Editing Hallucigenia

{{Short description|Genus of Cambrian animals}}
{{For|the album|Hallucigenia (album){{!}}''Hallucigenia'' (album)}}
{{Use dmy dates|date=December 2022}}
{{Automatic taxobox
| fossil_range      = {{Fossil range |Cambrian Stage 3|Middle Cambrian}}
| image             = USNM PAL 83935 Hallucigenia sparsa Image 1.jpg
| image_caption     = Fossil [[holotype]] of ''Hallucigenia sparsa'' from the [[Burgess Shale]]
| taxon             = Hallucigenia
| authority         = Conway Morris, 1977<ref name=ConwayMorris1977>{{Cite journal |title=A new metazoan from the Cambrian Burgess Shale of British Columbia |last=Conway Morris |first=S. |year=1977 |journal=Palaeontology |volume=20 |pages=623–640 |url=https://cdn.palass.org/publications/palaeontology/volume_20/pdf/vol20_part3_pp623-640.pdf |access-date=27 January 2021 |archive-date=31 March 2017 |archive-url=https://web.archive.org/web/20170331003831/http://cdn.palass.org/publications/palaeontology/volume_20/pdf/vol20_part3_pp623-640.pdf |url-status=dead }}</ref>
| display_parents   = 3
| subdivision_ranks = Species
| subdivision       = {{Species list
|H. sparsa|(Walcott, 1911) ([[Type species|type]])
|H. fortis|Hou & Bergström, 1995<ref name="HOU-1995" />
|H. hongmeia |Steiner et al 2012<ref name=Steiner12>{{Cite journal |first1=M. |last1=Steiner |first2=S. |last2=Hu |first3=J. |last3=Liu |first4=H. |last4=Keupp |journal=Bulletin of Geosciences |volume=87 |issue= 1 |pages=107–124 |year=2012 |doi=10.3140/bull.geosci.1280 |url=http://www.geology.cz/bulletin/fulltext/1280_Steiner.pdf |title=A new species of Hallucigenia from the Cambrian Stage 4 Wulongqing Formation of Yunnan (South China) and the structure of sclerites in lobopodians |doi-access=free}}</ref>
}}
| synonyms          = ''[[Canadia (annelid)|Canadia]] sparsa''
}}

'''''Hallucigenia''''' is a genus of [[lobopodian]] known from [[Cambrian]] aged [[Fossils of the Burgess Shale|fossils]] in [[Burgess Shale-type preservation|Burgess Shale-type deposits]] in Canada and China, and from isolated <!--claws and--> spines around the world.<ref name="NYT-20150702">{{cite news |last=Zimmer |first=Carl |author-link=Carl Zimmer |title=The Cambrian Explosion's Strange-Looking Poster Child |url=https://www.nytimes.com/2015/07/07/science/hallucigenia-cambrian-explosions-strange-looking-poster-child.html |date=2 July 2015 |work=[[The New York Times]] |access-date=2 July 2015 }}</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 [[Evolutionary grade|grade]] of [[Paleozoic]] [[panarthropod]]s from which the [[velvet worm]]s, [[water bear]]s, and [[arthropod]]s arose.<ref name="Ortega-Hernández-2015">{{Cite journal|last=Ortega-Hernández|first=Javier|date=2015-10-05|title=Lobopodians|journal=Current Biology|language=English|volume=25|issue=19|pages=R873–R875|doi=10.1016/j.cub.2015.07.028|issn=0960-9822|pmid=26439350|doi-access=free}}</ref><ref name="NYT-20150702" />

==Description==
[[File:20210000 Hallucigenia diagrammatic reconstruction.png|thumb|left|Reconstructions of ''H. fortis'', ''H. hongmeia'', and ''H. sparsa'' in scale.]]

