Hallucigenia

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Canadia sparsa

Hallucigenia is an extinct genus of animal found as fossils in the Middle Cambrian-aged Burgess Shale formation of British Columbia, Canada, represented by the species H. sparsa, and in the Lower Cambrian Maotianshan shale of China, represented by the species H. fortis. The genus name was coined by Simon Conway Morris when he re-examined the various specimens of Charles Walcott's Burgess Shale worm genus Canadia in 1979. Conway Morris found that what Walcott had called one genus in fact included several quite different animals. One of them was so unusual that nothing about it made much sense. Since the species clearly was not a polychaete worm, Conway Morris had to provide a new generic name to replace Canadia. Conway Morris named the species Hallucigenia sparsa because of its "bizarre and dream-like quality" (like a hallucination). Hallucigenia was initially considered by Stephen Jay Gould to be unrelated to any living species, but most palaeontologists now believe that the species was an ancestor of modern arthropods.[1][2]

Contents

Description and investigation

109 specimens of Hallucigenia are known from the Greater Phyllopod bed, where they comprise 0.3% of the community.[3] The 0.5 to 3 cm-long animal is wormlike — that is, long and narrow — with a poorly defined blob, or stain, on one end. This "blob" was arbitrarily designated the 'head' even though it had none of the features generally associated with heads: mouth, eyes, or other sensory organs. According to Morris' original interpretation, the animal has seven pincer-tipped tentacles lined up on one side and seven pairs of jointed spines on the other. Six of the tentacles were paired with spines, with one in front of the spines. There were also six smaller tentacles which may be configured in three pairs behind the seven larger ones. In addition, the body continued with a flexible, tube-like, tail-like extension behind the tentacles.

Faced with an animal that had no obvious head and two types of appendages, neither of which seemed appropriate for any reasonable form of locomotion, Morris assigned the blob as the head and hypothesized that the spines were legs and that the tentacles were feeding appendages. Morris was able to demonstrate a workable, if improbable, method of walking on the spines. 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. 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.[4] A picture of the animal as reconstructed by Morris can be found at Yvonne Navarro's website.[5].

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