Stegosaurus: The Dinosaur with Plates and Spikes

Stegosaurus is the dinosaur with the diamond-shaped plates running down its back and four spikes on the end of its tail. Its name means "roof lizard," because early paleontologists thought the plates lay flat against the body like roof tiles. They were wrong about that — and a few other things — but Stegosaurus remains one of the most recognizable dinosaurs in the fossil record. This guide covers what scientists currently know, drawn from Late Jurassic rock formations in North America and museum collections worldwide.

Quick facts#

• Lived: Late Jurassic, about 155 to 150 million years ago
• Length: 26 to 30 feet (8 to 9 meters) head to tail
• Height at hip: 9 feet (2.7 meters)
• Weight: 5 to 7 tons (10,000 to 14,000 pounds)
• Diet: Herbivore (plant-eater)
• Where found: Western North America — Colorado, Utah, Wyoming
• Defining features: Bony plates along the spine, four-spiked tail (the "thagomizer"), small head

What Stegosaurus looked like#

A bus-sized herbivore that walked on four legs. Front legs much shorter than back legs, giving the body a sloping profile rising from head to hips. The most famous features:

The back plates#

About 17 to 22 large bony plates, arranged in two staggered rows running down the back from the neck to the tail. The largest plates were over 2 feet tall and 2 feet wide. They were not flat against the body — they stuck straight up. The plates were made of bone, with networks of blood vessels running through them.

The thagomizer (the tail spikes)#

Four long spikes on the end of the tail, each up to 3 feet long. The cartoon-themed name "thagomizer" was coined by a Far Side cartoon strip in 1982, but paleontologists adopted it semi-officially because there was no existing term. Healed bone evidence on Allosaurus fossils suggests Stegosaurus actually used these spikes — they were genuine defensive weapons.

The small head#

A skull about the size of a horse's. The brain inside was famously small — about the size of a walnut — which gave Stegosaurus its undeserved reputation for stupidity. The brain was actually adequate for its lifestyle (slow movement, eating plants, occasional defense); it just looks tiny next to the bus-sized body.

What were the plates for?#

The longest-running debate in Stegosaurus research. Three main hypotheses, with the consensus shifting over decades:

1. Defense#

Early hypothesis: the plates protected the back from attack. Problem: the plates stuck up vertically, not covering the sides of the body. Most attacks would not have hit the plates. Current view: defense was probably not the primary function, though the visual impression of bony plates may have deterred some predators.

2. Temperature regulation (thermoregulation)#

The plates had networks of blood vessels running through them. By orienting the plates toward or away from sun and wind, Stegosaurus may have used them like radiators — absorbing heat in the morning, releasing it in the heat of midday. This hypothesis was dominant in the 1970s and 80s but has weakened as detailed bone studies suggest the blood-vessel networks may not have been extensive enough for major thermoregulation.

3. Display and species recognition#

Current consensus: the plates were primarily for display. They identified Stegosaurus to other Stegosaurus (mate selection, species recognition) and may have signaled health or status. The plates could have changed color when blood flowed through them, allowing Stegosaurus to "blush" or display.

Best modern theory: a mix of all three, with display being the dominant function. The Smithsonian's paleobiology research leans toward display as primary, with secondary thermoregulation and possibly minor defense roles.

What it ate#

Pure herbivore. Low-growing plants — ferns, cycads, horsetails, and other ground-level vegetation common in the Late Jurassic. The body design made low feeding the default: the head was held close to the ground, the small mouth could shear leaves but not chew tough material, and the body did not flex easily for high reaching.

The teeth were small and leaf-shaped. Like Brachiosaurus, Stegosaurus probably did not chew much — it swallowed plants whole and used gastroliths (stones in the stomach) to grind food during digestion.

Stegosaurus vs Allosaurus#

Stegosaurus shared its habitat with Allosaurus, the dominant Late Jurassic predator in western North America. Stegosaurus was the herbivore Allosaurus most often tried to eat — and the fossil record shows the battle was real.

