Hallucigenia
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Hallucigenia is a genus of lobopodian known from Cambrian aged fossils in Burgess Shale-type deposits in Canada and China, and from isolated spines around the world.<ref name="NYT-20150702">Template:Cite news</ref> The generic name reflects the type species' unusual appearance and eccentric history of study; when it was erected as a genus, H. sparsa was reconstructed as an enigmatic animal upside down and back to front.<ref name=ConwayMorris1977 /> Lobopodians are a grade of Paleozoic panarthropods from which the velvet worms, water bears, and arthropods arose.<ref name="Ortega-Hernández-2015">Template:Cite journal</ref><ref name="NYT-20150702" />
Description
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Hallucigenia is a Template:Cvt<ref name="Liu-2014">Template:Cite journal</ref><ref name="Smith-2015" /> long tubular animal with up to ten pairs of slender legs (lobopods). The first 2 or 3 leg pairs are slender and featureless,<ref name="Liu-2014" /><ref name="Smith-2015" /><ref name="Siveter-2018">Template:Cite journal</ref> while the remaining 7 or 8 pairs each terminate with 1 or 2 claws.<ref name="Steiner-2012" /><ref name="Smith-2015" /><ref name="Siveter-2018" /> Above the trunk region are 7 pairs of rigid conical sclerites (spines) corresponding to the 3rd–9th leg pairs.<ref name="Steiner-2012" /><ref name="Liu-2014" /><ref name="Smith-2015" /><ref name="Siveter-2018" /> The trunk is either featureless (H. sparsa)<ref name="Smith-2015" /> or divided by heteronomous annulations (H. fortis<ref name="HOU-1995">Template:Cite journal</ref> and H. hongmeia).<ref name="Steiner-2012" /><ref name="Smith-2014" /> The "head" and "tail" end of the animal are difficult to identify; one end extends some distance beyond the legs and often droops down as if to reach the substrate. Some specimens display traces of a simple gut.<ref name="Smith-2015" />
Research in the mid-2010s clarified that the longer end is a head with anteroventral mouth and at least a pair of simple eyes.<ref>Template:Cite journal</ref><ref name="Liu-2014" /><ref name="Smith-2015" /> The shape of head differs between species – elongated in H. sparsa, rounded in H. fortis,<ref name="Liu-2014" /><ref name="Smith-2015" /> while those of H. hongmeia remain unknown.<ref name="Steiner-2012" /> At least in H. sparsa, the head possesses radial teeth and pharyngeal teeth within the front of the gut.<ref name="Smith-2015">Template:Cite journal</ref><ref>Morelle, Rebecca, Face of bizarre sea creature Hallucigenia revealed, BBCNews, Science and Environment, 2015.06.25</ref>
HallucigeniaTemplate:'s spines are made up of one to four nested elements. The spine surface of H. sparsa is covered in an ornament of minute triangular "scales",<ref name="Caron2013">Template:Cite journal</ref> while the spine surface of Hallucigenia hongmeia is a net-like texture of microscopic circular openings, which can be interpreted as the remains of Papillae.<ref name="Steiner-2012">Template:Cite journal</ref>
History of study
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Hallucigenia sparsa was originally described by Charles Walcott as a species of the polychaete worm Canadia.<ref>WALCOTT, C. 1911. Cambrian Geology and Paleontology II. Middle Cambrian annelids. Smithsonian Miscellaneous Collections, 57(5): 109–145.</ref> In his 1977 redescription of the organism, Simon Conway Morris recognized the animal as something quite distinct, for which he proposed the name Hallucigenia because of the "bizarre and dream-like appearance of the animal." No specimen was available that showed both rows of legs, so 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.<ref name=ConwayMorris1977 /> This raised questions, such as how it would walk on the stiff legs, but it was accepted (with reservations) as the best available interpretation.<ref name="Gould">Template:Cite book</ref>
An alternative interpretation considered Hallucigenia to be an appendage of a larger, unknown animal. There had been precedent for this, as Anomalocaris had been originally identified as three separate creatures before being identified as a single huge (for its time) Template:Convert to Template:Convert creature.<ref name="Gould"/>
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 a lobopodian, a legged worm-like taxon which were still thought to be exclusively related to onychophoran (velvet worm), carnivorous animals that resemble a caterpillar and shoot a sticky substance from their papillae to ensnare their prey,<ref>Template:Cite journal</ref> at that time.<ref name="Ramsköld-1991" /><ref name="Ortega-Hernández-2015" /> They inverted it, interpreting the tentacles, which they believe to be paired, as walking structures and the spines as protective.<ref name="Ramsköld-1991" /> Further preparation of fossil specimens showed that "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.<ref name="Ramsköld-1992">Template:Cite journal</ref> 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.<ref name="Ramsköld-1991">Template:Cite journal</ref> This stain may be an artifact of decomposition.<ref name="Smith-2015" />
Affinity
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Template:External media Since the revisions around 1990s,<ref name="Ramsköld-1991" /><ref name="Ramsköld-1992" /><ref name="HOU-1995" /> Hallucigenia is unquestionably a lobopodian panarthropod, although the relationship with other panarthropods remains controversial. Hallucigenia has long been interpreted as a stem-group onychophoran (velvet worms) – a position that has found support from multiple phylogenetic analysis.<ref name="Smith-2014" /><ref name="Smith-2015" /><ref name="Yang-2015" /><ref>Template:Cite journal</ref> A key character demonstrating this affinity is the cone-in-cone construction of Hallucigenia claws, a feature shared only with modern onychophorans.<ref name="Smith-2014">Template:Cite journal</ref> On the other hand, some analysis rather support the position of Hallucigenia as a basal panarthropod outside of onychophoran stem-group.<ref>Template:Cite journal</ref><ref name="Siveter-2018" /><ref name="Caron-2020">Template:Cite journal</ref> Under this scenario, the cone-in-cone structure shared between Hallucigenia and onychophorans represent panarthropod plesiomorphy.<ref name="Siveter-2018" /><ref name="Caron-2020" /> Hallucigenia also exhibits certain characters inherited from the ancestral ecdysozoan, but lost in the modern onychophorans – in particular its distinctive foregut armature.<ref name="Smith-2015" />
Below is a cladogram for Hallucigenia according to Yang et al., 2015:<ref name="Yang-2015">Template:Cite journal</ref>
Diversity
[edit]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.<ref>Template:Cite journal</ref>
Three species of Hallucigenia have been described. The first specimen, Hallucigenia sparsa, was discovered in Canada. Two other species, H. fortis and H. hongmeia, are represented by the Maotianshan Shales' fossils of Chengjiang.<ref name="HOU-1995" /><ref name=Steiner12/>
Distribution
[edit]Hallucigenia was first described from the Burgess Shale in southeastern British Columbia, Canada. 109 specimens of Hallucigenia are known from the Greater Phyllopod bed, where they comprise 0.3% of the community.<ref name=Caron2006>Template:Cite journal</ref> 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.<ref name=Caron2013/>
See also
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[edit]Further reading
[edit]External links
[edit]Template:Commons category Template:Wikispecies
- Template:Cite web
- Hallucigenia sparsa at Smithsonian Museum of Natural History – (Internet Archive)
- Fossils found in Burgess Shale (in Spanish)
- A picture of Hallucigenia
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