Scelidosaurus

About Scelidosaurus

Characteristics of Scelidosaurus: The "Blueprint" of Thyreophorans

Scelidosaurus is an early thyreophoran with characteristics that can be called the "blueprint (basic design)" for both the Stegosauria and Ankylosauria groups that appeared in later eras. It lived during the Early Jurassic (about 193 to 191 million years ago).
It is a medium-sized dinosaur estimated to be about 4 meters in length and weighing 270 kg.

Armor and Skin: Osteoderms of Various Sizes

Recent research has revealed its defense system in detail. Its body surface was densely covered with osteoderms of various sizes.

• On the back of its head, it had a pair of large, curved, horn-like osteoderms, similar to those of bovid animals.
• From the neck to the back and tail, several rows of large, keeled bony plates ran symmetrically.
• On the sides of its body, sharp osteoderms curved sideways like goat horns were lined up.
• And the gaps between these large pieces of armor were densely filled with small polygonal or circular scales.

The surface of Scelidosaurus's osteoderms was porous, and a network of blood vessels passed inside. Therefore, it is speculated that they served not only as defensive armor but also as thermoregulatory organs (radiators) to release heat, and for display (intimidation or courtship) by engorging them with blood to change the color of the body surface.

Interestingly, there was no row of large osteoderms in the middle of the back (midline), but instead, a solid row was formed around the tail. It is thought that the tail, covered in heavy osteoderms, was used as a powerful defensive weapon to shake off predators.

Unexpected Ecology: Standing on Two Legs as Needed

Scelidosaurus was basically quadrupedal because it had heavy armor, but its hind legs were well developed compared to its front legs. This is because it strongly retained the characteristics of its bipedal ancestors.
From this, it is inferred that it was able to stand up on two legs or run on two legs (facultative bipedalism) when eating plants in high places or in an emergency. Fossil footprints of early ornithischians found in Poland also support the fact that they skillfully used both four and two legs.

Advanced Chewing and Breathing Mechanisms

Despite being an early dinosaur, Scelidosaurus had a mechanism specialized for a plant-based diet.

When eating hard plants, a force called "wishboning," where the jaw joint expands laterally, was exerted. The lower jaw of Scelidosaurus had a flexible joint between the left and right sides, and the jaw itself had a structure that allowed it to twist along its long axis. Furthermore, there were "cheek pouches" on the outside of the teeth, preventing finely crushed plants from falling out of its mouth.

It is also thought to have had a "hybrid breathing system" that combined breathing using the entire ribs to aid breathing in the broad chest, and breathing that ventilated the lungs by moving the internal organs through the movement of the pelvis (pubis).

A Miracle of Fossilization: Why it is found in marine strata

Scelidosaurus fossils are unearthed as extremely well-preserved articulated skeletons from Early Jurassic "marine strata (strata deposited in the sea)" in southern England. Why are terrestrial, heavily armored dinosaurs found in strata deposited in the sea?

Europe at the time was an "archipelago" environment with many islands floating in the sea. It is thought that a Scelidosaurus living in a rich coastal forest was swept out to sea by a massive flash flood or river overflow (flood) because it failed to escape in time, and drowned.

The carcass thrown into the sea swelled up like a balloon with decomposition gases from bacteria in its body, drifting on the sea surface belly-up (Bloat-and-float phenomenon). Eventually, when the gas escaped, the carcass sank to the quiet seabed. Because the deep seabed was anoxic, it was not eaten by scavengers, and the heavy osteoderm armor retained its original arrangement, resulting in a well-preserved fossil.

The "Rosetta Stone" of Dinosaur Research: A Naming Mistake and a Rediscovery 150 Years Later

Scelidosaurus became an important entity as the first dinosaur in the world to have an almost complete skeleton found, discovered in England in 1858.

Richard Owen, a prominent anatomist at the time, intended to name it "hind-limb lizard (skelos)" in Ancient Greek, looking at its strong legs. However, he mistakenly used the word for "rib of beef (skelis)," making a mistake that went down in the history of paleontology by getting it academically established as "rib of beef lizard."
Also, Owen conveniently used the skeleton of the quadrupedal Scelidosaurus to protect his own theory that "dinosaurs are giant quadrupedal reptiles."

Afterwards, this famous specimen slept in the storage of a museum in London for over 150 years without being studied in detail.

However, in the 2020s, Dr. David Norman of Cambridge University thoroughly restudied this fossil using the latest technology. As a result, it was strongly suggested that Scelidosaurus was not just a primitive dinosaur, but the most basal ancestor of later ankylosaurs (Ankylosauria).
If this hypothesis is correct, it means that ankylosaurs had undergone unique evolution from an older era than we had thought. Scelidosaurus fossils are currently in the spotlight again as a standard for correctly redrawing the phylogenetic tree of dinosaur evolution.