A famous specimen at the Denver Museum of Nature and Science includes an Allosaurus tail vertebra with a puncture wound exactly the size and shape of a Stegosaurus tail spike. Another specimen of Stegosaurus shows a healed bite wound from Allosaurus on its neck. Both fossils mean the animals survived their encounters at least once.

The tail spikes were not just decoration. Stegosaurus could swing the tail in a wide arc and probably swung it laterally to hit attackers. A 3-foot spike driven by a 6-ton body would have been a serious problem for Allosaurus.

Where Stegosaurus fossils are found#

Almost exclusively in the Morrison Formation, the same Late Jurassic rock formation that produced Brachiosaurus, Allosaurus, and most of the other classic Jurassic dinosaurs from North America. Specifically Colorado, Utah, and Wyoming.

The most famous specimens:

• "Sophie" at the Natural History Museum in London — one of the most complete Stegosaurus skeletons known
• Multiple specimens at the Denver Museum of Nature and Science — Colorado's state fossil is Stegosaurus
• The Carnegie Museum of Natural History in Pittsburgh has another extensive collection

For Florida families: no Stegosaurus in Florida. Like other Jurassic dinosaurs, Stegosaurus lived only in specific regions of the planet. North American Stegosaurus is a Mountain West story.

At a Jurassic Petting Zoo event#

The baby Stegosaurus is one of the five puppets in our petting zoo (Basic). The plates and spikes are the features kids gravitate toward — both because they are visually distinctive and because they are touchable in a way they would not be at a museum behind a barrier. The puppetry shows how the plates are oriented (sticking up, not flat) and lets kids feel how the spikes would have functioned.

For schools and families curious about meeting our Stegosaurus, see the experience page.

Frequently asked questions#

How big was Stegosaurus?#

About the length of a school bus (26 to 30 feet) and weighing as much as a large elephant (5 to 7 tons). Smaller than Brachiosaurus but still very large.

Was Stegosaurus really stupid?#

Its brain was small — about the size of a walnut — but "stupid" is the wrong way to think about it. Stegosaurus had the brain it needed for its lifestyle: eating low plants, walking slowly, occasionally fighting off predators. It is not a measure of intelligence the way mammal brain size is. The "second brain" myth (that Stegosaurus had a brain near its tail) was a 19th-century theory that has been disproven — what early paleontologists thought was a second brain was actually a glycogen body, common in birds.

How many plates did Stegosaurus have?#

About 17 to 22, arranged in two staggered rows along the back. The exact number varied slightly between individuals.

Did Stegosaurus have spikes anywhere besides the tail?#

The defining spikes are on the tail. Some closely related species (like Kentrosaurus) had spikes on the hips and shoulders as well. Stegosaurus itself had four tail spikes and the back plates.

Did Stegosaurus live in herds?#

Probably small family groups or pairs, based on the limited fossil evidence. Not large herds like some other herbivores. The exact social structure is debated.

When did Stegosaurus go extinct?#

Stegosaurus went extinct at the end of the Jurassic Period, about 150 million years ago — long before the asteroid impact at the end of the Cretaceous. The cause was environmental change as the planet's climate and ecosystems shifted. Other stegosaur species (like Kentrosaurus in Africa) survived a bit longer, but the whole group was effectively gone before the Cretaceous Period was well underway.

Why are the plates arranged in two rows instead of one?#

This was debated for over a century. Early reconstructions put them in a single row. Modern fossil evidence — including specimens where the plates were preserved roughly in place — shows two staggered rows. The staggered arrangement may have helped with display from any angle.

Meet a Stegosaurus#

Stegosaurus at a museum is impressive but behind glass. A baby Stegosaurus at a Jurassic Petting Zoo event lets kids actually touch the plates, observe the spikes, and see how the body design fits together. For South Florida schools and families, the experience page explains the format, or check date availability for your event.