Fossil holotype of Hallucigenia sparsa from the Burgess Shale

Hallucigenia is a genus of lobopodian known from Cambrian-aged fossils in Burgess Shale-type deposits in Canada and China, and from isolated spines around the world. The generic name reflects the type species' unusual appearance and eccentric history of study; when it was erected as a genus, H. sparsa was reconstructed as an enigmatic animal upside down and back to front. Lobopodians are a grade of Paleozoic panarthropods from which the velvet worms, water bears, and arthropods arose.

Hallucigenia is a 0.5–5.5 cm (3⁄16–2+3⁄16 in) long tubular animal with up to ten pairs of slender legs (lobopods). The first 2 or 3 leg pairs are slender and featureless, while the remaining 7 or 8 pairs each terminate with 1 or 2 claws. Above the trunk region are 7 pairs of rigid conical sclerites (spines) corresponding to the 3rd–9th leg pairs. The trunk is either featureless (H. sparsa) or divided by heteronomous annulations (H. fortis and H. hongmeia). The "head" and "tail" end of the animal are difficult to identify; one end extends some distance beyond the legs and often droops down as if to reach the substrate. Some specimens display traces of a simple gut.

Research in the mid-2010s clarified that the longer end is a head with anteroventral mouth and at least a pair of simple eyes. The shape of head differs between species – elongated in H. sparsa, rounded in H. fortis, while those of H. hongmeia remain unknown. At least in H. sparsa, the head possesses radial teeth and pharyngeal teeth within the front of the gut.

Hallucigenia's spines are made up of one to four nested elements. The spine surface of H. sparsa is covered in an ornament of minute triangular "scales", while the spine surface of Hallucigenia hongmeia is a net-like texture of microscopic circular openings, which can be interpreted as the remains of Papillae.

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Pub: 27 Oct 2023 09:12 UTC

Edit: 30 Aug 2025 16:23 UTC

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