Hallucigenia is a genus of lobopodian known from Cambrian-aged fossils in Burgess Shale-type deposits in Canada (Burgess Shale) and China, and from isolated <!--claws and--> 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. while the remaining 7 or 8 pairs each terminate with 1 or 2 claws. and H. hongmeia).
Hallucigenia 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.
History of study
thumb|Various interpretations of Hallucigenia sparsa throughout the history of study
Hallucigenia sparsa was originally described by Charles Walcott as a species of the polychaete worm Canadia. In his 1977 redescription of the organism, Simon Conway Morris recognized the animal as something quite distinct, for which he proposed the name Hallucigenia because of the "bizarre and dream-like appearance of the animal." No specimen was available that showed both rows of legs, so Conway Morris reconstructed the animal walking on its spines, with its single row of legs interpreted as tentacles on the animal's back. A dark stain at one end of the animal was interpreted as a featureless head. Only the forward tentacles could easily reach to the "head", meaning that a mouth on the head would have to be fed by passing food along the line of tentacles. Conway Morris suggested that a hollow tube within each of the tentacles might be a mouth.
thumb|Specimen with obvious spines
An alternative interpretation considered Hallucigenia to be an appendage of a larger, unknown animal. There had been precedent for this, as Anomalocaris had been originally identified as three separate creatures before being identified as a single huge (for its time) to creature. Ramskold and Hou also believe that the blob-like "head" is actually a stain that appears in many specimens, not a preserved portion of the anatomy. This stain may be an artifact of decomposition. A key character demonstrating this affinity is the cone-in-cone construction of Hallucigenia claws, a feature shared only with modern onychophorans. On the other hand, some analysis rather support the position of Hallucigenia as a basal panarthropod outside of onychophoran stem-group. Under this scenario, the cone-in-cone structure shared between Hallucigenia and onychophorans represent panarthropod plesiomorphy.
Diversity
In 2002, Desmond Collins informally suggested that new Hallucigenia fossils from the Burgess Shale showed male and female forms, one with "a rigid trunk, robust neck and a globular head" and the other thinner, and with a small head.
Three species of Hallucigenia have been described. The first specimen, Hallucigenia sparsa, was discovered in Canada. Two other species, H. fortis and H. hongmeia, are represented by the Maotianshan Shales' fossils of Chengjiang. Hallucigenia also forms a minor component of Chinese lagerstätten. Isolated hallucigeniid spines, however, are widely distributed in a range of Cambrian deposits, preserved both as carbonaceous and mineralized fossils.