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Stegosaurus: Body Like a Bus, Tiny Little Brain

Stegosaurus
A model of Stegosaurus stands in front of the visitor Center at Dinosaur National Monument, Utah. Wikimedia Commons (CC BY-SA 4.0)

Words can become fossils in their own right. Triceratops means "three-horned face" and Velociraptor translates to "speedy plunderer." Both genus names fit the dinosaurs they belong to; we know Triceratops had a trio of horns on its skull while Velociraptor was a lightly-built carnivore.

The case of Stegosaurus isn't so straightforward. Rooted in Greek, this Jurassic plant-gobbler's name means "roofed lizard," which made a lot more sense when the animal was first discovered over 140 years ago.

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It's About Time

Hollywood, take heed. Movies like "Fantasia" and the camp classic "Planet of Dinosaurs" show Stegosaurus duking it out with everyone's favorite bone-crusher, Tyrannosaurus rex. There's just one problem: Those two dinos never crossed paths in real life.

Tyrannosaurus had a fairly short reign that lasted from 68 to 65.5 million years ago. Stegosaurus came and went much, much earlier. The oldest specimens on record are around 155 million years of age — while the youngest were fossilized 150 million years before the present.

So the mighty T. rex actually lived closer to the dawn of mankind than it did to Stegosaurus's heyday.

The Jurassic Period, which lasted from 199.6 million to 145.5 million years ago, was drawing to a close when Stegosaurus roamed Earth. Although its range included Portugal, the beast is mainly known from fossil sites in western North America.

Stegosaurus belonged to a suborder of dinosaurs called — what else? — the stegosaurs.

Found in North America, Europe, Asia and mainland Africa, the stegosaurs walked on four legs and had long, beak-tipped skulls.

But it's the ornaments that really grab your attention. Spikes were a stegosaur mainstay, adorning the tails of every known species. Many of these creatures, like Africa's Kentrosaurus, also rocked big old spikes on the shoulders and lower back.

And where the back spikes came to an end, a much weirder feature took over. Stegosaurus and its kin are characterized by the vertical plates above their spines.

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"Show Me the Plates"

We may never know how these things functioned. Paleontologist Othniel Charles Marsh named Stegosaurus in 1877. He chose this name — which, again, means "roofed lizard" — because he figured the plates were sheets of armor that laid flat against the animal's backside.

Instead, later discoveries proved the objects stood upright, leaving the flanks on these dinosaurs exposed.

Because the plates contained blood vessels, experts used to think they helped stegosaurs chill out. Heat would supposedly dissipate from blood as it entered the tall, fanlike structures. After this chilled blood circulated elsewhere, it'd stay cool for a little while — lowering the overall body temperature.

Stegosaurus
A full skeleton of an adult Stegosaurus as well as a few bones from a baby specimen are displayed at the American Museum of Natural History in New York City.
InSapphoWeTrust/Flickr (CC BY-SA 2.0)

This hypothesis is no longer popular. According to a study published in 2005 in the journal Paleobiology, the blood-carrying networks in Stegosaurus plates were there to promote healthy bone growth — but they played no role in dissipating body heat.

Maybe Stegosaurus and its kin were just showing off. Made of bone and encased in horny sheaths, the plates could've made these dinos look bigger and more intimidating.

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The Business End

Capable of hitting 29.5 feet (9 meters) in length and weighing 5 tons (4.5 metric tons), Stegosaurus would loom large over today's land mammals. Even by stegosaur standards, it was a biggie; most of the dinosaurs in that group were only 13 to 23 feet (4 to 7 meters) long.

Yet in the late Jurassic, Stegosaurus lived in the shadows of behemoths. Sauropods — or "long-necked" dinosaurs — like the 60-foot (18-meter) Camarasaurus and the 80-foot (24-meter) Diplodocus were some of the creature's neighbors.

And while Stegosaurus didn't have to worry about T. rex, a rogue's gallery of Jurassic predators stalked its ecosystem.

Allosaurus was especially common. Measuring up to 28 feet (8.5 meters) long, this carnivore had serrated teeth and jaws that could open wide at a terrifying 79-degree angle.

Good thing Stegosaurus had four tail spikes at its disposal. We know they saw action once in a while; a study published in "The Armored Dinosaurs" by the Indiana University Press in 2001 that reviewed 51 of these dangerous-looking fossils found clear evidence of trauma on 10 percent of them.

Apparently, the "roofed lizard" hit below the belt. One Allosaurus pubic bone shows a deep wound thought to have been made by a Stegosaurus tail spike. All's fair in love and war, even Jurassic crotch-shots.

It seems there were plenty of injuries to go around; a Stegosaurus neck plate that some Allosaurus probably gnawed on has been recovered from the Cleveland-Lloyd Dinosaur Quarry in Utah.

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Food for Thought

Paleontologists think Stegosaurus browsed on low-lying vegetation like cycads. A 2016 computer simulation found the animal's bite force would've rivaled that of a sheep or cow.

However it processed food, this dinosaur didn't need much gray matter. Stegosaurus had a brain cavity that was long, narrow — and tiny. Tipping the scales at 20.8 ounces (80 grams) or so, the actual brain only made up about 0.001 percent of the creature's total body weight.

Note that we said "brain" and not "brains."

Absurd as it might sound, there's a rumor that Stegosaurus had a second brain located where the sun doesn't shine. Writing in 1881, Marsh drew attention to the enlarged cavity we find in the backbones above this dinosaur's hip region. Then he went and called it a "posterior braincase."

And the rest is history.

No one knows what this opening was for, although some researchers think it stored glycogen, a sugar that provides cells with energy. Regardless, there's no reason to think Stegosaurus — or any dinosaur — had multiple brains.

Stegosaurus has some pretty cool bragging rights, though. Not only is the Jurassic herbivore Colorado's official state fossil, but it was also the inspiration for Godzilla's dorsal plates. Take that, Kentrosaurus!

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