Hallucigenia was drawn upside down and backwards before anyone got it right
When Simon Conway Morris named a tiny Cambrian fossil in 1977, he pictured it tottering on stiff spines with tentacles waving from its back. The name he chose, Hallucigenia, honoured how dreamlike it looked. It took until 1991 to flip the creature over, and the mid-2010s to find which end was its head.
The animal is a lobopodian, a legged worm from the group of early panarthropods that gave rise to velvet worms, water bears and arthropods. It measured between half a centimetre and five and a half centimetres. Along its tube-shaped body ran up to ten pairs of slender legs, most ending in one or two claws, and above them seven pairs of rigid conical spines.
Its story started with a misfiling. Charles Walcott first described the Burgess Shale specimens as a species of the bristle worm Canadia. Conway Morris saw something stranger, but no fossil he had showed both rows of legs, so he read the single visible row as tentacles and the spines as stilts. A dark smudge at one end became a featureless head. Some researchers even wondered whether it was merely a piece of a bigger animal, as had happened with Anomalocaris, once split into three supposed creatures.
The correction came from China. Working with fossils of a related animal, Microdictyon, from the Maotianshan shales, Lars Ramskold and Hou Xianguang turned the reconstruction over in 1991: the tentacles were paired walking legs and the spines were armour. Careful preparation showed a hidden second row of legs lying at an angle beneath the rock surface. The smudge, they argued, was likely a decomposition stain. Later work established that the longer end really is a head, with a mouth underneath, at least one pair of simple eyes and, in H. sparsa, rings of teeth leading into the gut.
Three species are now known: H. sparsa from Canada, and H. fortis and H. hongmeia from the Chengjiang fossils. At one Burgess Shale bed, 109 specimens make up just 0.3 percent of the community, though loose spines turn up in Cambrian rocks worldwide. A 2014 study of its claws, built in nested cones like those of modern velvet worms, placed it as an early relative of that group, though the debate continues.
Source: Hallucigenia
