Hallucigenia

File:20210000 Hallucigenia diagrammatic reconstruction.png

Hallucigenia is a {{cvt|0.5|-|5.5|cm|in|frac=16}}{{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|url-access=subscription}} long tubular animal with up to ten pairs of slender legs (lobopods). The first 2 or 3 leg pairs are slender and featureless,{{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}} while the remaining 7 or 8 pairs each terminate with 1 or 2 claws. Above the trunk region are 7 pairs of rigid conical sclerites (spines) corresponding to the 3rd–9th leg pairs. The trunk is either featureless (H. sparsa) or divided by heteronomous annulations (H. fortis{{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}} and H. hongmeia). 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.

Research in the mid-2010s clarified that the longer end is a head with anteroventral mouth and at least a pair of simple eyes.{{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|url-access=subscription|doi-access=free}} The shape of head differs between species – elongated in H. sparsa, rounded in H. fortis, while those of H. hongmeia remain unknown. At least in H. sparsa, the head possesses radial teeth and pharyngeal teeth within the front of the gut.{{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}}Morelle, Rebecca, [https://www.bbc.com/news/science-environment-33262884 Face of bizarre sea creature Hallucigenia revealed], BBCNews, Science and Environment, 2015.06.25

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",{{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}} 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.{{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 }}

File:20210917 Hallucigenia sparsa interpretations.png

Hallucigenia sparsa was originally described by Charles Walcott as a species of the polychaete worm Canadia.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. 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. This raised questions, such as how it would walk on the stiff legs, but it was accepted (with reservations) as the best available interpretation.{{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 }}

File:HallucigeniaSparsa-ROM-June11-10.jpg

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.

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,{{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=}} at that time. 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 "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.{{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}} 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.{{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 }} This stain may be an artifact of decomposition.

File:H. sparsa.jpg

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|video1=[https://www.youtube.com/watch?v=9-Z9Ssgb0Kg&ab_channel=naturevideo&t=0s Hallucigenia: The worm with the missing head] Nature Video, YouTube.

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Since the revisions around 1990s, 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.{{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}} A key character demonstrating this affinity is the cone-in-cone construction of Hallucigenia claws, a feature shared only with modern onychophorans.{{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 }} On the other hand, some analysis rather support the position of Hallucigenia as a basal panarthropod outside of onychophoran stem-group.{{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 }}{{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|url-access=subscription}} Under this scenario, the cone-in-cone structure shared between Hallucigenia and onychophorans represent panarthropod plesiomorphy. Hallucigenia also exhibits certain characters inherited from the ancestral ecdysozoan, but lost in the modern onychophorans – in particular its distinctive foregut armature.

Below is a cladogram for Hallucigenia according to Yang et al., 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 }}

{{clade| style=font-size:100%;line-height:75%;width:300px;

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|1={{clade

|1=Microdictyon

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|2={{clade

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|1=Cardiodictyon

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|2={{clade

|1=Hallucigenia sparsa

|2=Hallucigenia fortis

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|3={{clade

|1=Hallucigenia hongmeia

|label2=Luolishaniidae

|2={{clade

|1=Acinocricus

|2=Collinsovermis

|3= Luolishania

|4= Collinsium

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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.{{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 }}

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.

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.{{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}} 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.

• Paleobiota of the Burgess Shale

{{Reflist}}

• {{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}}

{{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]

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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