Editing Opabinia

{{short description|Extinct stem-arthropod species found in Cambrian fossil deposits}}
{{Good article}}
{{Speciesbox
| fossil_range = [[Middle Cambrian]] ([[Wuliuan]]), {{fossilrange|505}}
| image = USNM PAL 57683 Opabinia regalis Image 3.jpg
| image_caption = Fossil specimen ([[holotype]]) on display at the [[National Museum of Natural History|Smithsonian]] in Washington, D.C.
| image_upright = 1.1
| parent_authority = [[Charles Doolittle Walcott|Walcott]], 1912
| taxon = Opabinia regalis
| authority = [[Charles Doolittle Walcott|Walcott]], 1912
}}

'''''Opabinia regalis''''' is an [[extinction|extinct]], [[stem group]] marine [[arthropod]] found in the [[Middle Cambrian]] [[Burgess Shale]] [[Lagerstätte]] (505 million years ago) of [[British Columbia]].<ref name=":1" /> ''Opabinia'' was a soft-bodied animal, measuring up to 7&nbsp;cm in body length, and had a segmented trunk with flaps along its sides and a fan-shaped tail. The head showed unusual features: five [[eye]]s, a mouth under the head and facing backwards, and a clawed [[proboscis]] that most likely passed food to its mouth. ''Opabinia'' lived on the seafloor, using the proboscis to seek out small, soft food.<ref name="Whittington1975">{{cite journal|author=Whittington, H. B.|date=June 1975|title=The enigmatic animal ''Opabinia regalis'', Middle Cambrian Burgess Shale, British Columbia|journal=[[Philosophical Transactions of the Royal Society B]]|volume=271|issue=910|pages=1–43 271|bibcode=1975RSPTB.271....1W|doi=10.1098/rstb.1975.0033|jstor=2417412|doi-access=}} Free abstract at {{cite journal|last1=Whittington|first1=H. B.|year=1975|title=The Enigmatic Animal ''Opabinia regalis'', Middle Cambrian, Burgess Shale, British Columbia|journal=[[Philosophical Transactions of the Royal Society B]]|volume=271|issue=910|pages=1–43|bibcode=1975RSPTB.271....1W|doi=10.1098/rstb.1975.0033|doi-access=}}</ref> Fewer than twenty good specimens have been described; 3 specimens of ''Opabinia'' are known from the Greater [[Phyllopod bed]], where they constitute less than 0.1% of the community.<ref name="Caron2006">{{Cite journal|last1=Caron |first1=Jean-Bernard|last2=Jackson |first2=Donald A.|s2cid=53646959|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}}</ref>

