How big was Stegosaurus?

Stegosaurus plates

Sophie has 18 plates, and a gap over the tail's base suggests a 19th stood there. The London mount shows 19, one a cast. They stood upright in two rows, left and right in turn, set in the skin, not joined to the skeleton.

The largest is 78.5 cm (2.6 ft) at its longest, in bone, part of it plaster. A smooth layer of horn most likely covered the plates, but its only print comes from one plate of a close relative, Hesperosaurus.

What they were for, nobody knows for sure. Most recent studies favour showing off, a sign by which Stegosaurus could recognise their own kind. Shedding heat, an idea of 1976, was at most a side use. Defence is the weakest idea: the plates were thin, and a tissue study found nothing that would let them move.

Stegosaurus tail spikes: the thagomizer

Sophie's four tail spikes, in two pairs, are 35 to 38 cm (14 to 15 in) of bone, their tips missing, and horn sheaths made them a little longer. They pointed out to the sides and back, close to level, as another skeleton with its tail bones in place suggests, though Sophie's mount holds them raised.

Most likely they were weapons. Of 51 stegosaur spikes from North America's Morrison Formation, about one in ten has a broken tip, some healed. One Allosaurus tail bone even has a hole that a spike tip fits. The nickname thagomizer began as a joke in a Far Side cartoon of the early 1980s, and scientists took it up informally.

Stegosaurus brain, and the “second brain”

For so big a body, Stegosaurus had a very small brain. The space for it in the skull has long been likened to a walnut, by the London museum to a plum, but a reptile's brain does not fill it, so the brain was smaller still.

In the 1880s and 1890s Othniel Marsh found a space in the backbone over the hips at least twenty times as big as the brain's, and called it, carefully, a rear brain case. There was no “second brain”. As in birds, the space most likely held a soft lump of tissue rich in glycogen, a stored sugar. What it did is not settled.

Sophie, the Stegosaurus skeleton in London

Sophie, a Stegosaurus stenops, was found in 2003 on a ranch near Shell, Wyoming, dug up in 2004 and prepared at the Sauriermuseum Aathal in Switzerland. London's Natural History Museum, whose scientists had seen a cast of it at a fair in Tucson in 2012, bought the skeleton with donors' help and has shown it since December 2014. The name honours a donor's daughter: whether Sophie was male or female, nobody knows.

With about 360 bones, Sophie is one of the most complete skeletons of Stegosaurus known. Missing are the left arm, both hands, most of the tail's base and some smaller bones, as well as skin and throat armour. The skull on show is a cast. How Sophie died, nobody knows: the bones show no bite or wound, and the scientists who described them consider starvation or an illness the likeliest causes.

The skeleton picture shows which of our bones are Sophie's own.

1.70 m

What a real Stegosaurus looked like

Stegosaurus vs T. rex and Triceratops

Stegosaurus never met T. rex or Triceratops. About 80 million years lay between the last Stegosaurus and the first T. rex or Triceratops, at least about 75. That is more than the 66 million years between the last T. rex and us, so T. rex is nearer to us in time than to Stegosaurus. Films that show them meeting are fiction. Stegosaurus's own neighbours included the long-necked Diplodocus and the meat-eater Allosaurus.

In the picture, our T. rex is twice as long as our Stegosaurus, whose tallest plate tip stands about as high as our Triceratops's back.

• Our Stegosaurus, 5.6 m

1.70 m

• Our Triceratops, 6.9 m
• Our T. rex, after SUE, about 11.4 m
• The person, 1.70 m

When and where Stegosaurus lived

Stegosaurus lived in western North America late in the Jurassic, and one skeleton comes from Portugal. The American finds lie in the Morrison Formation, whose dated layers are about 157 to 150 million years old, the top a little younger. Sophie lived about 150 million years ago, by the London museum's figure. Its fossils end before the Cretaceous, which began about 143 million years ago, and nobody knows why.

In the museum

Museum of Ages is a digital museum, made by a nonprofit. Free entry, no account. In the animal wing this animal will stand among the others, from the giants of long ago to the last of their kind. The Leonardo da Vinci wing shows what a finished wing looks like.

To the Leonardo wing

Use our pictures

The four pictures of the Stegosaurus on this page, the side view, the plates and spikes, the skeleton and the three with our Triceratops and our T. rex, are our own work under CC BY-SA 4.0, credit: Museum of Ages, and for the picture of all three also Triceratops outline after Dempsey 2025 (CC BY 4.0). They show a reconstruction, built in code and rendered in our own engine, not photographs. The pictures of the other pages listed here give their credits on those pages.