The stegosaurus is known for its unusual features: rows of large, bony plates along its back, spikes on its tail and a cranium so small that early paleontologists thought there must be another brain hidden elsewhere, like in a nerve bundle at the base of the dinosaur’s powerful tail.

But it was the purpose of those plates that really baffled scientists—a mystery they’ve been trying to solve for more than a century.

Initially it was thought the plates were for defense. But they cover only the animal’s back, leaving the rest of its body vulnerable.

Then it was thought they were used for thermoregulation. The stegosaurus could face them to the morning sun to absorb its heat. And later face them into the wind to cool down. But subsequent fossil finds didn’t show enough blood vessels in the plates for this to be likely.

The current hypothesis is that they were used to attract mates.

Fossils suggest that stegosaurus plates were attached to their skin, meaning the dinosaur may have been able to move them. The plates could have flopped and waggled when the animal walked. Some paleontologists think they may have been brightly colored.

Other stegosaurs might have found these features very attractive. Much like modern bird plumage.

Or perhaps different plate shapes and sizes were just used by the different stegosaur species to tell one other apart.

Perhaps we may never know. And with a brain that small, the stegosaur may not have either!