Scelidosaurus

Scelidosaurus (pron.: / ˌ s ɛ l ɨ d ɵ ˈ s ɔr ə s /; meaning "limb lizard", from Greek skelos/σκελος meaning 'leg' and saurus/σαυρος meaning 'lizard') is a genus of quadrupedal, lightly plated, herbivorous dinosaur about 4 metres (13 ft) long. It lived during the Early Jurassic Period, during the Hettangian to Sinemurian stages around 199.6 to 194  million years ago. Its fossils have been found in both England and in Arizona, in the United States. Scelidosaurus has been called the earliest complete dinosaur. This genus and related genera have been found on three continents.

Comparative anatomist Richard Owen described Scelidosaurus in 1859. Only one species, S. harrisonii, is considered valid today, although other species have been proposed over the years. One of the most primitive of the thyreophorans, the exact placement of Scelidosaurus within the suborder has been the subject of debate for nearly 150 years. This is not helped by the paucity of knowledge about its closest relatives.

Description

A full-grown Scelidosaurus was rather small, compared to most other dinosaurs. Some scientists have estimated a length of 4 metres (13 ft) Scelidosaurus was quadrupedal, with the hindlimbs considerably longer than the forelimbs. It may have reared up on its hind legs to browse on foliage from trees, but its forefeet were as large as its hind feet, indicating a mostly quadrupedal posture. Scelidosaurus had four toes, with the innermost digit being the smallest.

Skull

Unlike later ankylosaurs, the skull was low and triangular in shape, longer than it was wide, similar to that of primitive ornithischians. The head of Scelidosaurus was small, and it had a neck that was longer than that of most armoured dinosaurs.

Like other thyreophorans, Scelidosaurus was herbivorous, with very small, leaf-shaped cheek teeth suitable for cropping vegetation. It is believed Scelidosaurus fed with a puncture-crush system of tooth-on-tooth action, with simple up-and-down jaw movement. Unlike later ankylosaurs, Scelidosaurus still had the five pairs of fenestrae (skull openings) seen in primitive ornithischians, and its teeth were more leaf-shaped than later armoured dinosaurs.

Armour

The most obvious feature of Scelidosaurus is its armour, consisting of bony scutes embedded in the skin. These osteoderms were arranged in parallel rows down the animal's body. Osteoderms are also found in the skin of crocodiles, armadillos and some lizards. These osteoderms ranged in both size and shape; most were small, flat plates, but thicker scutes also occurred. The scutes were aligned in regular horizontal rows down the animal's neck, back, and hips, with smaller scutes arranged on the limbs and tail. The lateral scutes were conical, rather than the blade-like osteoderms of Scutellosaurus, and have been used to identify the genus. It also had a pair of distinctive three-pointed scutes behind the head. Compared to later ankylosaurs, Scelidosaurus was lightly armoured.

Fossilized skin impressions have also been found. Between the bony scutes, Scelidosaurus had rounded scales similar to those of a Gila monster. Between the large scutes, very small (5-10 millimetres [0.2-0.4 in]) flat "granules" of bone were distributed within the skin. In later ankylosaurs, these small scutes may have developed into larger scutes, fusing into the multi-osteodermal plate armour seen in genera such as Ankylosaurus.

Diet

Scelidosaurus and its Jurassic relatives were herbivorous. However, while other ornithischians possessed teeth capable of grinding plant material, Scelidosaurus had smaller, less complex teeth and a jaw capable of only simple up-and-down jaw movements. In this aspect, they resembled the stegosaurids, which also bore primitive teeth and simple jaws. Their diet would have consisted of leaved plants or fruits, as grasses did not evolve until late into the Cretaceous Period, after Scelidosaurus had become extinct.

Classification

Scelidosaurus was an ornithischian and has been classified at different times as an ankylosaur or stegosaur. This debate is still ongoing; at this time, it 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 favour, but is seen in older dinosaur books. Despite its ankylosaur classification, Scelidosaurus shared similarities with Stegosaurus, including a heavy body highest at the hips and bony plates down its back.

