Scelidosaurus is a genus of herbivorous armoured ornithischian dinosaur from the Jurassic of England.
Scelidosaurus lived during the Early Jurassic Period, during the Sinemurian to Pliensbachian stages around 191 million years ago. This genus and related genera at the time lived on the supercontinent Laurasia. Its fossils have been found near Charmouth in Dorset, England, and are known for their excellent preservation. Scelidosaurus has been called the earliest complete dinosaur. It is the most completely known dinosaur of the British Isles. Scelidosaurus is currently the only classified dinosaur found in Ireland. Despite this, a modern description is still lacking. After initial finds in the 1850s, comparative anatomist Richard Owen named and described Scelidosaurus in 1859. Only one species, Scelidosaurus harrisonii named by Owen in 1861, is considered valid today, although one other species was proposed in 1996.
Scelidosaurus was about 4 metres (13 ft) long. It was a largely quadrupedal animal, feeding on low scrubby plants, the parts of which were bitten off by the small, elongated head to be processed in the large gut. Scelidosaurus was lightly armoured, protected by long horizontal rows of keeled oval scutes that stretched along the neck, back and tail.
One of the oldest known and most "primitive" of the thyreophorans, the exact placement of Scelidosaurus within this group has been the subject of debate for nearly 150 years. This was not helped by the limited additional knowledge about the early evolution of armoured dinosaurs. Today most evidence indicates that Scelidosaurus is the sister taxon to the two main clades of Thyreophora, the Stegosauria and Ankylosauria.
A full-grown Scelidosaurus was rather small compared to most later non-avian dinosaurs, but it was a medium-sized species in the Early Jurassic. Some scientists have estimated a length of 4 metres (13 ft). In 2010, Gregory S. Paul gave a body length of 3.8 metres (12.5 ft) and a weight of 270 kilogrammes. Scelidosaurus was quadrupedal, with the hindlimbs longer than the forelimbs. It may have reared up on its hind legs to browse on foliage from trees, but its arms were relatively long, indicating a mostly quadrupedal posture. A trackway from the Holy Cross Mountains of Poland shows a scelidosaur like animal walking in a bipedal manner, hinting that Scelidosaurus may have been more proficient at bipedalism than previously thought.
The most obvious feature of Scelidosaurus is its armour, consisting of bony scutes embedded in the skin. These osteoderms were arranged in horizontal parallel rows down the animal's body. Osteoderms are today found in the skin of crocodiles, armadillos and some lizards. The osteoderms of Scelidosaurus ranged in both size and shape. Most were smaller or larger oval plates with a high keel on the outside, the highest point of the keel positioned more to the rear. Some scutes were small, flat and hollowed-out at the inside. The larger keeled scutes were aligned in regular horizontal rows. There were three rows of these along each side of the torso. The scutes of the lowest, lateral, row were more conical, rather than the blade-like osteoderms of Scutellosaurus.
Scelidosaurus harrisonii, named and described by Owen, is currently the only recognized species, based on several nearly complete skeletons. A potential second species from the Sinemurian-age Lower Lufeng Formation, Scelidosaurus oehleri, was described by David Jay Simmons in 1965 under its own genus, Tatisaurus. In 1996 Spencer G. Lucas moved it to Scelidosaurus. Although the fossils are fragmentary, this reassessment has not been accepted, and S. oehleri is today once again recognized as Tatisaurus.
Scelidosaurus was an ornithischian. It was the oldest ornithischian known until the description of Geranosaurus in 1911. During the twentieth century, it has been classified at different times as an ankylosaur or stegosaur. Alfred von Zittel (1902), William Elgin Swinton (1934), and Robert Appleby et al. (1967) identified the genus as a stegosaurian, though this concept then encompassed all armoured forms. In a 1968 paper, Romer argued it was an ankylosaur. In 1977, Richard Thulborn of the University of Queensland attempted to reclassify Scelidosaurus as an ornithopod similar to Tenontosaurus or Iguanodon. Thulborn argued Scelidosaurus was a lightly built bipedal dinosaur adapted for running. Thulborn's 1977 theories on the genus have since been rejected.
This debate is still ongoing; at this time, Scelidosaurus is considered to be either more closely related to ankylosaurids than to stegosaurids and, by extension, a true ankylosaur, or basal to the ankylosaur-stegosaur split. The stegosaur classification has fallen out of favor, but is seen in older dinosaur books. Cladistic analyses have invariably recovered a basal position for Scelidosaurus, outside of the Eurypoda.
Like most other thyreophorans, Scelidosaurus is known to be herbivorous. However, while some later ornithischian groups possessed teeth capable of grinding plant material, Scelidosaurus had smaller, less complex leaf-shaped teeth suitable for cropping vegetation and jaws capable of only vertical movement, due to a short jaw joint. Paul Barrett concluded that Scelidosaurus fed with a puncture-crush system of tooth-on-tooth action, with a precise but simple up-and-down jaw movement, in which the food was mashed between the inner side of the upper teeth and the outer side of the lower teeth, without the teeth actually touching each other as shown by very long vertical wear facets on the lower teeth alone. In this aspect, it resembled the stegosaurids, which also bore primitive teeth and simple jaws. Its diet would have consisted of ferns or conifers, as grasses did not evolve until late into the Cretaceous Period, after Scelidosaurus was long extinct.