Hylaeosaurus

Hylaeosaurus

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Hylaeosaurus is the most obscure of the three animals used by Sir Richard Owen
Richard Owen
Sir Richard Owen, FRS KCB was an English biologist, comparative anatomist and palaeontologist.Owen is probably best remembered today for coining the word Dinosauria and for his outspoken opposition to Charles Darwin's theory of evolution by natural selection...

 to first define the new group Dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...

ia, in 1842. The original specimen, recovered by Gideon Mantell
Gideon Mantell
Gideon Algernon Mantell MRCS FRS was an English obstetrician, geologist and palaeontologist...

 from the Tilgate
Tilgate, Crawley
Tilgate is a neighbourhood within the town of Crawley in West Sussex, England. The area contains a mixture of privately developed housing, self build groups and ex-council housing...

 Forest in the south of England
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...

 in 1832, now resides in the Natural History Museum
Natural History Museum
The Natural History Museum is one of three large museums on Exhibition Road, South Kensington, London, England . Its main frontage is on Cromwell Road...

 of London
London
London is the capital city of :England and the :United Kingdom, the largest metropolitan area in the United Kingdom, and the largest urban zone in the European Union by most measures. Located on the River Thames, London has been a major settlement for two millennia, its history going back to its...

, where it is still encased in the limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....

 block in which it was found. Despite never having been prepared, it is still the best specimen that exists of this genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 of primitive, armored
Armour (zoology)
Armour in animals is external or superficial protection against attack by predators, formed as part of the body , usually through the hardening of body tissues, outgrowths or secretions. It has therefore mostly developed in 'prey' species...

 ankylosauria
Ankylosauria
Ankylosauria is a group of herbivorous dinosaurs of the order Ornithischia. It includes the great majority of dinosaurs with armor in the form of bony osteoderms. Ankylosaurs were bulky quadrupeds, with short, powerful limbs. They are first known to have appeared in the early Jurassic Period of...

n dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...

.

Description and environment


Hylaeosaurus lived about 136 million years ago, in the late Valanginian
Valanginian
In the geologic timescale, the Valanginian is an age or stage of the Early or Lower Cretaceous. It spans between 140.2 ± 3.0 Ma and 136.4 ± 2.0 Ma...

 age
Faunal stage
In chronostratigraphy, a stage is a succession of rock strata laid down in a single age on the geologic timescale, which usually represents millions of years of deposition. A given stage of rock and the corresponding age of time will by convention have the same name, and the same boundaries.Rock...

 of the early Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

 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
Iguanodon
Iguanodon is a genus of ornithopod dinosaur that lived roughly halfway between the first of the swift bipedal hypsilophodontids and the ornithopods' culmination in the duck-billed dinosaurs...

and the Megalosaurus
Megalosaurus
Megalosaurus is a genus of large meat-eating theropod dinosaurs of the Middle Jurassic period of Europe...

. Modern estimates range up to 6 metres (19.7 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
Nodosaurus
Nodosaurus was a genus of herbivorous ankylosaurian dinosaur from the Late Cretaceous, the fossils of which are found in North America. Two incomplete specimens have been discovered in Wyoming and Kansas, and no skulls...

than an Ankylosaurus
Ankylosaurus
Ankylosaurus is a genus of ankylosaurid dinosaur, containing one species, A. magniventris...

, and a beak
Beak
The beak, bill or rostrum is an external anatomical structure of birds which is used for eating and for grooming, manipulating objects, killing prey, fighting, probing for food, courtship and feeding young...

, which it probably used to crop low-lying vegetation.

History of discovery



The first Hylaeosaurus fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

s were discovered in Sussex
Sussex
Sussex , from the Old English Sūþsēaxe , is an historic county in South East England corresponding roughly in area to the ancient Kingdom of Sussex. It is bounded on the north by Surrey, east by Kent, south by the English Channel, and west by Hampshire, and is divided for local government into West...

. Additional remains have been discovered on the Isle of Wight
Isle of Wight
The Isle of Wight is a county and the largest island of England, located in the English Channel, on average about 2–4 miles off the south coast of the county of Hampshire, separated from the mainland by a strait called the Solent...

 (also part of Great Britain
Great Britain
Great Britain or Britain is an island situated to the northwest of Continental Europe. It is the ninth largest island in the world, and the largest European island, as well as the largest of the British Isles...

), and in Ardennes
Ardennes
The Ardennes is a region of extensive forests, rolling hills and ridges formed within the Givetian Ardennes mountain range, primarily in Belgium and Luxembourg, but stretching into France , and geologically into the Eifel...

, France
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...

, though the remains from France may actually belong to a Polacanthus
Polacanthus
Polacanthus, deriving its name from the Ancient Greek poly-/πολυ- "many" and acantha/ακανθα "thorn" or "prickle", was an early armored, spiked, plant-eating ankylosaur from the early Cretaceous period. Early depictions often gave it a very vague head as it was only known from the rear half of the...

. 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
Gideon Mantell
Gideon Algernon Mantell MRCS FRS was an English obstetrician, geologist and palaeontologist...

 originally claimed the name Hylaeosaurus meant "forest
Forest
A forest, also referred to as a wood or the woods, is an area with a high density of trees. As with cities, depending where you are in the world, what is considered a forest may vary significantly in size and have various classification according to how and what of the forest is composed...

 lizard", after the Tilgate Forest in which it was discovered. Later, he claimed that it meant "Weald
Weald
The Weald is the name given to an area in South East England situated between the parallel chalk escarpments of the North and the South Downs. It should be regarded as three separate parts: the sandstone "High Weald" in the centre; the clay "Low Weald" periphery; and the Greensand Ridge which...

en lizard" ("wealden" being another word for forest), in reference to the Wealden Group
Wealden Group
The Wealden Group is a group in the lithostratigraphy of southern England. The Wealden group consists of paralic to continental facies sedimentary rocks of Valanginian to Barremian age and thus forms part of the English Lower Cretaceous. It is composed of alternating sands and clays...

, the name for the early Cretaceous geological
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...

 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".

Classification


Hylaeosaurus armatus was first named by Gideon Mantell in 1833, and is currently considered the only species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 in the genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

. 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 dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...

s 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
Polacanthus
Polacanthus, deriving its name from the Ancient Greek poly-/πολυ- "many" and acantha/ακανθα "thorn" or "prickle", was an early armored, spiked, plant-eating ankylosaur from the early Cretaceous period. Early depictions often gave it a very vague head as it was only known from the rear half of the...

is same species, but there are a number of differences in their bone structure (osteology
Osteology
Osteology is the scientific study of bones. A subdiscipline of anatomy, anthropology, and archeology, osteology is a detailed study of the structure of bones, skeletal elements, teeth, morphology, function, disease, pathology, the process of ossification , the resistance and hardness of bones , etc...

).

Hylaeosaurus is traditionally considered to be a primitive nodosaurid, in the Polocanthinae subfamily, like Gastonia
Gastonia (dinosaur)
Gastonia is a genus of nodosaurid dinosaur from the Early Cretaceous of North America, around 125 million years ago. Closely related to Polacanthus, it has a sacral shield and large shoulder spikes. It is also the first polacanthine dinosaur to have been mounted for display at the Denver Museum of...

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
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

 and Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

, and then vanished shortly after, replaced by more advanced ankylosauria
Ankylosauria
Ankylosauria is a group of herbivorous dinosaurs of the order Ornithischia. It includes the great majority of dinosaurs with armor in the form of bony osteoderms. Ankylosaurs were bulky quadrupeds, with short, powerful limbs. They are first known to have appeared in the early Jurassic Period of...

ns.

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