Editing Opabinia (section)

==Classification==
{{cladogram|style=font-size:75%; line-height:40%
|title=
|align= right
|caption= Summarized phylogeny between ''Opabinia'' and other [[Ecdysozoan]] taxa.<ref name=":3" />
|cladogram=
{{clade| style=width:30em;font-size:100%;line-height:100%
|label1=<small>[[Ecdysozoa]]</small>
|1={{clade
|label1=<small>[[Cycloneuralia]]</small>
|1=[[Priapulida]] [[File:Adult priapulid 2.jpg|50px]] and relatives
|label2=<small>[[Panarthropoda]]</small>
|2={{clade|state3=double
  |1=[[Onychophora]] [[File:Velvet worm.png|80px]]
  |2=[[Tardigrada]] [[File:SEM image of Milnesium tardigradum in active state - journal.pone.0045682.g001-2 (white background).png|60px]]
  |label3=†|3=[[Lobopodia]]n grade [[File:H. sparsa (white background).jpg|80px]]
  |4={{clade
    |label1=†|1=''[[Pambdelurion]]'' [[File:20191112_Pambdelurion_whittingtoni.png|80px]]
    |label2=†|2=''[[Kerygmachela]]'' [[File:20191022 Kerygmachela kierkegaardi without lobopods.png|80px]]
    |3={{clade
      |label1=†|1='''''Opabinia''''' [[File:20191108 Opabinia regalis.png|130px]]
      |label2=†|2=''[[Utaurora]]'' [[File:20220212 Utaurora comosa.png|80px]]
      |label3=†|3=[[Radiodonta]] [[File:20210626_Anomalocaris.png|70px]]
      |4=[[Euarthropoda]] [[File:Aptostichus simus Monterey County.jpg|70px]]
}} }} }} }} }} }}
Considering how paleontologists' reconstructions of ''Opabinia'' differ, it is not surprising that the animal's classification was highly debated during the 20th century.<ref name="Budd1996"/> [[Charles Doolittle Walcott]], the original [[wikt:describer|describer]], considered it to be an [[anostraca]]n [[crustacean]] in 1912.<ref name=":4" /> The idea was followed by [[G. Evelyn Hutchinson]] in 1930, providing the first reconstruction of ''Opabinia'' as an anostracan swimming upside down.<ref>Hutchinson, George Evelyn (1930). [https://repository.si.edu/handle/10088/15851 Restudy of some Burgess shale fossils]. ''Proceedings of the United States National Museum'' '''78''' (2854): 1–24. doi:10.5479/si.00963801.78-2854.1.</ref> [[:it:Alberto Simonetta|Alberto Simonetta]] provided a new reconstruction of ''Opabinia'' in 1970 very different to those of Hutchinson's, with lots of [[arthropod]] features (''e.g. ,''[[Tergite|dorsal exoskeleton]] and jointed limbs) which are reminiscent of ''[[Yohoia]]'' and ''[[Leanchoilia]]''.<ref>Simonetta AM. 1970 Studies on non trilobite arthropods of the Burgess Shale (Middle Cambrian). Palaeontogr. Ital. 66, 35–45.</ref> [[Leif Størmer]], following earlier work by [[Percy Raymond]], thought that ''Opabinia'' belonged to the so-called "trilobitoids" ([[trilobites]] and similar taxa). After his thorough analysis [[Harry B. Whittington]] concluded that ''Opabinia'' was not arthropod in 1975, as he found no evidence for arthropodan jointed limbs, and that nothing like the flexible, probably fluid-filled, proboscis was known in arthropods.<ref name="Whittington1975"/> Although he left ''Opabinia'''s classification above the [[Family (biology)|family]] level open, the annulated but not articulated body and the unusual lateral flaps with gills persuaded him that it may have been a representative of the ancestral stock from the origin of [[annelid]]s and arthropods,<ref name="Whittington1975"/> two distinct animal phyla ([[Lophotrochozoan]] and [[Ecdysozoan]], respectively) which were still thought to be close relatives (united under [[Articulata hypothesis|Articulata]]) at that time.<ref>{{Cite journal|last=Edgecombe|first=Gregory D.|date=2009|title=Palaeontological and Molecular Evidence Linking Arthropods, Onychophorans, and other Ecdysozoa|journal=Evolution: Education and Outreach|language=en|volume=2|issue=2|pages=178–190|doi=10.1007/s12052-009-0118-3|issn=1936-6434|doi-access=free}}</ref>

