Dacentrurus

Dacentrurus (meaning "tail full of points"), originally known as Omosaurus, was a large stegosaur of the Late Jurassic Period (154 - 150 mya) of Europe. Its type species, Omosaurus armatus, was named in 1875, based on a skeleton found in England. In 1902 the genus was renamed into Dacentrurus because the name Omosaurus had already been used for a crocodilian. After 1875, half a dozen other species would be named but perhaps only Dacentrurus armatus is valid.

Finds of this animal have been limited and much of its appearance is uncertain. It was a heavily built quadrupedal herbivore, adorned with plates and spikes.

Description
Dacentrurus was a large stegosaurid, some specimen have been estimated to reach lengths between 7–8 m (23–26 ft)[1][2] and to weigh up to 5 t (5.5 short tons).[3] Many books claim that Dacentrurus was a small stegosaur, when in fact finds such as a 1.5 metres (4.9 ft) wide pelvis indicate that Dacentrurus was among the largest of them. For a stegosaur the gut was especially broad,[3] and a massive rump is also indicated by exceptionally wide dorsal vertebrae centra.[1] The hindlimb was rather short,[3] but the forelimb relatively long, largely because of a long lower arm.[1]

Although Dacentrurus is considered to have the same proportions as Stegosaurus, its plate and spike configuration is known to be rather different, as it probably had both two rows of small plates on its neck and two rows of longer spikes along its tail.[4] The holotype specimen of Dacentrurus armatus contained a small blunt asymmetrical neck plate and also included a tail spike which could have been part of a thagomizer. The tail spike had sharp cutting edges on its front and rear side.

Dacentrurus has sometimes been portrayed with a spike growing near the shoulder, similarly to a ''Kentrosaurus'. Whether this portrayal is accurate or not is not yet determined.

Discovery and species
On 23 May 1874, James Shopland of the Swindon Brick and Tyle Company reported to Professor Richard Owen that their clay pit, the Swindon Great Quarry below Old Swindon Hill at Swindon in Wiltshire, had again produced a fossil skeleton. Owen sent out William Davies to secure the specimen, which proved to be encased in an eight feet high clay nodule. During an attempt to lift it in its entirety, the loam clump crumbled into several pieces. These were eventually transported to London in crates with a total weight of three tonnes. The bones were subsequently partially uncovered by Owen's preparator, the mason Caleb Barlow.[5]

Owen named and described the remains in 1875 as the type species Omosaurus armatus. The generic name is derived from Greek ὦμος, omos, "upper arm", in reference to the robust humerus. The specific name armatus can mean "armed" in Latin and in this case refers to a large spike that Owen assumed was present on the upper arm.[6]

The holotype, BMNH 46013, was found in a layer of the Kimmeridge Clay Formation dating from the late Kimmeridgian. The main nodule fragment contains the pelvis; a series of six posterior dorsal vertebrae, all sacrals and eight anterior caudal vertebrae; a right femur and some loose vertebrae. In all, thirteen detached vertebrae are present in the material. Also an almost complete left forelimb was contained by another loam clump. Additional elements include a partial fibula with calcaneum, a partial tibia, a right neck plate and a left tail spike.

Several other species would be named within the genus Omosaurus. Part of the British Museum of Natural History collection was specimen BMNH 46321, a pair of spike bases found in the Kimmeridge Clay by William Cunnington near the Great Western Railway cutting near Wootton Bassett. These Owen in 1877 named Omosaurus hastiger, the epithet meaning "spike-bearer" or "lance-wielder", the spikes by him seen as placed on the wrist of the animal.[7] In 1887, John Whitaker Hulke named Omosaurus durobrivensis based on specimen BMNH R1989 found near Felton[disambiguation needed], the specific name being derived from Durobrivae.[8] This in 1956 became the separate genus Lexovisaurus. In 1893, Harry Govier Seeley named Omosaurus phillipsii, based on a femur, specimen YM 498, the epithet honouring the late John Phillips.[9] Seeley suggested this may be the same taxon as Priodontognathus phillipsii Seeley 1869, which has led to the misunderstanding, due to its having the same specific name, that Priodontognathus was simply subsumed by him under Omosaurus. This interpretation however, is incorrect as both species have different holotypes. Seeley in 1901 named another species: Omosaurus leedsi based on specimen CAMSM (SMC) J.46874, a plate found in Northamptonshire, the epithet honouring Alfred Nicholson Leeds.[10] In 1910 Friedrich von Huene named Omosaurus vetustus, based on specimen OUM J.14000, a femur found in the west bank of Cherwell River, the epithet meaning "the ancient one".[11] In 1911 Franz Nopcsa named Omosaurus lennieri, the epithet honouring Gustave Lennier, based on a partial skeleton in 1899 found in the Kimmeridgian Argiles d'Octeville near Cap de la Hève in Normandy, France.[12] The specimen would be destroyed during the allied bombing of Caen in 1944.

Even as the last two Omosaurus species were named, it had become known that the name Omosaurus had been preoccupied by a crocodilian, Omosaurus perplexus Leidy 1856.[13] In 1902 Frederick Augustus Lucas renamed the genus into Dacentrurus. The name is derived from Greek δα~, da~, "very" or "full of", κέντρον, kentron, "point", and οὐρά, oura, "tail".[14] Lucas only gave a new combination name for the type species Omosaurus armatus: Decentrurus armatus, but in 1915 Edwin Hennig moved most Omosaurus species to Dacentrurus, resulting in a Dacentrurus hastiger, Dacentrurus durobrivensis, Dacentrurus phillipsi and a Dacentrurus lennieri.[15] Nevertheless it would be common for researchers to use the name Omosaurus instead until the middle of the twentieth century.[16] D. vetustus, earlier indicated as Omosaurus (Dacentrurus) vetustus by von Huene, was included with Lexovisaurus as a Lexovisaurus vetustus in 1983.