Stegosaurus: the plated, spiked grazer of the Late Jurassic

When and where

Stegosaurus lived during the Late Jurassic, between 155 and 150 million years ago, across the Morrison Formation of the western United States and the Lourinhã Formation of Portugal. The two regions sat on opposite sides of a then-narrow North Atlantic, and the same genus on both shores tells you the ocean had not yet widened enough to act as a faunal barrier. In North America the genus shared its range with Allosaurus, Diplodocus, and Brachiosaurus, making it part of one of the densest large-dinosaur ecosystems known.

How we know

Othniel Charles Marsh named Stegosaurus armatus in 1877 during the so-called Bone Wars, working from material found at Garden Park, Colorado. The most complete specimen, "Sophie," was excavated in Wyoming in 2003 and acquired by London's Natural History Museum in 2013, where the mount now stands as the most complete Stegosaurus ever found, with 85 per cent of the bones preserved. CT scans of Sophie's skull confirmed the long-running estimate that the brain weighed only around 80 grams, walnut-sized for an animal weighing several tonnes. The popular myth of a "second brain" in the hip came from an enlarged neural canal there, now interpreted as a glycogen body, the same structure modern birds carry. Source: Wikipedia entry on Stegosaurus.

What set it apart

Two features carry the silhouette: 17 staggered kite-shaped plates running in a double row along the back, and a four-spike tail. The plates were not solid armour, since their thin walls are full of vascular grooves, suggesting they served either thermal regulation or display rather than physical defence. The tail spikes, by contrast, were combat weapons, and Allosaurus vertebrae have been found with Stegosaurus-spike-shaped puncture wounds. The arrangement of the spikes, two pairs angled outward, was named the "thagomizer" by cartoonist Gary Larson in 1982 and adopted by working paleontologists shortly after. Adult Stegosaurus reached 9 metres long and 2 to 7 tonnes depending on growth stage and species.

For collectors and classrooms

A scaled Stegosaurus figurine works on a shelf because the plate-and-spike profile reads at a glance. Look for one with the plates staggered in two rows, not lined up symmetrically, since recent fossil articulation supports the staggered pose. Browse a hand-painted figurine.