In 1977, British palaeontologist Simon Conway-Morris discovered the fossil of a truly weird animal, which he named Hallucigenia because of its “bizarre and dream-like quality”. He wasn’t kidding. The creature was so strange that it took fourteen years for scientists to work out which way up it stood. And now, nearly four decades after its original discovery, we finally know—clearly and conclusively—which end is the head.

Conway-Morris guessed that the creature used its spines like stilts for walking along the ocean floor, while the tentacles plucked food out of the overlying water. Then, in 1991, a set of related Chinese fossils showed that Conway-Morris’ reconstruction was upside-down. The “tentacles” were legs. The “mouths” on their tips were claws. And the