Imagine learning that scientists had got Tyrannosaurus rex upside-down all this time and, in reality, it slithered on its “back” and waved its legs—sorry, tentacles—in the air. It’d be a bit embarrassing, but it would also never happen. We have some complete Tyrannosaurus skeletons, we can compare these to similar modern animals, and we can simulate the forces that acted upon them. It’s obvious which way up the animal stood.

But not all prehistoric creatures are so easily reconstructed. In 1977, the British palaeontologist Simon Conway-Morris stumbled across a truly bizarre finger-sized fossil, unlike anything that had ever been seen before. It had been found 66 years earlier in the Burgess Shale—a legendary fossil field in the Canadian Rockies—and classified as