In 1985, [[Derek Briggs]] and Whittington published a major redescription of ''[[Anomalocaris]]'', also from the Burgess Shale.<ref>{{Cite journal|last1=Whittington|first1=Harry Blackmore|last2=Briggs|first2=Derek Ernest Gilmor|date=1985-05-14|title=The largest Cambrian animal, Anomalocaris, Burgess Shale, British-Columbia|url=https://royalsocietypublishing.org/doi/abs/10.1098/rstb.1985.0096|journal=Philosophical Transactions of the Royal Society of London. B, Biological Sciences|volume=309|issue=1141|pages=569–609|doi=10.1098/rstb.1985.0096|bibcode=1985RSPTB.309..569W}}</ref> Soon after that, Swedish palaeontologist [[:sv:Jan Bergström (paleozoolog)|Jan Bergström]], noting in 1986 the similarity of ''Anomalocaris'' and ''Opabinia'', suggested that the two animals were related, as they shared numerous features (''e.g.,'' lateral flaps, gill blades, stalked eyes, and specialized frontal appendages). He classified them as primitive arthropods, although he considered that arthropods are not [[monophyletic|a single phylum]].<ref name="Bergström1986 "/>

In 1996, [[Graham Budd]] found what he considered evidence of short, un-jointed legs in ''Opabinia''.<ref name="Budd1996" /> His examination of the gilled lobopodian ''[[Kerygmachela]]'' from the [[Sirius Passet]] [[lagerstätte]], about {{ma|518}} and over 10M&nbsp;years older than the Burgess Shale, convinced him that this specimen had similar legs.<ref name="Budd1993">{{cite journal
| author=Budd, G. E. | s2cid=4341971 | year=1993 | title=A Cambrian gilled lobopod from Greenland
| journal=Nature | volume=364 | pages=709–711
| doi=10.1038/364709a0 | issue=6439
|bibcode = 1993Natur.364..709B }}</ref><ref name="Budd1997StemGroupArthropods">{{Cite book
 | first =G. E.
 | last =Budd
 | editor-last =Fortey
 | editor-first =R. A.
 | editor-link = Richard Fortey
 | editor2-last =Thomas
 | editor2-first =R. H.
 | chapter =Stem Group Arthropods from the Lower Cambrian Sirius Passet Fauna of North Greenland
 | title =Arthropod Relationships – Special Volume Series 55
 | year =1997
 | publisher =Systematics Association
}}</ref> He considered the legs of these two [[genus|genera]] very similar to those of the Burgess Shale lobopodian ''[[Aysheaia]]'' and the modern [[onychophora]]ns (velvet worms), which are regarded as the bearers of numerous [[Plesiomorphy|ancestral traits]] shared by the ancestors of arthropods. After examining several sets of features shared by these and similar lobopodians he drew up a "broad-scale reconstruction of the arthropod [[stem group|stem-group]]", ''i.e.,'' of arthropods and what he considered to be their evolutionary basal members.<ref name="Budd1996" /> One striking feature of this family tree is that modern [[tardigrada|tardigrade]]s (water bears) may be ''Opabinia'<nowiki/>''s closest living [[evolution]]ary relatives.<ref name="Budd1996"/> On the other hand, Hou ''et al.'' (1995, 2006) suggested ''Opabinia'' is a member of unusual [[cycloneuralia]]n worms with [[Convergent evolution|convergent]] arthropod features.<ref>{{Cite journal|last1=Xian-Guang|first1=Hou|last2=Bergström|first2=Jan|last3=Ahlberg|first3=Per|date=1995-09-01|title=Anomalocaris and other large animals in the lower Cambrian Chengjiang fauna of southwest China|url=https://doi.org/10.1080/11035899509546213|journal=GFF|volume=117|issue=3|pages=163–183|doi=10.1080/11035899509546213|bibcode=1995GFF...117..163X |issn=1103-5897}}</ref><ref name=":5" />

Although Zhang and Briggs (2007) disagreed with Budd's diagnosis that ''Opabinia''{{'}}s "triangles" were legs, the resemblance they saw between ''Opabinia''{{'}}s lobe+gill arrangement and arthropods' [[biramous]] limbs led them to conclude that ''Opabinia'' was very closely related to arthropods. In fact they presented a family tree very similar to Budd's except that theirs did not mention tardigrades.<ref name="ZhangBriggs2007" /> Regardless of the different morphological interpretations, all major restudies since 1980s similarly concluded that the resemblance between ''Opabinia'' and arthropods (''e.g.,'' stalked eyes, dorsal segmentation, posterior mouth, fused appendages, gill-like limb branches) are taxonomically significant.<ref name="Bergström1986" /><ref name="Budd1996" /><ref name="ZhangBriggs2007" /><ref name="Budd2011" /><ref name=":1" />

