Hallucigenia is one of the more unusual organisms found within the Burgess Shale and its exact phylogeny is still subject to conjecture. Long assumed to have no living descendants it is now thought to be indirectly related to modern velvet worms.
Hallucigenia was originally considered so odd that it could only be imagined as part of an hallucination, thus its unusual name. However, later examinations showed that researchers had been looking at the remains upside down and what were initially thought to be uniquely styled legs were in fact spines that grew up from its back (Conway Morris, 1977). This rearranging of the information helped to explain Hallucigenia’s anatomy though it is still regarded as bizarre.
In addition to the spines and lopobod style legs, more recent examinations of Hallucigenia fossils have identified a series of unusual feeding appendages and possible eye spots (Smith &Caron, 2015).
That soft bodied creatures such as Hallucigenia should have left an impression in the fossil record is remarkable in itself. By their nature fossilised remains are rare and they tend to be of hardened anatomical parts such as shell. It is because of this bias in the fossil record that palaeontologists suspect we are only ever looking at a small proportion of the life that was present at any one time.
It takes especially rare conditions to allow for the preservation of soft bodied tissue. Essentially creatures have to be caught up in super fine anoxic silt (a low oxygen environment). These sediments are known as 'lagerstatten' and only a few have been found that contain palaeozoic life. The most famous of these would be the Burgess Shale but they also include the Maotianshan Shale of China and the Emu Bay Shale of South Australia. One of the most remarkable observations about these rare and limited deposits is the great amount of life that they contain. The Burgess Shale alone has unearthed well over 65,000 specimens.
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