Its name is suggestive of a past age of psychochemical experimentation -- this fossil cautions against overweening confidence of paleontologists who think they know how ancient animals lived

Hallucigenia, one of the weirdest and rarest of the fossils from Walcott's Quarry, has probably received more column space in the scientific and popular press than any of the other Burgess Shale fossils. In 1977, it was the subject of one of Simon Conway Morris' first papers in his revision of the worm-like Burgess Shale fossils. He named this small fossil Hallucigenia and reconstructed it as a cylindrical trunk balanced on seven pairs of slender spikes with a single row of seven tentacles along its back. The head is ill-defined and the anus is located at the tip of a long flexible tail. Other paleontologists and biologists immediately questioned the orientation of this reconstruction, pointing out that no other animal has ever been known to walk on rigid spines. The critics suggested that the tentacles made more sense as legs than did the spikes, but Conway Morris effectively responded that a single row was simply not plausible as legs. Because only a few specimens were available for study, no more data was available and the discussion came to an impasse. Conway Morris' interpretation of Hallucigenia remained -- a bizarre animal without fossil or living relatives.

Then in the late 1980s, Lars Ramsköld, a Swedish dentist turned paleontologist who was studying similar fossils from China, examined most of the available museum specimens of Hallucigenia and proposed that the spines were dorsal and protective in function. If these spines were indeed located along the back of the animal, then the single row of tentacles must be legs, and if they were legs, they must be paired. To test his idea, Ramsköld asked the curator of the Walcott Collection in Washington to let him prepare the type specimen of Hallucigenia sparsa -- that is, to excavate into the shale around the fossil. This was an audacious request. To put it in perspective, it is comparable to an art historian asking the curator of Rembrandt paintings at the Riiksmuseum in Amsterdam to allow him to flake away some of the paint of "The Night Watch" to determine if Rembrandt had painted over the face of the original captain of the company. To his surprise, Ramsköld got permission. After he cautiously removed a few shale chips he discovered that a second row of tentacles lay beneath the first row. By this simple and boldly executed test, Ramsköld's scientific intuition had been confirmed. Virtually all paleontologists now view Hallucigenia as a small animal that walked on seven pairs of slender flexible legs and protected by seven pairs of long thin spines along its back.

By flipping this animal and providing it with legs, Ramsköld also gave it a home. The Onychophora, a phylum of caterpillar-looking animals now living in tropical rainforests, was represented by a single Burgess Shale fossil -- the stumpy-legged Aysheaia which had just been redescribed by Harry Whittington. Aysheaia lacks dorsal spines. It is now joined by Hallucigenia with its long slender spines. Other Cambrian onychophorans with short spines provide a link between the two.

Further reading:

Conway Morris, S.
1998:  The Crucible of Creation: The Burgess Shale and the Rise of Animals. Oxford University Press, 242 p.