Scelidosaurus, meaning “limb lizard,” is a genus of herbivorous dinosaur from the Hettangian and Sinemurian stages of the Early Jurassic Period (208 to 194 million years ago). Its fossils have been discovered in both England and the United States — in Arizona. Scelidosaurus has been called the earliest complete dinosaur known.
The type specimen was discovered by James Harrison of Charmouth, England, while scouring the cliffs of Black Ven (between Charmouth and Lyme Regis) for raw material for a manufacturer of cement in 1858. During his search he uncovered a few fragmentary fossils of limb bones. He sent them to Professor Richard Owen of the Natural History Museum in London for analysis. The find, with which more came from the same area later on, revealed a nearly complete skeleton.
Sir Richard Owen named the dinosaur Scelidosaurus in 1859; however, a complete description was not given until 1863. Unfortunately, partial remains of a theropod dinosaur were mixed in with Scelidosaurus, which were not discovered until 1868. Richard Lydekker selected the knee joint as the lectotype (a type species previously undesignated by the original author) of Scelidosaurus in 1888.
There has been continued debate over the classification of Scelidosaurus for more than a century. Scelidosaurus was classified as a stegosaur by Von Zittel (1902); Swinton (1934); and Appleby (1967). It was also reclassified as an ornithopod similar to Tenontosaurus or Iguanodon by Richard Thulborn of University of Queensland in 1977. Thulborn’s theories have since been rejected.
B.H. Newman in 1968 applied to have Lydekker’s selection of the knee joint as the lectotype officially annulled by the International Commission on Zoological Nomenclature, as the joint was from a megalosaur. The knee joint, along with a femur and a partial tibia, were reassigned to Merosaurus in 1995.
Further discoveries of fossils in 1989 in the Kaventa Formation of northern Arizona were classified as Scelidosaurus fossils and established a geographic connection between Arizona fossils and those found in Europe. Some scientists have disputed the findings as being from Scelidosaurus, however.
After nearly 150 years of debate, only one species, S. harrisonii, is considered valid today.
An adult Scelidosaurus was relatively small at 13 feet long, compared to other similar dinosaurs. It was a quadrupedal, with the hindlimbs considerably longer than the forelimbs. It may have reared up on its hind legs to browse on high foliage. However, its forefeet were as large as the hind feet, indicating a mostly quadrupedal posture. It had four toes, with the innermost digit being the smallest.
The skull was low and triangular in shape, longer than it is wide, similar to primitive ornithischian skulls. Its head was small, and the neck was longer than that of most armored dinosaurs. The most obvious feature of Scelidosaurus was its armor, consisting of bony scutes embedded in the skin. The scutes were arranged in parallel rows down the body. These scutes –or osteoderms — are also found in the skin of modern crocodiles, armadillos and some lizards.
The osteoderms varied in size and shape throughout the body. Most were small, flat plates, but thicker scutes also occurred. They were aligned in regular horizontal rows down the animal’s neck, back, and hips, with smaller scutes arranged on the limbs and tail. Scelidosaurus was identified as a unique genus because of its lateral conical scutes. It also had a pair of distinctive three-pointed scutes behind the head. Scelidosaurus was less armored than its relative: the Ankylosaurus.
Like other thyreophorans, Scelidosaurus was herbivorous. It had 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. Its teeth were more leaf-shaped than later armored dinosaurs.
Like stegosaurus, Scelidosaurus may have swallowed gastroliths to aid processing of food (because of the lack of chewing ability), in the same manner used by modern birds and crocodiles. 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.
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 Center in Charmouth, England. The center houses both a model and a cast of Scelidosaurus, fossils of which were collected in the area.