Dinosaurs A to Z
Stegosauridae

Stegosaurus STEG-uh-SOR-us

"Roofed lizard"

Stegosaurus silhouette
You 1.8 m (5.9 ft) tall
Stegosaurus 9 m (29.5 ft) long
as long as 6 people standing fingertip to fingertip
silhouette · Andrew Farke (CC-BY) via PhyloPic
Length
9 m (29.5 ft) — About 9 m long, 4 m tall at the shoulder, ~3.5 tonnes.
Period
Late Jurassic (155–145 Mya)
Place
North America · Western United States · Portugal
Food
Herbivore — Low-growing ferns, cycads, and horsetails. Probably stripped soft plants with a beak.
Clade
Stegosauridae Ornithischia Thyreophora Stegosauria Stegosaurus and close relatives.

A plate-backed, spike-tailed Jurassic plant-eater that is one of the most recognizable dinosaurs.

Stegosaurus is the dinosaur that looks like nothing else alive today — a plant-eater the size of a bus, with a double row of upright bony plates along its back and four spikes on its tail. It walked on four legs but the back legs were much longer than the front, so its body sloped forward from the hips.

Stegosaurus lived in the Late Jurassic, about 150 million years ago. That puts it closer in time to Tyrannosaurus rex than T. rex is to us — children’s books that show them fighting are mixing two periods 85 million years apart.

More about Stegosaurus

Discovery

Named by Othniel Charles Marsh · Dinosaur Ridge National Landmark, western United States

Stegosaurus remains were first identified during the so-called Bone Wars by Othniel Charles Marsh. The first known skeletons were fragmentary and scattered, so it took many years before the animal's true posture and plate arrangement were understood.

Portrait of Othniel Charles Marsh
Othniel Charles Marsh The paleontologist who first identified Stegosaurus remains during the Bone Wars, the intense fossil-collecting rivalry of the period.
How did <em>Stegosaurus</em> actually eat with such an odd jaw?
Its teeth and jaws were very different from those of other plant-eating ornithischianA 'bird-hipped' dinosaur. All were plant-eaters (Triceratops, Stegosaurus). dinosaurs, which had grinding teeth and flexible jaws. Stegosaurus instead had small, peg-shaped teeth and a jaw likely limited to a simple up-and-down motion. This points to a different feeding strategy that is not yet well understood.
What were the plates really for?
The function of the plates and spikes has been the subject of much speculation. The current general view is that the plates were used mainly for display and secondarily for thermoregulation, while the tail spikes were most likely for defense against predators. These interpretations remain debated rather than settled.

What we know

  • Two staggered rows of large bony plates ran along the back.
  • Four long spikes on the tail — paleontologists call this the 'thagomizer'.
  • Skull was tiny relative to body — the brain was about the size of a walnut.
  • Lived alongside Allosaurus, Apatosaurus, and Diplodocus in the Morrison Formation.

What we guess

  • What the plates were for. Old idea: armor. Newer ideas: heat regulation, species display, or threat advertising. Probably all three at once.
  • Whether the plates were brightly colored in life — possible but unprovable.
  • How exactly the tail spikes were used. Allosaurus vertebrae with stegosaur-spike-shaped wounds suggest defense, not just display.