Prehistoric Animals


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The Stegosaurus is a large herbivorous dinosaur that occupies an important place in the popular imagination because of its morphology which is iconic in the animal kingdom: its back is covered with two rows of triangular bony plates mounted vertically on both sides of his spine and is ended by a small tail adorned with four spikes named thagomizer. The anecdote of the origin of this strange word is rather interesting; it comes from a famous Far Side comic dating back to 1982. It shows a group of cavemen gathering around a tail picture of Stegosaurus. One of them points to the spikes of the tail and exclaims: "Now this side is called thagomizer!" The word "thagomizer" was coined by Far Side creator Gary Larson, but has remained very popular with paleontologists since that time. It should also be mentioned that the Stegosaurus figurine is the one that seems the coolest on the corner of a desk.

Since it was initially believed that the plates were lying on the body of the animal like the scales of crocodiles, it was given the name of "roofed reptile" in reference to this feature that is reminiscent of roof shingles. After discovering a specimen whose appearance was preserved in the mud, it was realized that the plates were most likely pointing upwards. Since then, Stegosaurus has been represented with the appearance that people know it today.

Very small brain

This dinosaur lived in the American midwest at the same time as the great carnivorous predators like Allosaurus and Ceratosaurus and the titanic sauropods like Diplodocus and Brachiosaurus. Stegosaurus brain is infamous for being tiny relative to the rest of his body. One wonders how a four-ton dinosaur could possibly survive and grow with a peanut in the noggin. The amount of gray matter available was very limited. Fortunately for the Stegosaurus, it only had to be slightly smarter than the plants it swallowed and only needed rudimentary reflexes to avoid predator attacks. So, although it is one of the most stupid dinosaurs, its intelligence was enough to flourish in the wild states of North America's Upper Jurassic.

A second brain in the anus

Funny enough, the naturalists of the time had a lot of trouble getting used to the idea that such a massive animal could have such a small brain size. They went so far as to hypothesize that the Stegosaurus had to have additional gray matter located elsewhere on his body. More specifically, it was assumed that it had a second brain in the region of the hips: in the anus. However, fossil evidences never materialized and this theory gradually fell into oblivion with the years passing by. When you think about it today with contemporary knowledge, it seems completely absurd. Yet at the time it was a perfectly plausible idea. The lesson of this fun story is that intelligence does not always guarantees success.

One of the first dinosaurs to have cheeks

A little known fact of this dinosaur is that it is one of the first to have developed cheeks. This anatomical quirck noted by extrapolating the shape and arrangement of its teeth gave it the ability to chew and predigrate his food before swallowing, much like humans do. Because of that, it could consume more plant material than other dinosaurs with no cheeks. And like many other herbivores of the Mesozoic era, the Stegosaurus voluntarily swallowed small rocks called gastroliths that helped crush the rough plant material which greatly facilitated the process of digestion. The scientific term that is generally used to describe this kind of eating behavior is lithophagy. Given his enormous size and cold-blooded metabolism, this animal undoubtedly had to eat hundreds of pounds of cycads and ferns every day to sustain itself.

What was the bony plates on his back?

If it weren’t for the two rows of pointed triangular plates alternately disposed on each edge of its spine, this dinosaur would not be distinguished from other second rank herbivores like the Iguanodon. This peculiarity makes this animal special and is the reason why it is so well known. It took a very long time before the true position of these plates were discovered and even today their exact function remains unknown. What is known with certainty is that these structures were giant versions of osteoderms found on modern crocodiles and that they did not attached to the backbone of the animal but rather on his thick skin which made them more flexible and gave them a greater range of motion. So what purpose could they serve?

Sexual selection characteristic

Like pterosaur ridges and mammoth tusks, the bony plates of Stegosaurus are thought to be primarily a sexual selection feature. Males with larger, sharper plates were more attractive to females during mating season.

Thermal regulator

Another theory that is often put forward is that they allowed the Stegosaurus to regulate the temperature of his body. If this dinosaur was indeed cold-blooded, as most Mesozoic herbivores are presumed to be, it may have used these bone protuberances to absorb sunlight during the day and dissipate body heat during the night. Studies have shown that the outer layers had blood vessels which supports this thesis.

Visual deterrent to predators

Since it lived at around the same time and in the same environment as the fearsome carnivore Allosaurus, Stegosaurus had to constantly protect his rear so as not to fall prey to this predator. The plates could have made it look bigger from a distance which would have had a deterrent effect on those considering feeding on it. It was certainly more true for newborns and juveniles than adults because they had less elaborate means of defense.

Active defensive role

But in addition to making it unattractive to predators, the plates could have also played an active defensive role. Since they were relatively mobile, it is not excluded that they could in deadly fights turn towards the attacker and injure his jaw or neck. This hypothesis is, however, defended only by the most marginal palaeontologists.

Communication tool

The last plausible theory is that the plates could "blush" to warn other members of a herd about imminent danger or to signal a nearby food source. This theory is well defended by the high degree of vascularization in the outer layers of these bony protusions.

A combination of these features

Since evolution tends to adapt specific anatomical characteristics to serve several functions, it is likely that all these hypotheses or a combination of these proves to be true.


Stegosaurus fossil

The Stegosaurus is another classic dinosaur discovered in the American West during the "Fossil War" of the late 19th century. The first fossil, the holotype, was named in 1877 by famous paleontologist Othniel C. Marsh, who originally thought he was dealing with a gigantic prehistoric turtle. He also thought that the plates laid flat on the back of the beast. But as the fossils started to accumulate, Marsh realized his mistake and assigned the vestiges of Stegosaurus to an Upper Jurassic dinosaur.

Vague description

The original description of the Stegosaurus was that of a low-slung dinosaur with a relatively small brain as well as triangular plates along its spine that served as a body armor. There was also mentions of sharp spikes garnishing its tail. This description was so vague that it led researchers to create nearly a dozen species of Stegosaurus to accommodate fossil discoveries. This list includes Stegosaurus armatus, Stegosaurus ungulatus, Stegosaurus stenops, Stegosaurus duplex, etc. The first specimen is Stegosaurus armatus, which was nearly 30 feet long and had 17 small triangular patches and four pikes on the tail. It is this species that most people know. Over time, some of these classifications turned out to be completely separate dinosaur species or different life stages of the same animal.

Initial reconstructions were difficult and the first illustrations do not resemble the image associated with Stegosaurus.

Allosaurus vs Stegosaurus

Allosaurus vs Stegosaurus

Allosaurus and Stegosaurus were perpetual rivals on the plains and woodlands of upper Jurassic North America. They are the equivalent of Tyrannosaurus and Triceratops during the Cretaceous era. Allosaurus was the apex predator of that time and was feared by all the herbivore fauna. But the Stegosaurus was defensively well adapted to this hostile environment; its center of gravity was low and it could swing its tail as a deadly weapon.

Signs of severe thagomizer injuries were found on the vertebrae of Allosaurus specimens. Some of these wounds did not even heal, indicating that they were fatal to the theropod. Bite marks whose appearance are characteristic of Allosaurus jaw we’re also discovered in the necks of some Stegosaurus fossils. The fight for survival was fierce in these wild ecosystems as the bones can attest today.


The Stegosaurus is a close cousin of the Ankylosaurus, which was literally built like a tank and flourished tens of millions of years later during the Cretaceous period. These two families of dinosaurs are classified among the group of thyreophorans.

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