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Hallucigenia, fossil reconstruction  and imagination

Hallucigenia reconstruction turned upside down !

Most people realise that reconstructions of animals from their fossilised remains is a risky business, and that when there are only a few fossil specimens, opportunities for misinterpretation are many. Add to this an investigator who declares `I have a natural temptation to emphasise the unusual', then the probability of error is greatly increased. All these ingredients are present in the case of an organism named Hallucigenia, first described in 1977 by Conway Morris in the journal Palaeontology.

Everyone familiar with the reconstruction will admit that Hallucigeniawas weird, even surreal. Instead of legs, this worm-like animal had seven pairs of sharply pointed spines. Above its body swayed seven tentacles with pincers at their tips, having a function that is entirely unknown and the subject of speculation. So strange was this animal that some have suggested that the fossilised remains probably represent an appendage of an undiscovered larger organism, but even if this were the case, the situation would be most bizarre!

Hallucigenia comes from to the famous Burgess Shale assemblage of fossils, considered to be of great importance in understanding the evolution of life on Earth. The story of these fossils is recounted in graphic detail by Stephen Jay Gould in his book Wonderful Life, published by Penguin (1991). Gould argues the case for the `stunning disparity and uniqueness of anatomy' that is to be found early in the history of modern multi-cellular life. We might ask - is it feasible to select just one of the Burgess Shale organisms which epitomises the meaning of the whole? It looks as though there is a positive answer, for on 16 May 1991, the science editor of the Daily Telegraph wrote:

However, new evidence has come to light which suggests that Hallucigeniaexists only in our imagination. Lars Ramiskold of the Swedish Museum of Natural History and Hou Xianguang from the Nanjing Institute of Geology and Palaeontology have been studying worm-like fossils from Southern China. They consider that the fossils have a body plan similar to the velvet worms of modern rain forests, but that they were armoured - bearing protective spines. After comparing the characteristics of the Hallucigeniafossils with those of their own, they were able to conclude that the reconstruction was wrong, that Hallucigenia was actually an armoured velvet worm, and that it belongs to the phylum Onychophora.

Their revised reconstruction was published in Nature (16 May 1991). The walking spines were really for protection and were attached to the animal's back. The tentacles are reinterpreted as legs, with the acknowledgement that only a single row is visible in the Burgess Shale specimens. The authors point out politely that `Hallucigenia has therefore previously been oriented upside down'.

The Hallucigenia story has implications which go far beyond the details of the revision. Particular care must be taken when data is limited, for there are fewer constraints on speculative thought. If we arrange the Burgess Shale organisms into two groups, `well-documented' and `poorly documented', we find that a great deal of the evidence for disparity of anatomy belongs to the poorly-documented category which, as Hallucigeniahas shown, is more likely to be the subject of revision. 

Perhaps the most valuable lessons we can learn from these developments concerns way scientific work is undertaken. The presuppositions and interests of scholars are important, because of their effects on the way data is weighed and their relevance assessed. Scientists are not the impartial students of nature they are often theought to be! When it comes to the development of evolutionary ideas and interpretations, the world-views of the people involved appear to be of paramount importance. 

David J. Tyler (1991)

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