WHAT WERE THE SPINAL PLATES ON STEGOSAURUS FOR?

Stegosaurus is one of the most recognisable dinosaurs, for one main reason: the big, triangle-shaped plates lining its neck, back and tail. They are arranged in two rows of alternating pairs, and at the tip of the tail they transition into a line of foreboding spikes, each more than 30cm long. The largest plates, located over the back and hips, are the size of coffee tables. Seen in side view, Stegosaurus cuts a unique profile. But it is hardly alone – such plates are a defining feature of a wider group called the stegosaurs, comprised of Stegosaurus and its closest fossil relatives like Kentrosaurus and Huayangosaurus.

Ever since the first Stegosaurus skeletons were found in western North America during the ‘Bone Wars’ of the 1870s, palaeontologists have debated the function of the plates. One idea was that they were defensive structures – armour to fend off the bites of the hatchet-skulled apex predator Allosaurus. However, some questioned whether protective plating sticking up from the back, rather than guarding the flanks, would actually be effective. More recently, researchers have posited other explanations. Stegosaurs vary widely in the size, shape and distribution of their plates, which hints that they may have been used as display structures, to signify species membership, attract mates, or intimidate rivals. Histological thin sections of the plates show their outer layer is engulfed with channels for blood vessels and nerves, which may have enabled them to act as solar panels and heat dumps, to help control metabolism. However, the dense vascularisation may have simply been used to help the sheath that once covered the bony plate – made of keratin, the same stuff as our fingernails – grow rapidly or even change colour, which would be useful for display.

Although there is no firm answer, it seems display and perhaps thermoregulation are more likely functions than defence.