''Hallucigenia'' is a {{cvt|0.5|-|5.5|cm|in|frac=16}}<ref name="Liu-2014">{{Cite journal|last1=Liu|first1=Jianni|last2=Dunlop|first2=Jason A.|date=2014-03-15|title=Cambrian lobopodians: A review of recent progress in our understanding of their morphology and evolution|url=https://www.sciencedirect.com/science/article/pii/S003101821300285X|journal=Palaeogeography, Palaeoclimatology, Palaeoecology|series=Cambrian Bioradiation|language=en|volume=398|pages=4–15|doi=10.1016/j.palaeo.2013.06.008|bibcode=2014PPP...398....4L|issn=0031-0182}}</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">{{Cite journal|last1=Siveter|first1=Derek J.|last2=Briggs|first2=Derek E. G.|last3=Siveter|first3=David J.|last4=Sutton|first4=Mark D.|last5=Legg|first5=David|date=2018|title=A three-dimensionally preserved lobopodian from the Herefordshire (Silurian) Lagerstätte, UK|journal=Royal Society Open Science|volume=5|issue=8|pages=172101|doi=10.1098/rsos.172101|pmc=6124121|pmid=30224988}}</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&ndash;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">{{Cite journal|last1=HOU|first1=XIANGUANG|last2=BERGSTRÖM|first2=JAN|date=1995-05-01|title=Cambrian lobopodians-ancestors of extant onychophorans?|journal=Zoological Journal of the Linnean Society|volume=114|issue=1|pages=3–19|doi=10.1111/j.1096-3642.1995.tb00110.x|issn=0024-4082}}</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 eye]]s.<ref>{{Cite journal|last1=Ma|first1=Xiaoya|last2=Hou|first2=Xianguang|last3=Aldridge|first3=Richard J.|last4=Siveter|first4=David J.|last5=Siveter|first5=Derek J.|last6=Gabbott|first6=Sarah E.|last7=Purnell|first7=Mark A.|last8=Parker|first8=Andrew R.|last9=Edgecombe|first9=Gregory D.|date=2012-09-01|title=Morphology of Cambrian lobopodian eyes from the Chengjiang Lagerstätte and their evolutionary significance|url=https://www.sciencedirect.com/science/article/pii/S1467803912000254|journal=Arthropod Structure & Development|language=en|volume=41|issue=5|pages=495–504|doi=10.1016/j.asd.2012.03.002|pmid=22484085|issn=1467-8039}}</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">{{cite journal|last1=Smith|first1=Martin R.|last2=Caron|first2=Jean-Bernard|title=''Hallucigenia''{{'}}s head and the pharyngeal armature of early ecdysozoans|journal=Nature|date=2015|volume=523|issue=7558|pages=75–78|doi=10.1038/nature14573|pmid=26106857|bibcode=2015Natur.523...75S|s2cid=205244325|url=http://dro.dur.ac.uk/20476/1/20476.pdf}}</ref><ref>[[Rebecca Morelle|Morelle, Rebecca]], [https://www.bbc.com/news/science-environment-33262884 Face of bizarre sea creature Hallucigenia revealed], BBCNews, Science and Environment, 2015.06.25</ref>

''Hallucigenia''{{'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">{{cite journal |first1=Jean-Bernard |last1=Caron |first2=Martin R. |last2=Smith |first3=Thomas H. P. |last3=Harvey |title=Beyond the Burgess Shale: Cambrian microfossils track the rise and fall of hallucigeniid lobopodians |journal=Proceedings of the Royal Society B: Biological Sciences |volume=280 |issue=1767 |pages=20131613 |date=September 2013 |pmid=23902914 |doi=10.1098/rspb.2013.1613 |pmc=3735267}}</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">{{cite journal |last1=Steiner |first1=Michael |date=2012 |title=A new species of ''Hallucigenia'' from the Cambrian Stage 4 Wulongqing Formation of Yunnan (South China) and the structure of sclerites in lobopodians |url=http://www.geology.cz/bulletin/fulltext/1280_Steiner.pdf |journal=Bulletin of Geosciences |volume=87 |issue=1 |pages=107–124 |doi=10.3140/bull.geosci.1280|doi-access=free }}</ref>

==History of study==
[[File:20210917 Hallucigenia sparsa interpretations.png|left|thumb|Various interpretations of ''Hallucigenia sparsa'' throughout the history of study]]

''Hallucigenia sparsa'' was originally described by [[Charles Doolittle Walcott|Charles Walcott]] as a species of the [[polychaete worm]] ''[[Canadia (annelid)|Canadia]]''.<ref>WALCOTT, C. 1911. [https://repository.si.edu/handle/10088/34820 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">{{cite book |last=Gould |first=Stephen Jay |author-link=Stephen Jay Gould |title=Wonderful life: the Burgess Shale and the nature of history |publisher=W.W. Norton |location=New York |year=1989 |pages=154–157 |isbn=978-0-393-02705-1 |url-access=registration |url=https://archive.org/details/wonderfullifebur00goul }}</ref>