When the first thorough examination of ''Opabinia'' in 1975 revealed its unusual features, it was thought to be unrelated to any known [[phylum]],<ref name=":0" /> or perhaps a relative of arthropod and [[annelid]] ancestors.<ref name="Whittington1975" /> However, later studies since late 1990s consistently support its affinity as a member of basal arthropods, alongside the closely related [[Radiodonta|radiodonts]] (''[[Anomalocaris]]'' and relatives) and [[Lobopodia#Gilled lobopodians|gilled lobopodians]] (''[[Kerygmachela]]'' and ''[[Pambdelurion]]'').<ref name="Budd1996" /><ref>{{Cite journal|last=Budd|first=Graham E.|date=1998|title=The morphology and phylogenetic significance of Kerygmachela kierkegaardi Budd (Buen Formation, Lower Cambrian, N Greenland)|url=https://www.cambridge.org/core/journals/earth-and-environmental-science-transactions-of-royal-society-of-edinburgh/article/abs/morphology-and-phylogenetic-significance-of-kerygmachela-kierkegaardi-budd-buen-formation-lower-cambrian-n-greenland/AF165229724342F0BD90933A037CB05F|journal=Earth and Environmental Science Transactions of the Royal Society of Edinburgh|language=en|volume=89|issue=4|pages=249–290|doi=10.1017/S0263593300002418|s2cid=85645934 |issn=1473-7116}}</ref><ref name="ZhangBriggs2007" /><ref name="Budd2011" /><ref name=":1">{{Cite journal|last=Briggs|first=Derek E. G.|date=2015-04-19|title=Extraordinary fossils reveal the nature of Cambrian life: a commentary on Whittington (1975) 'The enigmatic animal Opabinia regalis, Middle Cambrian, Burgess Shale, British Columbia'|journal=Philosophical Transactions of the Royal Society B: Biological Sciences|volume=370|issue=1666|pages=20140313|doi=10.1098/rstb.2014.0313|pmid=25750235|pmc=4360120|bibcode=2015RSPTB.37040313B }}</ref><ref name=":2">{{Cite journal|last=Ortega-Hernández|first=Javier|date=2016|title=Making sense of 'lower' and 'upper' stem-group Euarthropoda, with comments on the strict use of the name Arthropoda von Siebold, 1848|url=http://dx.doi.org/10.1111/brv.12168|journal=Biological Reviews|language=en|volume=91|issue=1|pages=255–273|doi=10.1111/brv.12168|pmid=25528950|s2cid=7751936}}</ref><ref name=":3">{{Cite journal|date=2017-05-01|title=Origin and evolution of the panarthropod head – A palaeobiological and developmental perspective|journal=Arthropod Structure & Development|language=en|volume=46|issue=3|pages=354–379|doi=10.1016/j.asd.2016.10.011|issn=1467-8039|last1=Ortega-Hernández|first1=Javier|last2=Janssen|first2=Ralf|last3=Budd|first3=Graham E.|pmid=27989966|doi-access=free|bibcode=2017ArtSD..46..354O }}</ref>

In the 1970s, there was an ongoing debate about whether [[multicellular|multi-celled]] animals appeared suddenly during the Early Cambrian, in an event called the [[Cambrian explosion]], or had arisen earlier but without leaving fossils. At first ''Opabinia'' was regarded as strong evidence for the "explosive" hypothesis.<ref name=":0" /> Later the discovery of a whole series of similar [[lobopodian]] animals, some with closer resemblances to arthropods, and the development of the idea of [[stem group]]s, suggested that the Early Cambrian was a time of relatively fast [[evolution]], but one that could be understood without assuming any unique evolutionary processes.<ref name="Budd2003" />

== History of discovery ==

In 1911, [[Charles Doolittle Walcott]] found in the [[Burgess Shale]] nine almost complete fossils of ''Opabinia regalis'' and a few of what he classified as ''Opabinia ? media'', and published a description of all of these in 1912.<ref name=":4">WALCOTT, C. D. 1912. [https://repository.si.edu/bitstream/handle/10088/23430/SMC_57_Walcott_1910_6_145-245.pdf?sequence=1&isAllowed=y Middle Cambrian Branchiopoda, Malacostraca, Trilobita and Merostomata]. Smithsonian Miscellaneous Collections, 57: 145-228.</ref> The generic name is derived from [[Mount Hungabee#Gallery|Opabin pass]] between [[Mount Hungabee]] and [[Mount Biddle]], southeast of [[Lake O'Hara]], British Columbia, [[Canada]].<ref name=":4" /> In 1966–1967, [[Harry B. Whittington]] found another good specimen,<ref>{{cite book
| author=Gould, S. J. | title=Wonderful Life | publisher= Hutchinson Radius
| location=London | year=1990 | isbn=978-0-09-174271-3 | page=77 and p. 189 | author-link= Stephen Jay Gould
}}</ref> and in 1975 he published a detailed description based on very thorough [[dissection]] of some specimens and photographs of these specimens lit from a variety of angles. Whittington's analysis did not cover ''Opabinia ? media''; Walcott's specimens of this species could not be identified in his collection.<ref name="Whittington1975" /> In 1960 Russian [[paleontologist]]s described specimens they found in the [[Norilsk]] region of [[Siberia]] and labelled ''Opabinia norilica'',<ref>Miroshnikov, L. D. and Krawzov, A. G. (1960). Rare paleontological remains and traces of life in late Cambrian deposits of the northwestern Siberian platform. Palaeontology and biostratigraphy of the Soviet Arctic, 3, pp. 28–41.</ref> but these fossils were poorly preserved, and Whittington did not feel they provided enough information to be classified as members of the [[genus]] ''Opabinia''.<ref name="Whittington1975"/>