Scelidosaurus gives its name to the Scelidosauridae, a group of primitive ornithischians close to the ancestry of ankylosaurs and stegosaurs. Aside from Scelidosaurus, other members of the clade include Bienosaurus and possibly Scutellosaurus. Originally proposed by Edward Drinker Cope in 1869, the family was resurrected by Chinese paleontologist Dong Zhiming in 2001 after study of Bienosaurus, which shares close affinities with Scelidosaurus. The scelidosaurids have been found in Early Jurassic formations, and may have persisted into the Late Jurassic. Their fossils have been found in China, England, and Arizona. Some paleontologists consider the Scelidosauridae paraphyletic, but Benton (2004) lists the group as monophyletic.

Fossil records of thyreophorans more basal than Scelidosaurus are sparse. The more primitive Scutellosaurus, also found in Arizona, was an earlier genus which was facultatively bipedal. A trackway of a possible early armoured dinosaur, from around 195 million years ago, has been found in France. Ancestors of these primitive thyreophorans evolved from early ornithischians similar to Lesothosaurus during the Late Triassic.

Species

S. harrisonii, 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, S. oehleri, was described by D.J. 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.

History of discovery

While James Harrison of Charmouth (Early Sinemurian stage, 196.5-194 million years ago), West Dorset of England was quarrying the cliffs of Black Ven (between Charmouth and Lyme Regis), possibly for raw material for the manufacture of cement, in 1858, he found a few fragmentary fossils of limb bones. He sent them to Professor Richard Owen of the Natural History Museum (originally the British Museum (Natural History), London). These, with later finds from the same spot, revealed a nearly complete skeleton. Scelidosaurus was named by Sir Richard Owen in 1859; however, a complete description did not appear until 1863. Unfortunately, mixed in with the Scelidosaurus fossils were the partial remains of a theropod dinosaur; this was not discovered until 1968. In 1888, Richard Lydekker selected the knee joint as the lectotype of Scelidosaurus.

For many years, the enigmatic fossils of Scelidosaurus caused some debate over the classification of the genus. Von Zittel (1902), Swinton (1934), and Appleby et al. (1967) identified the genus as a stegosaur. 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.

In 1968, B. H. Newman applied to have Lydekker's selection of the knee joint as the lectotype officially rescinded by the International Commission on Zoological Nomenclature, as the joint was from a megalosaur. Wells et al. informally reassigned these bones, consisting of a femur and partial tibia, to " Merosaurus" in 1995.

In 1989, scutes identified as belonging to Scelidosaurus sp., which were found in the Kayenta Formation (Glen Canyon Group) of northern Arizona, helped to determine the age of the strata was around 199.6-196.5 million years ago. These scutes also established a geographic tie-in between Arizona's Glen Canyon and Europe, where fossils of Scelidosaurus had previously been discovered. Some scientists have disputed the assignment to Scelidosaurus, though.

In 2000, Martill et al. announced the preservation of soft tissue in a specimen of Scelidosaurus. These fossils consist of eight caudal vertebrae in a cut slab of carbonate mudstone, which was judged to date from the late Hettangian to Sinemurian stages. Parts of the fossil were preserved in such a way that an envelope of preserved soft tissue is visible around the vertebrae, and show the presence of an epidermal layer over the scutes. The authors concluded that the osteoderms of all basal armoured dinosaurs were covered in a tough, probably keratinous layer of skin.

In popular culture

Although Scelidosaurus is nowhere near as well known as its sister taxa Ankylosaurus or Stegosaurus, the genus has appeared infrequently in popular media. One instance is in Nintendo's Jurassic Park III: Park Builder video game, where the player controls a menagerie of dinosaurs, including Scelidosaurus. The dinosaur is also one of the main exhibits at the Charmouth Heritage Coast Centre in Charmouth, England. The centre houses both a model and a cast of Scelidosaurus, fossils of which were collected in the area. The children's show Harry and His Bucket Full of Dinosaurs features a Scelidosaurus named Sid as one of Harry's dinosaur friends.