[[File:HallucigeniaSparsa-ROM-June11-10.jpg|thumb|Specimen with obvious spines]]

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) {{convert|1.12|ft|cm|1|abbr=~|adj=mid||order=flip}} to {{convert|1.24|ft|cm|1|adj=mid|-long|order=flip}} 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>{{Cite journal |last=Gould |first=Stephen, Jay |date=1992 |title=The Reversal of Hallucigenia. |url= |journal=Natural History |volume=101 |issue=1 |pages=12 |via=}}</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">{{cite journal |first1=Lars |last1=Ramsköld |date=April 1992 |title=The second leg row of Hallucigenia discovered |journal=Lethaia |volume=25 |issue=2 |pages=221–4 |doi=10.1111/j.1502-3931.1992.tb01389.x}}</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">{{cite journal |last1=Ramsköld |first1=L. |last2=Hou |first2=X.-G. |title=New early Cambrian animal and onychophoran affinities of enigmatic metazoans |journal=Nature |volume=351 |pages=225–8 |year=1991 |bibcode=1991Natur.351..225R |doi=10.1038/351225a0 |issue=6323 |s2cid=4309565 }}</ref> This stain may be an artifact of decomposition.<ref name="Smith-2015" />

==Affinity==
[[File:H. sparsa.jpg|thumb|Restoration of ''H. sparsa'']]
{{external media
|float=right
|width=228px
|video1=[https://www.youtube.com/watch?v=9-Z9Ssgb0Kg&ab_channel=naturevideo&t=0s Hallucigenia: The worm with the missing head]  ''Nature Video'', YouTube.
}}
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>{{Cite journal|last1=Zhang|first1=Xi-Guang|last2=Smith|first2=Martin R.|last3=Yang|first3=Jie|last4=Hou|first4=Jin-Bo|date=2016|title=Onychophoran-like musculature in a phosphatized Cambrian lobopodian|journal=Biology Letters|volume=12|issue=9|pages=20160492|doi=10.1098/rsbl.2016.0492|issn=1744-9561|pmc=5046927|pmid=27677816}}</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">{{cite journal | last1 = Smith | first1 = M. R. | last2 = Ortega-Hernández | first2 = J. | year = 2014 | title = ''Hallucigenia''{{'}}s onychophoran-like claws and the case for Tactopoda | url = http://dro.dur.ac.uk/19108/1/19108.pdf| journal = Nature | volume = 514 | issue = 7522| pages = 363–366 | doi = 10.1038/nature13576 | pmid=25132546| bibcode = 2014Natur.514..363S | s2cid = 205239797 }}</ref> On the other hand, some analysis rather support the position of ''Hallucigenia'' as a basal panarthropod outside of onychophoran stem-group.<ref>{{Cite journal|last1=Caron|first1=Jean-Bernard|last2=Aria|first2=Cédric|date=2017-01-31|title=Cambrian suspension-feeding lobopodians and the early radiation of panarthropods|journal=BMC Evolutionary Biology|volume=17|issue=1|pages=29|doi=10.1186/s12862-016-0858-y|issn=1471-2148|pmc=5282736|pmid=28137244 |doi-access=free }}</ref><ref name="Siveter-2018" /><ref name="Caron-2020">{{Cite journal|last1=Caron|first1=Jean-Bernard|last2=Aria|first2=Cédric|date=2020|title=The Collins' monster, a spinous suspension-feeding lobopodian from the Cambrian Burgess Shale of British Columbia|url=https://onlinelibrary.wiley.com/doi/abs/10.1111/pala.12499|journal=Palaeontology|language=en|volume=63|issue=6|pages=979–994|doi=10.1111/pala.12499|s2cid=225593728|issn=1475-4983}}</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">{{cite journal |last1=Yang |first1=Jie |date=June 2015 |title=A superarmored lobopodian from the Cambrian of China and early disparity in the evolution of Onychophora |journal= Proceedings of the National Academy of Sciences|volume=112 |issue=28 |pages=8678–8683 |doi=10.1073/pnas.1505596112 |pmid=26124122 |pmc=4507230 |bibcode=2015PNAS..112.8678Y |doi-access=free }}</ref>