== Occurrence ==

All the recognized ''Opabinia'' specimens found so far come from the "[[Phyllopod bed]]" of the Burgess Shale, in the [[Canadian Rockies]] of British Columbia.<ref name="Whittington1975"/>

In 1997, Briggs and Nedin reported from [[South Australia]] [[Emu Bay Shale]] a new specimen of ''[[Myoscolex]]'' that was much better preserved than previous specimens, leading them to conclude that it was a close relative of ''Opabinia''<ref name="BriggsNedin1997">{{cite journal
| title=The Taphonomy and Affinities of the Problematic Fossil Myoscolex from the Lower Cambrian Emu Bay Shale of South Australia
| author1=Briggs, D. E. G. |author-link = Derek Briggs|author2=Nedin, C. | journal=Journal of Paleontology | volume=71 | issue=1
|date=January 1997 | pages=22–32 | doi= 10.1017/S0022336000038919| jstor=1306537
| bibcode=1997JPal...71...22B | s2cid=131851540 }}</ref>—although this interpretation was later questioned by Dzik, who instead concluded that ''Myoscolex'' was an [[annelid]] worm.<ref>{{cite journal
| title=Anatomy and relationships of the Early Cambrian worm ''Myoscolex'' | author=Dzik, J.
| year=2004 | journal=Zoologica Scripta | volume=33 | issue=1 | pages=57–69
| doi=10.1111/j.1463-6409.2004.00136.x
| s2cid=85216629
}}</ref>

== Morphology ==

<gallery mode="packed" heights="150">
File:20191108 Opabinia regalis.png|Restoration
File:20210222 Opabinia size.png|Size estimation
</gallery>



''Opabinia'' looked so strange that the audience at the first presentation of Whittington's analysis laughed.<ref name="Whittington1975"/> The length of ''Opabinia regalis'' from head (excluding proboscis) to tail end ranged between {{convert|4|cm|in}} and {{convert|7|cm|in}}.<ref name="Whittington1975" /> One of the most distinctive characters of ''Opabinia'' is the hollow [[proboscis]], whose total length was about one-third that of the body, and projected down from under the head. The proboscis was [[:wikt:striated|striated]] like a [[vacuum cleaner]]'s hose and flexible, and it ended with a claw-like structure whose terminal edges bore 5 spines that projected inwards and forwards. The bilateral symmetry and lateral (instead of vertical as reconstructed by Whittington 1975<ref name="Whittington1975" />) arrangement of the claw suggest it represents a pair of fused frontal appendages, comparable to those of [[Radiodonta|radiodonts]] and [[Lobopodia#Gilled lobopodians|gilled lobopodians]].<ref name="Bergström1986" /><ref name="Budd1996" /><ref name=":5">{{Cite book|last1=Xianguang|first1=Hou|chapter=Dinocaridids – anomalous arthropods or arthropod-like worms?|last2=Bergström|first2=Jan |title=Originations, Radiations and Biodiversity Changes – evidences from the Chinese fossil record.|pages=139–158|editor1-last=Jiayu|editor1-first=Rong|editor2-last=Zongjie|editor2-first=Fang|editor3-last=Zhanghe|editor3-first=Zhou|editor4-last=Renbin|editor4-first=Zhan|editor5-last=Xiangdong|editor5-first=Wang|editor6-last=Xunlai|editor6-first=Yuan|date=2006|citeseerx=10.1.1.693.5869}}</ref><ref>{{Cite journal|last=Chipman|first=Ariel D.|date=2015-12-18|title=An embryological perspective on the early arthropod fossil record|journal=BMC Evolutionary Biology|volume=15|issue=1|pages=285|doi=10.1186/s12862-015-0566-z|issn=1471-2148|pmc=4683962|pmid=26678148 |doi-access=free |bibcode=2015BMCEE..15..285C }}</ref> The head bore five stalked eyes: two near the front and fairly close to the middle of the head, pointing upwards and forwards; two larger eyes with longer stalks near the rear and outer edges of the head, pointing upwards and sideways; and a single eye between the larger pair of stalked eyes, pointing upwards. It has been assumed that the eyes were all [[compound eye|compound]], like other [[arthropod]]s' lateral eyes,<ref name="Whittington1975"/> but this reconstruction, which is not backed up by any evidence,<ref name=Whittington1975/> is "somewhat fanciful".<ref name=Paulus2000>{{Cite journal
 | last = Paulus | first = H. F.
 | year = 2000
 | title = Phylogeny of the Myriapoda-Crustacea-Insecta: a new attempt using photoreceptor structure*
 | journal = Journal of Zoological Systematics & Evolutionary Research
 | volume = 38
 | issue = 3
 | pages = 189–208
 | doi = 10.1046/j.1439-0469.2000.383152.x
| doi-access = free
 }}</ref> The mouth was under the head, behind the proboscis, and pointed ''backwards'', so that the [[digestive tract]] formed a U-bend on its way towards the rear of the animal. The proboscis appears to have been sufficiently long and flexible to reach the mouth.<ref name="Whittington1975"/>