Since the 2010s, the suggested close relationship between ''Opabinia'' and tardigrades/cycloneuralians is no longer supported, while the affinity of ''Opabinia'' as a stem-group arthropod alongside [[Radiodonta]] (a clade that includes ''[[Anomalocaris]]'' and its relatives<ref>{{Cite journal|last=Collins|first=Desmond|date=1996|title=The "evolution" of Anomalocaris and its classification in the arthropod class Dinocarida (nov.) and order Radiodonta (nov.)|url=https://www.cambridge.org/core/journals/journal-of-paleontology/article/abs/evolution-of-anomalocaris-and-its-classification-in-the-arthropod-class-dinocarida-nov-and-order-radiodonta-nov/BBC7E5F260A34413AD31BBDE89207870|journal=Journal of Paleontology|language=en|volume=70|issue=2|pages=280–293|doi=10.1017/S0022336000023362|bibcode=1996JPal...70..280C |s2cid=131622496 |issn=0022-3360}}</ref>) and [[Lobopodia#Gilled lobopodians|gilled lobopodians]] is widely accepted,<ref name=":1" /><ref name=":2" /><ref name=":3" /> as consistently shown by multiple phylogenetic analyses,<ref>{{Cite journal|last1=Legg|first1=David A.|last2=Sutton|first2=Mark D.|last3=Edgecombe|first3=Gregory D.|last4=Caron|first4=Jean-Bernard|date=2012-12-07|title=Cambrian bivalved arthropod reveals origin of arthrodization|journal=Proceedings of the Royal Society B: Biological Sciences|volume=279|issue=1748|pages=4699–4704|doi=10.1098/rspb.2012.1958|pmc=3497099|pmid=23055069}}</ref><ref>{{Cite journal|last=Legg|first=David|date=2013|title=Multi-Segmented Arthropods from the Middle Cambrian of British Columbia (Canada)|url=https://www.cambridge.org/core/journals/journal-of-paleontology/article/abs/multisegmented-arthropods-from-the-middle-cambrian-of-british-columbia-canada/9CA5AE5D58832452EDA388ED438774CB|journal=Journal of Paleontology|language=en|volume=87|issue=3|pages=493–501|doi=10.1666/12-112.1|bibcode=2013JPal...87..493L |s2cid=86725173|issn=0022-3360}}</ref><ref>{{Cite journal|last1=Legg|first1=David A.|last2=Vannier|first2=Jean|date=2013|title=The affinities of the cosmopolitan arthropod ''Isoxys'' and its implications for the origin of arthropods|url=https://onlinelibrary.wiley.com/doi/abs/10.1111/let.12032|journal=Lethaia|language=en|volume=46|issue=4|pages=540–550|doi=10.1111/let.12032|bibcode=2013Letha..46..540L |issn=1502-3931}}</ref><ref>{{Cite journal|last1=Legg|first1=David A.|last2=Sutton|first2=Mark D.|last3=Edgecombe|first3=Gregory D.|date=2013-09-30|title=Arthropod fossil data increase congruence of morphological and molecular phylogenies|journal=Nature Communications|language=en|volume=4|issue=1|pages=2485|doi=10.1038/ncomms3485|pmid=24077329|bibcode=2013NatCo...4.2485L|issn=2041-1723|doi-access=free}}</ref><ref>{{Cite journal|last1=Smith|first1=Martin R.|last2=Ortega-Hernández|first2=Javier|date=2014|title=Hallucigenia's onychophoran-like claws and the case for Tactopoda|url=https://www.nature.com/articles/nature13576|journal=Nature|language=en|volume=514|issue=7522|pages=363–366|doi=10.1038/nature13576|pmid=25132546|bibcode=2014Natur.514..363S|s2cid=205239797|issn=1476-4687}}</ref><ref>{{Cite journal|last1=Van Roy|first1=Peter|last2=Daley|first2=Allison C.|last3=Briggs|first3=Derek E. G.|date=2015|title=Anomalocaridid trunk limb homology revealed by a giant filter-feeder with paired flaps|url=https://www.nature.com/articles/nature14256|journal=Nature|language=en|volume=522|issue=7554|pages=77–80|doi=10.1038/nature14256|pmid=25762145|bibcode=2015Natur.522...77V|s2cid=205242881|issn=1476-4687}}</ref><ref>{{Cite journal|last1=Smith|first1=Martin R.