeosaurus armatus. Mantell published alithograph of his find inThe Geology of the South-East of England;[5] and another drawing in the fourth edition ofThe Wonders of Geology, in 1840.
Hylaeosaurus is the most obscure of the three animals used by Sir Richard Owen to first define the new groupDinosauria, in 1842, the other genera beingMegalosaurus andIguanodon. Not only hasHylaeosaurus received less public attention, despite being included in the life-sized models byBenjamin Waterhouse Hawkins placed in theCrystal 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 weregastralia or belly-ribs.
The original specimen, recovered by Gideon Mantell from theTilgate Forest, was in 1838 acquired by theNatural History Museum ofLondon. It has the inventory numberNHMUK PV OR 3775 (earlier BMNH 3775). It was found in a layer of theTunbridge Wells Sand Formation dating from theValanginian, about 137 million years old. Thisholotype 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.[6] 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.[7]
Several finds from the mainland of Britain have been referred toHylaeosaurus armatus.[8][9] However, in 2011Paul Barrett andSusannah Maidment concluded that only the holotype could with certainty be associated with the species, in view of the presence ofPolacanthus in the same layers.[10]
Additional remains have been referred toHylaeosaurus, from theIsle of Wight, (theArdennes of)France,[11]Bückeberg Formation,Germany,[12]Spain[13] andRomania.[14] The remains from France may actually belong toPolacanthus and the other references are today also considered dubious.[6][15] However, possible remains were reported from Germany in 2013: a spike, specimen DLM 537 and the lower end of a humerus, specimen GPMM A3D.3, which were referred to aHylaeosaurus sp.[16]
Hylaeosaurus armatus Mantell 1833 is currently considered the only validspecies in thegenus. However, three others have been named. In 1844, Mantell namedHylaeosaurus oweni based on the same specimen asH. armatus, wanting to honour Richard Owen.[17] This has been sunk as ajunior objective synonym ofH. armatus.[8] In 1956Alfred Romer renamedRegnosaurus intoHylaeosaurus northhamptoni.[18]Polacanthus Owen 1865 was byWalter Coombs in 1971 renamed intoHylaeosaurus foxii.[19] These last two names have found no acceptance;H. foxii remained an invalidnomen ex dissertatione. It has also been suggested thatPolacanthus is simply the same species asHylaeosaurus armatus and thus a junior synonym, but there are a number of differences in theirosteology.
Sometimes bones from theHylaeosaurus material have later been made separate species. In 1928Franz Nopcsa made specimen NHMUK PV OR 2584, a left scapula referred by Mantell toH. armatus,[20] part of the type material ofPolacanthoides ponderosus.[21] Though in 1978 synonymised withHylaeosaurus,[22]Polacanthoides is today considered anomen dubium,[23] an indeterminate member of theThyreophora.
Gideon Mantell originally estimated thatHylaeosaurus was about 7.6 metres (25 ft) long, or about half the size of the other two original dinosaurs,Iguanodon andMegalosaurus. At the time, he modelled the animal after a modern lizard. Modern estimates range up to 6 metres (20 ft) in length.[24]Gregory S. Paul in 2010 estimated the length at 5 metres (16 ft), the weight at 2 tonnes (2.0 long tons; 2.2 short tons).[25] Some estimates are considerably lower: in 2001Darren Naish e.a. gave a length of 3–4 metres (9.8–13.1 ft).[15]
Many details about the build ofHylaeosaurus are unknown, especially if the material is strictly limited to the holotype. Maidment gave twoautapomorphies, 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.[10] Even these traits are not very distinctive: Mantell and Owen had attributed the lack of fusion toontogeny and the total number of spines cannot be observed.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 abeak used to crop low-lying vegetation.
In 2001 the skull and lower jaws remains were described byKenneth Carpenter. The damaged and shifted skull elements provided little information. Thequadrate is laterally bowed. Thequadratojugal has a high attachment point on the shaft of the quadrate. A triangularpostorbital horn was present.[26] In 2020, it was concluded that the presumed quadrate was in fact the jugal.
Several distinguishing traits were established in 2020. On the shoulder blade, there is a sharp angle of 120° between the acromion and the proximal plate. 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. The top edge of the proximal plate is curved sideways. The sides of the centra of the neck vertebrae show a horizontal ridge. Apart from these autapomorphies, the undersides of the side processes of the back vertebrae are exceptionally concave.
The spines at the shoulder are curved to the rear, long, flattened, narrow and pointed. Their underside shows a shallow trough. The front spine is the longest at 42.5 centimetres; to the rear the spines become gradually shorter and wider. A fourth spine, of about the same build but more forward-pointing, is present immediately behind the skull. In 2013Sven Sachs andJahn Hornung suggested a configuration in which there were five lateral neck spines, the new German spine having a morphology adapted to fit in the third position.[16]
Hylaeosaurus was the firstankylosaur discovered. Until well into the twentieth century its exact affinities would remain uncertain. In 1978 Coombs assigned it to theNodosauridae within theAnkylosauria.[22] This is still a usual classification,Hylaeosaurus being recovered as a basal nodosaurid in most exactcladistic analyses, sometimes more precisely as a member of thePolacanthinae, and thus being related toGastonia andPolacanthus. However, in the 1990s, the polacanthines were sometimes seen as basalankylosaurids, because they were mistakenly believed to have small tail-clubs. A more popular alternative today is that they formed aPolacanthidae, a basal group outside of the nodosaurids + ankylosauridsclade.
A 2012 study findingHylaeosaurus to be a basal nodosaurid but not a polacanthine is shown in thiscladogram:[27]
It was defined as a non-ankylosaurineankylosaurid in a phylogenetic analysis by Zhenget al. (2018).[28][29] Below is a simplified cladogram from that study:[28]
In one of two phylogenetic analyses by Xinget al. (2024)Hylaeosaurus is resolved as a non-euankylosaur ankylosaur. Below is a simplified cladogram from that study. Zhenget al. (2018) dataset +Datai (14-taxon deletion):[29]