{{clade| style=font-size:100%;line-height:75%;width:300px;
|label1=
|1={{clade
    |1=''[[Microdictyon]]''
    |label1= 
    |2={{clade
        |label1=
        |1=''[[Cardiodictyon]]''
        |label2=
        |2={{clade
            |1='''''Hallucigenia sparsa'''''
            |2='''''Hallucigenia fortis'''''
                |label1= 
                |label2=
                |3={{clade
                    |1='''''Hallucigenia hongmeia'''''
                    |label2=[[Luolishaniidae]] 
                    |2={{clade
                        |1=''[[Acinocricus]]''
                        |2=''[[Collinsovermis]]'' 
                        |3= ''[[Luolishania]]''
                        |4= ''[[Collinsium]]''
                        }}
                    }}
                }}
            }}
        }}
    }}

==Diversity==
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>{{cite journal |last=Collins |first=Desmond |year=2002 |title=Hallucigenia unveiled. Abstracts, Palaeontological Association, 46th annual meeting |journal=Palaeontology Newsletter |volume=51 |pages=85–6 |url=http://downloads.palass.org/annual_meeting/2002/confabs2002.pdf |archive-url=https://web.archive.org/web/20160303230105/http://downloads.palass.org/annual_meeting/2002/confabs2002.pdf |archive-date=3 March 2016 }}</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==
''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>{{cite journal|last1=Caron |first1=Jean-Bernard|last2=Jackson |first2=Donald A.|title=Taphonomy of the Greater Phyllopod Bed community, Burgess Shale|journal=PALAIOS |volume=21 |issue=5 |pages=451–65 |date=October 2006 |doi=10.2110/palo.2003.P05-070R |jstor=20173022 |bibcode=2006Palai..21..451C |s2cid=53646959}}</ref> ''Hallucigenia'' also forms a minor component of Chinese [[lagerstätte]]n. Isolated hallucigeniid spines, however, are widely distributed in a range of Cambrian deposits, preserved both as [[small carbonaceous fossil|carbonaceous]] and [[small shelly fossil|mineralized]] fossils.<ref name=Caron2013/>

==See also==
* [[Paleobiota of the Burgess Shale]]

==References==
{{Reflist}}

==Further reading==
* {{cite journal |first1=Martin R. |last1=Smith |first2=Javier |last2=Ortega-Hernández |year=2014 |title=''Hallucigenia''{{'s}} onychophoran-like claws and the case for Tactopoda |journal=Nature |volume=514 |issue=7522 |pages=363–6 |doi=10.1038/nature13576 |pmid=25132546 |bibcode=2014Natur.514..363S |s2cid=205239797 |url=http://dro.dur.ac.uk/19108/1/19108.pdf}}
** {{cite press release |date=17 August 2014 |title=Evolutionary misfit: Misunderstood worm-like fossil finds its place in the Tree of Life |website=ScienceDaily |url=https://www.sciencedaily.com/releases/2014/08/140817220058.htm}}

==External links==
{{Commons category|Hallucigenia}}
{{Wikispecies|Hallucigenia}}
* {{cite web |date=2011 |title=''Hallucigenia sparsa'' |work=Burgess Shale Fossil Gallery |publisher=Virtual Museum of Canada |url=http://burgess-shale.rom.on.ca/en/fossil-gallery/view-species.php?id=60 |archive-url=https://web.archive.org/web/20201112025257/http://burgess-shale.rom.on.ca/en/fossil-gallery/view-species.php?id=60 |archive-date=12 November 2020 |url-status=dead |access-date=21 January 2023}}
* [https://web.archive.org/web/20060427054038/http://www.nmnh.si.edu/paleo/shale/phallu.htm ''Hallucigenia sparsa'' at Smithsonian Museum of Natural History] – ([[Internet Archive]])
* [https://web.archive.org/web/20040506033303/http://www.arrakis.es/~owenwang/articulos/pasados.htm Fossils found in Burgess Shale] (in Spanish)
* [http://www.yvonnenavarro.com/hallug.htm A picture of Hallucigenia]

{{Lobopodia}}
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[[Category:Burgess Shale fossils]]
[[Category:Cambrian genus extinctions]]
[[Category:Maotianshan shales fossils]]
[[Category:Taxa named by Simon Conway Morris]]
[[Category:Wheeler Shale]]
[[Category:Xenusia]]