page_title: Hallucigenia
section_title: History of study
hierarchical_section_title: Hallucigenia / History of study
caption_attribution_description: Specimen of Hallucigenia Sparsa on display in the Royal Ontario Museum, Toronto.
context_page_description: Hallucigenia is a genus of Cambrian xenusiids known from articulated 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 upside down and back to front. Hallucigenia is now recognized as a "lobopodian worm". It is considered by some to represent an early ancestor of the living velvet worms, although other researchers favour a relationship closer to arthropods.
context_section_description: Hallucigenia was originally described by 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, establishing the new genus. No specimen was available that showed both rows of legs, and as such 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. This raised questions, such as how it would walk on the stiff legs, but it was accepted as the best available interpretation.
  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) 1-metre-long (3 ft) creature. Given the uncertainty of its taxonomy, Hallucigenia was tentatively placed within the phylum Lobopodia, a catch-all taxon containing numerous odd "worms with legs".
  In 1991, Lars Ramskold and Hou Xianguang, working with additional specimens of a "hallucigenid", Microdictyon, from the lower Cambrian Maotianshan shales of China, reinterpreted Hallucigenia as an Onychophoran (velvet worm). They inverted it, interpreting the tentacles, which they believe to be paired, as walking structures and the spines as protective. Further preparation of fossil specimens showed that the 'second legs' were buried at an angle to the plane along which the rock had split, and could be revealed by removing the overlying sediment. 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 artefact of decomposition.
caption: Specimen with obvious spines
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