The main part of the body was typically about {{convert|5|mm|in}} wide and had 15 segments, on each of which there were pairs of flaps (lobes) pointing downwards and outwards. The flaps overlapped so that the front of each was covered by the rear edge of the one ahead of it. The body ended with what looked like a single conical segment bearing three pairs of overlapping tail fan blades that pointed up and out, forming a tail like a V-shaped double fan.<ref name="Whittington1975"/>

Interpretations of other features of ''Opabinia'' fossils differ. Since the animals did not have [[biomineralization|mineralized]] armor nor even tough organic [[exoskeleton]]s like those of other arthropods, their bodies were flattened as they were buried and fossilized, and smaller or internal features appear as markings within the outlines of the fossils.<ref name="Whittington1975"/><ref name="Budd1996"/>

<gallery mode="packed" heights="150">
File:20210809 Opabinia regalis flap gill interpretation.png|Various interpretations on the flap and gill structures of ''Opabinia regalis'' <br>A: Whittington (1975),<ref name="Whittington1975" /> B: Bergström (1986),<ref name="Bergström1986" /> C: Budd (1996),<ref name="Budd1996" /> D: Zhang & Briggs (2007),<ref name="ZhangBriggs2007" /> E: Budd & Daley (2011)<ref name="Budd2011" />
File:20210807 Opabinia regalis trunk cross section.png|''Opabinia'' cross-section based on Budd and Daley (2011)<ref name="Budd2011" />
</gallery>

