Dino of the Week: Scelidosaurus

Type Species: Scelidosaurus harrisonii

Classification: Dinosauria – Ornithischia – Thyreophora – Scelidosauridae

Time Period: Early Jurassic

Location: British Isles and North America

Diet: Herbivore

The thirteen-foot-long herbivore Scelidosaurus lived among the scattered wooded islands of the British Isles during the Sinemurian and Pliensbachian stages of the Early Jurassic some 196-183 million years ago. It also 'migrated' into northern Laurasia, or North America as we know it now, as its remains have recently been discovered in Arizona. Scelidosaurus is the most completely known dinosaur from the British Isles and the only classified dinosaur from Ireland. For 150 years Scelidosaurus has been the epicenter of a fierce debate regarding its placement in the ‘dinosaur family tree.’ Some scientists argue that it’s an early ankylosaurs; others that it’s an early stegosaur; still others that it’s an early thyrophoroidean; and still others insist that it’s its own type of dinosaur with its own family, the Scelidosauridae (perhaps along with Scutellosaurus and Emausaurus). The general consensus at the moment – though, you can imagine, it is quite subject to change! – is that it belongs to Scelidosauridae, a thyreophoran sister taxon of the ankylosaurs and stegosaurs.

Scelidosaurus had a small head with a horny, beak-like front to its mouth. It was quadrupedal, with its rear legs longer and stronger than its forelegs, so that its back sloped up towards the hips (these features would be developed and exaggerated later on by the stegosaurs). Though primarily a low browser feasting on ferns and cycads, Scelidosaurus may have been able to rear up on its hind limbs to reach higher foliage such as conifers, but this is debated; even if it could become bipedal for eating, it wouldn’t have been able to move on two legs, and the bipedal stance would be awkward and cumbersome. Thus, though Scelidosaurus was capable of a bipedal stance, it likely only did so when the environment forced its hand. Scelidosaurus fed with a puncture-crush system of tooth-on-tooth action with a precise up-and-down jaw movement without the teeth actually touching each other. The later stegosaurs, with their primitive teeth and simple jaws, fed likewise. Its tail was stiff due to tendons that lay alongside its backbone and which became ossified with bony minerals.

The most notable facet of Scelidosaurus was the ‘light armor’ it sported: its neck, body, and tail were studded with small, pebble-like scales and large bony plates called scutes. The scutes were generally longer at the front of the body and diminishing in size towards the rear, especially at the thighs. Scelidosaurus’ bony armor – called osteoderms – are seen in modern crocodiles, armadillos, and some lizards. Compared to the later ankylosaurs, Scelidosaurus’ armor was ‘amateur hour,’ for it lacked continuous plating, spikes, or pelvic shields. Scelidosaurus’ osteoderms were arranged in horizontal parallel rows down the length of its body. The osteoderms 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. Between these main series, one or two rows of smaller oval keeled scutes were present. There were in total four rows of large scutes on the tail: one at the top midline, one at the midline of the underside, and one at each tail side. Whether the midline tail scutes continued over the torso and neck to the front is unknown and unlikely for the neck, though Scelidosaurus is often pictured this way. The neck had at each side two rows of large scutes. The osteoderms of the lower neck row were very large, flat, and plate-like. The first osteoderms of the top neck rows formed a pair of unique three-pointed scutes directly behind the head. These points seem to have been connected by tendons to the rear joint processes. Some of the latest Scelidosaurus specimens show different osteoderms, including scutes on which the keel is more like a thorn or spike. These specimens also have little horns on the rear corners of the head. These differences could indicate a variant species of Scelidosaurus or could be instances of sexual dimorphism (i.e. perhaps males and females had different patterns and types of armor). 

Scelidosaurus is one of the dinosaurs for which we have skin impressions. Between the bony scutes, Scelidosaurus had rounded, non-overlapping scales like the modern Gila monster. Between the large scutes, very small, flat ‘granules’ of bone were distributed within the skin. In the later ankylosaurs, these small scutes may have developed into larger scutes, fusing into the multi-osteodermal plate armor seen in species such as Ankylosaurus.