Research reviewed 30 July 2026. Scientific interpretations may change as new evidence is published.
Stegosaurus was a large Jurassic plant eater with two rows of tall back plates and a muscular tail carrying four sharp spikes. Fossils show that the spikes could be used in defence, while the purpose of the plates is still debated.
| Fact | Evidence based answer |
|---|---|
| Scientific name | Stegosaurus stenops |
| Time period | Late Jurassic, about 152 to 145 million years ago |
| Fossil locations | Mainly the western United States |
| Diet | Herbivore |
| Estimated length | Commonly about 7 to 9 metres |
| Estimated mass | Several tonnes, depending on size and species |
| Famous features | Large plates and four tail spikes |
Stegosaurus was an armoured dinosaur, but its protection differed from that of Ankylosaurus. Thin plates stood along its neck, back and tail, while four tail spikes formed a defensive weapon.
The name means "roof lizard". Early palaeontologists imagined flat plates like roof tiles, but complete fossils showed two upright, alternating rows.
Stegosaurus stenops is the best known species. A well preserved skeleton called Sophie has helped researchers investigate growth, mass and posture.
Stegosaurus lived about 152 to 145 million years ago. Most fossils come from the Morrison Formation in Colorado, Utah and Wyoming.
Its landscape included rivers, floodplains, woodland and seasonal dryness. Neighbours included giant sauropods, Allosaurus and Ceratosaurus.
The formation covers a wide area and several million years, so not every species necessarily met.
Stegosaurus had a small head, short neck and long hind legs. Its back sloped down towards a head held close to the ground.
It walked on four sturdy legs. Limb joints and trackways suggest low speed, while broad feet supported its weight and the tail stayed raised.
The bony plates were too thin to form a complete shield. Blood vessel grooves show they had an active blood supply. Display, recognition and heat control are possible functions.
The four tail spikes are informally called a thagomizer. Strong muscles could swing them at attackers. An Allosaurus tail bone with a spike shaped injury suggests such blows occurred.
Stegosaurus was a herbivore with a beak at the front of its jaws and small, leaf shaped teeth farther back. Its jaws were better suited to simple up and down bites than the complex grinding action of later duck billed dinosaurs.
It probably browsed low vegetation such as ferns, cycads and soft shoots. The low head position suggests that much of its food grew near the ground. Scientists use tooth wear, jaw mechanics and available fossil plants to study diet, but no menu can be exact.
Food was likely broken into pieces and swallowed for digestion in a large gut. There is no good evidence that Stegosaurus chewed plants as thoroughly as a modern grazing mammal.
Stegosaurus is known from skulls, teeth, vertebrae, limbs, plates, spikes and fairly complete skeletons. Plates preserved near their original position reveal their arrangement.
Sophie is a 5.6 metre young adult with an estimated mass of about 1.6 tonnes. Larger adults reached 7 to 9 metres and several tonnes.
Bone tissue reveals growth, joints suggest movement and injuries record ancient events. Soft tissues, colour and behaviour rarely survive directly.
The main debate concerns the plates. Display is plausible because they were highly visible. Blood vessels may have aided temperature control, but do not prove cooling was the main purpose.
Scientists ask whether plate arrangements varied by species, age or individual. A keratin covering could also have changed their shape and colour.
Researchers continue to refine estimates of mass, growth and neck movement as evidence improves.
Stegosaurus did not have a second brain. A widened spinal space may have held tissue linked with energy storage.
Its plates did not lie flat. Fossils show upright, alternating rows.
Stegosaurus lived tens of millions of years before Tyrannosaurus, so they never met.
Display and recognition are leading ideas, while temperature control is also possible. Scientists have not proved one single function.
Yes. The tail was muscular, and an injured Allosaurus bone provides evidence that a spike could cause serious damage.
The familiar Stegosaurus stenops arrangement had four large spikes near the tail tip.
No. It had one brain in its skull. The space near its hips was not a second brain.
It ate plants, probably including low growing ferns, cycads and soft shoots available in its habitat.
Its heavy body and limb structure suggest a fairly slow walking animal, although an exact top speed cannot be measured.
Adults commonly reached about 7 to 9 metres. Individuals varied, and the well known Sophie skeleton is smaller.
No. Stegosaurus lived much earlier, during the Late Jurassic Period.
