What Was Stegosaurus? Plates, Spikes and a Brain the Size of a Plum
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The first Stegosaurus was dug out of a Colorado hillside in 1877 and named roof lizard because its describer thought the plates had lain flat on its back like tiles. They stood upright, in two staggered rows from neck to tail, and what they were for has been argued about ever since, along with the four spikes on the tail, the tiny head on the end of the arched back, and the report, repeated in textbooks for a century, that the animal had a second brain in its hips. It is the state fossil of Colorado, the most recognisable dinosaur after Tyrannosaurus, and the one about which the most confident claims have been withdrawn.
The animal
Stegosaurus lived in the Late Jurassic, about 155 to 145 million years ago, in the floodplains of the American West that are now the Morrison Formation, alongside Allosaurus, Diplodocus and Brachiosaurus, and relatives are known from Portugal and China. The largest species reached about nine metres and five tonnes. It walked on four legs with the hind pair much longer than the front, so that the back arched up to a peak over the hips and the head hung low near the ground, and its skull was small and narrow with a toothless beak and rows of small leaf-shaped teeth for cropping low vegetation, ferns, cycads and horsetails, which it swallowed without much chewing and fermented in a large gut. It could not have raised its head much above a metre, and its whole design was for feeding at knee height.
The plates
There were seventeen plates in two rows, the largest about 60 centimetres across, thin, and not attached to the skeleton at all but embedded in the skin, which is why they were found scattered and why their arrangement took decades to settle. What they were for:
- •Display: the plates are thin, arranged to be seen from the side, and different in shape between the two known species, which is the pattern of a signalling organ; they would have made the animal look larger and told other stegosaurs which species and how mature it was
- •Heat regulation: the plates are riddled with channels for blood vessels, which suggested radiators or solar panels, but later studies found the vessels of a kind that supply growing bone rather than exchange heat, and the idea has faded
- •Not armour: they are too thin to stop a bite and cover only the top of the animal, though they may have made it awkward to attack from the side
The tail
The tail spikes were weapons. Four spikes up to 90 centimetres long stuck out sideways from the end of the tail, and a study of their strength, and of an Allosaurus vertebra with a puncture wound that matches one exactly, shows that Stegosaurus swung its tail at attackers and connected; about ten percent of the spikes found have damage from use. The spikes are called the thagomizer, a name given in a 1982 Far Side cartoon in which a caveman lecturer explains that it is named after the late Thag Simmons, and adopted by palaeontologists at a conference some years later, so that the joke is now the technical term.
The brain that was not there
Stegosaurus's brain was about the size of a plum, some 80 grams in a five-tonne animal, the smallest for its size of any dinosaur, and Othniel Marsh, who named the animal, noticed a large cavity in the vertebrae over the hips, twenty times the volume of the skull's braincase, and proposed in 1881 that it held a second brain to run the back half of the animal. The idea went into every book for a century. The cavity is real, and it is now thought to have held a glycogen body, a mass of energy-storing tissue that birds have in the same place and whose function is uncertain even in them, along with the nerve plexus that controls the hind legs and tail, which is enlarged in any animal with a big tail. There was one brain, and it was small; the animal that ran on it survived for ten million years, which is a long time to be stupid.
Since
Stegosaurus stood at the Natural History Museum in London as a plaster cast for decades and since 2014 as Sophie, the most complete skeleton known, 85 percent of an animal found in Wyoming in 2003, and the scan of that skeleton settled the plates' arrangement and gave the first reliable estimate of the animal's weight. Its relatives, the stegosaurs, were a family that flourished in the Jurassic and dwindled in the Cretaceous, giving way to the armoured ankylosaurs, and the last of them died out sixty million years before the asteroid; the roof lizard whose roof stood on end is a Jurassic animal through and through, the contemporary of the giant sauropods rather than of Triceratops, with which it is so often drawn.
The takeaway
Stegosaurus was a nine-metre, five-tonne plant-eater of the Late Jurassic American West, low-headed and high-hipped, with two rows of thin bony plates along its back that were most likely for display, and four tail spikes that it used as a weapon, as an Allosaurus bone punctured by one shows. Its plum-sized brain was the smallest of any dinosaur for its size, the cavity over its hips held a glycogen body rather than a second brain, and it was extinct long before the Cretaceous.