|last2=Caron|first2=Jean-Bernard|date=2015|title=Hallucigenia's head and the pharyngeal armature of early ecdysozoans|url=https://www.nature.com/articles/nature14573|journal=Nature|language=en|volume=523|issue=7558|pages=75–78|doi=10.1038/nature14573|pmid=26106857|bibcode=2015Natur.523...75S|s2cid=205244325|issn=1476-4687}}</ref><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 |bibcode=2017BMCEE..17...29C }}</ref><ref>{{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><ref name=":6">{{Cite journal|last1=Zeng|first1=Han|last2=Zhao|first2=Fangchen|last3=Niu|first3=Kecheng|last4=Zhu|first4=Maoyan|last5=Huang|first5=Diying|date=2020|title=An early Cambrian euarthropod with radiodont-like raptorial appendages|url=https://www.nature.com/articles/s41586-020-2883-7|journal=Nature|language=en|volume=588|issue=7836|pages=101–105|doi=10.1038/s41586-020-2883-7|pmid=33149303|bibcode=2020Natur.588..101Z|s2cid=226248177|issn=1476-4687}}</ref><ref>{{Cite journal|last1=Anderson|first1=Evan P.|last2=Schiffbauer|first2=James D.|last3=Jacquet|first3=Sarah M.|last4=Lamsdell|first4=James C.|last5=Kluessendorf|first5=Joanne|last6=Mikulic|first6=Donald G.|date=2021|title=Stranger than a scorpion: a reassessment of ''Parioscorpio venator'', a problematic arthropod from the Llandoverian Waukesha Lagerstätte|url=https://onlinelibrary.wiley.com/doi/abs/10.1111/pala.12534|journal=Palaeontology|language=en|volume=64|issue=3|pages=429–474|doi=10.1111/pala.12534|bibcode=2021Palgy..64..429A |s2cid=234812878|issn=1475-4983}}</ref><ref>{{Cite journal|last1=Moysiuk|first1=Joseph|last2=Caron|first2=Jean-Bernard|date=2020|title=Exceptional multifunctionality in the feeding apparatus of a mid-Cambrian radiodont|journal=Paleobiology|volume=47 |issue=4 |language=en|pages=704–724|doi=10.1017/pab.2021.19|bibcode=2021Pbio...47..704M |s2cid=236552819 |issn=0094-8373|doi-access=free}}</ref> as well as new discoveries such as the presence of arthropod-like gut glands<ref name="Budd2011" /><ref name=":7">{{Cite journal|last1=Vannier|first1=Jean|last2=Liu|first2=Jianni|last3=Lerosey-Aubril|first3=Rudy|last4=Vinther|first4=Jakob|last5=Daley|first5=Allison C.|date=2014-05-02|title=Sophisticated digestive systems in early arthropods|journal=Nature Communications|language=en|volume=5|issue=1|pages=3641|doi=10.1038/ncomms4641|pmid=24785191|bibcode=2014NatCo...5.3641V|s2cid=205324774 |issn=2041-1723|doi-access=free}}</ref> and the intermediate taxon ''[[Kylinxia]]''.<ref name=":6" />

In [[2022 in arthropod paleontology|2022]], Paleontologists described a similar looking animal which was discovered in Cambrian-aged rocks of [[Utah]].<ref name=":8">{{Cite journal|last1=Pates|first1=Stephen|last2=Wolfe|first2=Joanna M.|last3=Lerosey-Aubril|first3=Rudy|last4=Daley|first4=Allison C.|last5=Ortega-Hernández|first5=Javier|date=2022-02-09|title=New opabiniid diversifies the weirdest wonders of the euarthropod stem group|journal=Proceedings of the Royal Society B: Biological Sciences|volume=289|issue=1968|pages=20212093|doi=10.1098/rspb.2021.2093|pmc=8826304|pmid=35135344}}</ref> The fossil was named ''[[Utaurora|Utaurora comosa]],'' and was found within the [[Wheeler Shale]].<ref name=":8" /> The stem-arthropod was actually first described in 2008, but at the time it was originally considered a specimen of ''Anomalocaris.''<ref>{{Cite journal|last1=Briggs|first1=Derek E. G.|last2=Lieberman|first2=Bruce S.|last3=Hendricks|first3=Jonathan R.|last4=Halgedahl|first4=Susan L.|last5=Jarrard|first5=Richard D.|date=2008|title=Middle Cambrian arthropods from Utah|url=https://www.cambridge.org/core/journals/journal-of-paleontology/article/abs/middle-cambrian-arthropods-from-utah/B78C8E548B4E0BF042A1AAE08FD4725B|journal=Journal of Paleontology|language=en|volume=82|issue=2|pages=238–254|doi=10.1666/06-086.1|bibcode=2008JPal...82..238B |s2cid=31568651 |issn=0022-3360}}</ref> This discovery could suggest there were other animals that looked like ''Opabinia,'' and its family may have been more diverse.<ref name=":8" />{{Clear}}