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Hallucigenia

Hallucigenia sparsa is an extinct animal genus so named because when Simon Conway Morris[?] re-examined Walcott[?]'s Burgess Shale genus Canadia[?] in 1979, he found that it included several quite different animals. One of them was so unusual that nothing about it made much sense. Since it clearly was not a polychaete worm, Morris had to provide a new generic name to replace Canadia[?]. Morris named it Hallucigenia because of it's 'bizarre and dream-like quality' (like a hallucination).

The 2cm or so animal is wormlike -- long and narrow -- with a poorly defined blob on one end that was arbitrarily assigned as the 'head' even though it had -- so far as is known -- none of the features generally associated with heads -- mouth, eyes, other sensory organs, etc. The animal has seven pincher tipped tentacles lined up on one side and seven pairs of jointed spines on the other. Six of the tentacles are paired with spines. One is forward of the spines. There are also six smaller tentacles which may be configured in three pairs in back of the seven larger ones. There is a flexible, tube-like body extension behind the tentacles.

Faced with an animal that has no obvious head and two types of appendages neither of which seems appropriate for any reasonable form of locomotion, Morris somewhat arbitrarily assigned the blob as the head and hypothesized that the spines were 'feet' 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 can 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 is a less than satisfactory reconstruction, but it was accepted as the best available. A picture of the animal as reconstructed by Morris can be found at:

http://www.yvonnenavarro.com/hallug.htm

An alternative interpretation favored by some paleontologists was that Hallucigenia is an appendage of some larger, unknown, animal.

In 1991 Ramiskold and Hou Xianguang working with additional specimens of a 'Hallucigenid' from the Lower Cambrian Maotianshan shales of China reinterpreted Hallucigenia as an Onychophore. They inverted it, interpreting the tentacles, which they believe to be paired, as the walking structures and the spines as protective. Inexplicably none of the 30 or so known Burgess Shale specimens shows any obvious sign of pairing in the large tentacles. Nor do their Chinese counterparts. The pairing is based on dissection through the fossil to reveal what is probably a second tentacle structure. Ramiskold and Hou also believed that the 'head' is actually a stain that appears in many specimens, not a preserved portion of the anatomy.

Ramiskold and Hou's is the accepted modern interpretation, but is far from problem free. Unlike its contemporary Aysheaia, Hallucigenia has very little resemblance to modern Onychophora. The possibly paired pincher tipped tentacles bear little resemblance to the paired annulated legs of the Onychophora. It is unknown what the spines were made of and how much 'protection' they offered. They do not seem to be preserved independent of the soft shelled animals as carbonate or chitinous shells would probably be. It is not easy to explain why 30 or more specimens -- each hypothecated to have seven pairs of rather long flexible, feet do not show even one example of paired "feet". But at least this reconstruction can plausibly walk and the spines serve a reasonable purpose. A picture of this reconstruction as well as a photograph of an actual fossil can be seen here (http://www.nmnh.si.edu/paleo/shale/phallu.htm).

Some paleontologists accept Ramiskold and Hou's interpretation of the animals legs, spines, and head, but believe that Hallucigenia might be an 'armored lobopod' related to Anomalocaris rather than (or as well as) being related to the Onychophora. It remains possible that Hallucigenia is an appendage of a larger creature.



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