Scelidosaurus lived during the Early Jurassic Period, during the Sinemurian to Pliensbachian stages around 191 million years ago, at the time when Europe formed an island archipelago.

Scelidosaurus was lightly armoured, protected by long horizontal rows of keeled oval scutes that stretched along the neck, back and tail.

The front snout bones, the premaxillae, have a common central rough extension, in life bearing a small upper beak.

In fact, the upper temporal fenestrae were very large, forming conspicuous round openings in the top of the rear skull, serving as attachment areas for the powerful muscles that closed the lower jaws.

The crowns of the maxillary and dentary teeth have denticles on their edges and a swollen basis[5] The ascending branches of the paired premaxillae notched the combined nasal bones, whereas the opposite was usual in ornithischians.

These elements had been sketched by Norman in the seventies prior to the incident and interpreted as parts of the pterygoids, but in 2020 he concluded that they were special bones covering the roof of the nasal cavity, which he named the "epivomers".

At it rear side, the femur mid-shaft featured a well-developed drooping fourth trochanter, a process for the attachment of the retractor tail muscle, the Musculus caudofemoralis longus.

[5][12] Their edges develop grooves and eventually interlock, resulting in collar-like structures made of interconnected sections adorned with bony projections.

The bony plates of the neck display diverse shapes, ranging from small and pointed to flat and cap-like, tall and ridged, or broad and blade-like.

Smaller elements fill spaces between these primary rows, while the body’s remaining areas are covered by tightly packed, tiny bony structures, many of which supported keratinized scales.

The tail exhibits a distinct arrangement, with four rows of tall, hollow, ridged bony elements along its top, bottom, and sides.

The variation in bony armor across this animal, particularly in the neck and sides, suggests that larger projections were likely covered by keratin, while smaller, hollow forms may have served other functional or adaptive roles.

[14] The lemma text contained a diagnosis, implicating that the genus was validly named and was not a nomen nudum, despite the fact that the definition was vague and no specimens were identified.

Norman explained this by Owen's excessive workload in this period, including several administrative functions, polemics with fellow-scientists and the study of a large number of even more interesting newly discovered extinct animals, such as Archaeopteryx.

Unfortunately, mixed in with the Scelidosaurus fossils had been the partial remains of a theropod dinosaur and the femur and tibia thus belonged to such a carnivore; this was not discovered until 1968 by Bernard Newman.

[24] The new lectotype skeleton had been uncovered in the Black Ven Marl or Woodstone Nodule Bed, marine deposits of the Charmouth Mudstone Formation, dating from the late Sinemurian stage, about 191 million years ago.

[20] Owen in 1861 described a second, partial, skeleton of a juvenile animal, that later was added to the collection of Elizabeth Philpot and today is registered in the Lyme Regis Museum as specimen LYMPH 1997.37.4-10.

[27] Found by geologist James Frederick Jackson (1894–1966) of Charmouth, it is from a slightly younger layer, the Stonebarrow Marl Member dating to the early Pliensbachian, about 190 million years old.

[25] In 1985 Simon Barnsley, David Costain and Peter Langham excavated a partial skeleton including a very complete skull and skin impressions,[28] which was sold to the Bristol Museum where it is registered as specimen BRSMG CE12785.

These include a 3.1 metres (ten feet) long skeleton found by David Sole in 2000, perhaps the most complete non-avian dinosaur exemplar ever discovered in the British Isles.

Histologist Robin Reid recognized the first specimen as dinosaurian due to its bone texture and structure, and reported it in 1989, suspecting it belonged to Scelidosaurus or a similar animal.

The assignment of the femoral fragment was upheld, with a clear ornithischian identity and with size and morphology specifically very similar to Scelidosaurus and unlike close relative Scutellosaurus.

However, the tibia was reinterpreted as that of an indeterminate neotheropod, the pentagonal object as a mere piece of basalt resembling a fossil, and Grey's specimen as belonging to an ichthyosaur.

The fossil, with inventory number BRSMG CF2781, was in the early 1990s, in an already prepared state, discovered in the legacy of the late Professor John Challinor, who had used it to illustrate his lectures.

It consists of a series of eight caudal vertebrae in a cut slab of carbonate mudstone, which was judged to date from the late Hettangian to Sinemurian stages.

In the later twentieth century, the term was used for an assembly of "primitive" ornithischians close to the ancestry of ankylosaurs and stegosaurs, such as Scutellosaurus, Emausaurus, Lusitanosaurus and Tatisaurus.

[3] Today, paleontologists usually consider the Scelidosauridae paraphyletic, thus not forming a separate branch or clade; however, Benton (2004) lists the group as monophyletic.

Alfred von Zittel (1902), William Elgin Swinton (1934), and Robert Appleby et al. (1967) identified the genus as a stegosaurian,[40] though this concept then encompassed all armoured forms.

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,[9][41] or basal to the ankylosaur-stegosaur split.

Raising itself on its hindlimbs alone, could have vertically increased the feeding envelope and was perhaps anatomically possible, but Norman doubted it was a relevant part of its behaviour.