Temporal range: Cambrian Stage 3–Middle Cambrian
|Artist's rendering of Hallucigenia according to the modern interpretation|
|Fossil of Hallucigenia from the Burgess shale|
Conway Morris, 1977
Hallucigenia is a genus of Cambrian animals known from articulated fossils in exceptional Burgess Shale-type deposits in Canada and China, and from isolated spines around the world. Its quirky name reflects its unusual appearance and eccentric history of study; when it was erected as a genus, the animal was reconstructed upside down and back to front. Hallucigenia is now recognized as a "lobopodian worm" and is considered to represent an early ancestor of the living velvet worms, close relatives of arthropods.
Hallucigenia is a 0.5—3.5 cm long tubular organism with seven or eight pairs of slender legs, each terminating with a pair of claws. Above each leg is a rigid conical spine. The 'head' and 'tail' end of the organism are difficult to identify; one end extends some distance beyond the legs and often droops down as if to reach the floor. Although some specimens display traces of a gut, the internal anatomy has not been formally described.
Hallucigenia's spines are made up of one to four nested elements. Their surface is covered in an ornament of minute triangular 'scales'.
History of study
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. A picture of the animal as reconstructed by Morris can be found here.
An alternative interpretation considered Hallucigenia to be an appendage of a larger, unknown animal. There had been precedent for this, as the species Anomalocaris had been originally identified as three separate creatures before being identified as a single huge (for its time) 3-foot-long (0.91 m) creature. Given the uncertainty of its taxonomy, Hallucigenia was tentatively placed within the phylum Lobopodia, a catch-all clade 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.
Hallucigenia has a similar body form to the onychophorans, with the prominent exception of its spines. Ironically, these spines provide the most compelling evidence for a link to Onychophora: their nested construction has no analogue except for the claws of velvet worms. Nevertheless, the homology between these superficially different structures has yet to be conclusively demonstrated.
In 2002, Desmond Collins informally suggested that new Hallucigenia fossils from the Burgess Shale showed male and female forms, one with "a rigid trunk, robust neck and a globular head" and the other thinner, and with a small head.
Further species are represented from the Chengjiang.
109 specimens of Hallucigenia are known from the Greater Phyllopod bed, where they comprise 0.3% of the community. Hallucigenia also forms a minor component of Chinese lagerstätten. Isolated hallucigeniid spines, however, are widely distributed in a range of Cambrian deposits, preserved both as carbonaceous and mineralized fossils.
- Conway Morris, S. (1977). "A new metazoan from the Cambrian Burgess Shale of British Columbia" (PDF). Palaeontology 20: 623–640. http://palaeontology.palass-pubs.org/pdf/Vol%2020/Pages%20623-640.pdf.
- Steiner, M.; Hu, S.; Liu, J.; Keupp, H. (2012). "A new species of Hallucigenia from the Cambrian Stage 4 Wulongqing Formation of Yunnan (South China) and the structure of sclerites in lobopodians". Bulletin of Geosciences 87: 107–124. doi:10.3140/bull.geosci.1280.
- Caron, J. -B.; Smith, M. R.; Harvey, T. H. P. (2013). Proceedings of the Royal Society B: Biological Sciences 280 (1767). doi:10.1098/rspb.2013.1613.
- Gould, Stephen Jay (1989). Wonderful life: the Burgess Shale and the nature of history. New York: W.W. Norton. ISBN 0-393-02705-8.
- Ramsköld, L. (1992). "The second leg row of Hallucigenia discovered". Lethaia 25 (2): 221–224. doi:10.1111/j.1502-3931.1992.tb01389.x.
- Ramsköld, L.; Hou, X.-G. (1991). "New early Cambrian animal and onychophoran affinities of enigmatic metazoans". Nature 351 (6323): 225–8. Bibcode:1991Natur.351..225R. doi:10.1038/351225a0.
- Connor, Steve (16 December 2002). "Scientists see the light on the 'weirdest' fossil". The Independent. Retrieved 23 October 2009.
- Caron, J. -B.; Jackson, D. A. (October 2006). "Taphonomy of the Greater Phyllopod Bed community, Burgess Shale". PALAIOS 21 (5): 451–465. doi:10.2110/palo.2003.P05-070R.
- "Hallucigenia sparsa". Burgess Shale Fossil Gallery. Virtual Museum of Canada. 2011.
- http://www.nmnh.si.edu/paleo/shale/phallu.htm - (Internet Archive)
- http://www.arrakis.es/~owenwang/articulos/pasados.htm (Spanish)