Half a billion years ago, an underwater avalanche of mud buried a number of living creatures, preserving imprints of soft parts as well as bones. Zoom forward 500 million years, to 1909, and the palaeontologist Charles Walcott discovers a rock—later known as the Burgess Shale—in what has now become British Columbia, Canada. In it were fossils extraordinary both for their type and their well-preserved state. Many of them were species never before known to science, some with absolutely bizarre physiological features, such as Opabinia, a sea-dwelling creature with five eyes, a backwards-facing mouth on the underside of its head, and a proboscis used to feed. But perhaps none of the creatures were more puzzling than Hallucigenia.
This creature is so weird, it seems like it was taken from a dream, hence the name. When Simon Conway Morris examined it in 1977, he figured it had seven pairs of legs and tentacles extending from its back, with a blob-like head at one end. Then, in the late 1980s, the Swedish paleontologist Lars Ramsköld hypothesized that the “feet” were actually protective spikes on the back of the creature. He discovered a second pair of tentacles, flipping Hallucigenia upside down and making the tentacles on the back into a kind of legs. The “head” is now believed to be a dark spot, an artifact of the fossilization that doesn’t correspond to a feature of the creature.
There are still fossils from the Burgess Shale that are not fully understood. (Images originally from here.)