Stegosaurus

About Stegosaurus

Features of Stegosaurus: Dorsal Plates (Osteoderms)

The most prominent feature of Stegosaurus is the large, kite-shaped bone plates (osteoderms) arranged along its back, numbering up to 17-19. Years of research have revealed that these plates stood vertically in "two alternating rows".

They were once thought to be "armor" to protect against predators, but because the inside of the plates was highly porous like a sponge and lacked strength, their role as defensive weapons has been dismissed.

Instead, two leading roles are considered: "thermoregulation" and "visual display". Traces of numerous blood vessels inside the plates suggest they may have functioned as radiators to release heat in the wind or warm the blood in the sunlight. It is also thought that by pumping large amounts of blood into the plates to make them flush red, they were used as a display to recognize peers, appeal to the opposite sex, or intimidate enemies.

Furthermore, recent studies have found that there are two distinct shapes of plates: a "wide and round type" and a "narrow and tall type", leading to the prevailing theory that this indicates a gender difference (sexual dimorphism).

Powerful Weapon: "Tail Spikes"

The tip of its tail was equipped with four massive spikes reaching 60 to 90 cm in length. From the articulated state of fossils, it is known that these spikes protruded "horizontally (outwards)" rather than pointing upwards.

Stegosaurus had a flexible tail and could swing its spiked tail at high speed using massive muscles. Indeed, an Allosaurus tail bone has been found with a puncture hole perfectly matching the shape of this spike, providing evidence that it was used as a counterattack weapon against carnivorous dinosaurs. Additionally, many spikes have been found showing signs of healing after being broken by severe impact.

Armor Protecting the Throat of Stegosaurus

While the plates on its back tend to get all the attention, Stegosaurus had specialized armor called "gular armor" from its throat to the underside of its neck. This consisted of countless small, pebble-like bones embedded in its skin, resembling chainmail.

For Stegosaurus, the underside of its neck was a vulnerable spot easily targeted by carnivorous dinosaurs like Allosaurus. The gular armor is thought to have served as a physical shield to prevent fatal injuries when bitten from below, and also as protection for the throat against hard, sharp plant leaves.

Diet and Digestion Mechanism

Stegosaurus had a small head and cropped low-growing plants such as ferns and cycads using a horny beak at the tip of its snout. It was once thought that "its bite force was weak and it swallowed food whole", but recent studies of tooth wear have shown that it bit its upper and lower teeth together to grind plant fibers to some extent.

Furthermore, the "gastroliths" (stomach stones) found inside Stegosaurus are angular in shape. Because the gastroliths of animals that grind food by rubbing stones together in a muscular stomach, like birds, become rounded, the opposite shape indicates that Stegosaurus did not have a powerful stomach. It is thought that they sent food into an "extremely long intestine (hindgut)" and digested it slowly through microbial fermentation, much like modern cows or elephants.

Small Brain and the Misconception of a "Second Brain"

The brain of Stegosaurus was remarkably small relative to its massive body, weighing only about 80 grams (roughly the size of a modern dog's brain). However, recent inner ear studies using CT scans have revealed that their sensory organs underwent unique evolution.

From the orientation of the semicircular canals in the inner ear, it was discovered that their natural head posture was constantly pointing downwards (at a roughly 45-degree angle). This is a result of specializing in eating low-growing plants near the ground. They are also thought to have communicated with herd members by picking up low-frequency sounds that could travel long distances in obstacle-filled forests.

In the past, seeing a large cavity in the hip bone (sacrum) led to the widely believed theory that "they had a second brain in their hips to move their giant bodies." However, this theory is now completely debunked. This cavity was not housing a brain, but rather an organ called the "glycogen body", similar to what is seen in modern birds, which is thought to have stored energy or supplied nutrients to the nervous system.

Growth and Herd Life

It is known that the timing of development for Stegosaurus's back plates and tail spikes differed depending on its growth stage. When young, a vascular network was first formed in the back plates. This suggests that the need for thermoregulation accompanying rapid volume increase, or the function as a display organ upon sexual maturity, was prioritized during youth.
On the other hand, the bones of the tail spikes, which served as powerful physical weapons, only hardened (ossified densely) when they became fully grown adults. In other words, the tail spikes of juvenile and young Stegosaurus were useless as defensive weapons.

Fossilized footprints discovered in the US suggest that they formed herds and traveled in the same direction. Because young individuals still had incomplete tail weapons, it is believed that they protected themselves from carnivorous dinosaurs like Allosaurus by adopting a formation where massive, robust adults stood on the outside of the herd, guarding the young on the inside.

First Discovery and the Miraculous Specimen "Sophie"

The name Stegosaurus was first described in a paper in 1877. When originally discovered by paleontologist Othniel Charles Marsh, Stegosaurus was thought to be an aquatic turtle-like reptile. The genus name Stegosaurus means "roof lizard" in Greek, derived from the imagination that its back plates lay flat covering its back like a turtle's shell or roof tiles. Later, better-preserved fossils were found, and it began to be restored with its plates standing upright as we see today.

The initial type species (the reference species) was "Stegosaurus armatus" based on fragmentary fossils, but it took an unusual turn when the type species was later changed to the better-preserved "Stegosaurus stenops".

For many years, its overall appearance had been inferred from a combination of fragmentary fossils, but in 2014, the Natural History Museum in London unveiled the most complete Stegosaurus specimen ever, with about 85% of its bones intact. This specimen is affectionately known as "Sophie".

Calculations based on Sophie's skeleton using the latest 3D digital technology revealed that her living weight was about 1.6 tons (roughly the size of a small rhinoceros). Also, although its walking speed was as slow as a human's brisk walk (about 6 km/h), it is estimated that it was good at "sharp turns" on the spot by using its short front legs as an axis and bracing with its long hind legs. This allowed it to quickly point its thagomizer (tail spikes) at enemies even if carnivorous dinosaurs tried to flank it.

The Relationship Between Gachapin and Stegosaurus

The fins growing on the back of the Japanese hero "Godzilla" were modeled after the dorsal plates of Stegosaurus.
Also, in 2012, with the cooperation of the Fukui Prefectural Dinosaur Museum, it was estimated that the ancestor of "Gachapin," a popular character in a Japanese children's TV show series, belonged to the <Ornithischia - Thyreophora - Stegosauria>.