Whittington (1975) interpreted the gills as paired extensions attached dorsally to the bases of all but the first flaps on each side, and thought that these gills were flat underneath, had overlapping layers on top.<ref name="Whittington1975" /> Bergström (1986) revealed the "overlapping layers" were rows of individual blades, interpreted the flaps as part of dorsal coverings ([[tergite]]) over the upper surface of the body, with blades attached underneath each of them.<ref name="Bergström1986">{{cite journal
| author=Bergström, J. | year=1986 | title=''Opabinia'' and ''Anomalocaris'', unique Cambrian arthropods
| journal=Lethaia | volume=19 | issue=3 | pages=241–246 | doi=10.1111/j.1502-3931.1986.tb00738.x
| bibcode=1986Letha..19..241B }}</ref><ref name="Budd2011">{{Cite journal | last1 = Budd | first1 = G. E. | last2 = Daley | first2 = A. C. | title = The lobes and lobopods of ''Opabinia regalis'' from the middle Cambrian Burgess Shale | journal = Lethaia | volume = 45 | pages = 83–95 | year = 2011 | doi = 10.1111/j.1502-3931.2011.00264.x }}</ref> [[Graham Budd|Budd]] (1996) thought the gill blades attached along the front edges on the dorsal side of all except the first flaps. He also found marks inside the flaps' front edges that he interpreted as internal channels connecting the gills to the interior of the body, much as Whittington interpreted the mark along the proboscis as an internal channel.<ref name="Budd1996">{{cite journal
| author=Budd, G. E. | year=1996
| title=The morphology of ''Opabinia regalis'' and the reconstruction of the arthropod stem-group
| journal=Lethaia | volume=29 | issue=1 | pages=1–14 | doi=10.1111/j.1502-3931.1996.tb01831.x
| bibcode=1996Letha..29....1B
}}</ref> Zhang and [[Derek Briggs|Briggs]] (2007) however, interpreted all flaps have posterior spacing where the gill blades attached.<ref name="ZhangBriggs2007" /> Budd and Daley (2011) reject the reconstruction by Zhang & Briggs, showing the flaps have complete posterior edges as in previous reconstructions. They mostly follow the reconstruction by Budd (1996) with modifications on some details (e.g. the first flap pair also have gills; the attachment point of gill blades located more posteriorly than previously thought).<ref name="Budd2011" />[[File:20210809 Opabinia regalis digestive system.png|thumb|[[Digestive system]] (yellow highlight) of ''Opabinia'', showing pairs of [[diverticula]] individualized from the triangular extension]]
Whittington (1975) found evidence of near-triangular features along the body, and concluded that they were internal structures, most likely sideways extensions of the gut ([[diverticula]]).<ref name="Whittington1975" /> Chen ''et al.'' (1994) interpreted them as contained within the lobes along the sides.<ref name="ChenRamsköldZhou1994">{{cite journal
| author1=Chen, J-Y. |author2=Ramsköld, L. |author3=Gui-qing Zhou, G-Q.
|s2cid=1913482 | title=Evidence for Monophyly and Arthropod Affinity of Cambrian Giant Predators
| journal=Science |date=May 1994 | volume=264 | issue=5163| pages=1304–1308
| doi=10.1126/science.264.5163.1304
| pmid=17780848
|bibcode = 1994Sci...264.1304C }}</ref> Budd (1996) thought the "triangles" were too wide to fit within ''Opabinia''{{'}}s slender body, and that [[Cross section (geometry)|cross-section]] views showed they were attached separately from and lower than the lobes, and extended below the body.<ref name="Budd1996" /> He later found specimens that appeared to preserve the legs' exterior cuticle. He therefore interpreted the "triangles" as short, fleshy, conical legs (lobopods). He also found small mineralized patches at the tips of some, and interpreted these as claws. Under this reconstruction, the gill-bearing flap and lobopod were homologized to the outer gill branch and inner leg branch of arthropod [[biramous]] limbs seen in ''[[Marrella]]'', [[trilobite]]s, and [[crustacean]]s.<ref name="Budd1996" /> Zhang and Briggs (2007) analyzed the chemical composition of the "triangles", and concluded that they had the same composition as the gut, and therefore agreed with Whittington that they were part of the digestive system. Instead they regarded ''Opabinia''{{'}}s lobe+gill arrangement as an early form of the arthropod limbs before it split into a biramous structure.<ref name="ZhangBriggs2007">{{cite journal
| author1=Zhang, X. |author2=Briggs, D. E. G. | year=2007
| title=The nature and significance of the appendages of ''Opabinia'' from the Middle Cambrian Burgess Shale
| journal=Lethaia | volume=40 | issue=2 | pages=161–173 | doi=10.1111/j.1502-3931.2007.00013.x
|bibcode=2007Letha..40..161Z }}</ref> However, this similar chemical composition is not only associated with the digestive tract; Budd and Daley (2011) suggest that it represents mineralization forming within fluid-filled cavities within the body, which is consistent with hollow lobopods as seen in unequivocal lobopodian fossils. They also clarify that the gut diverticula of ''Opabinia'' are series of circular gut glands individualized from the "triangles". While they agreed on the absence of terminal claws, the presence of lobopods in ''Opabinia'' remain as a plausible interpretation.<ref name="Budd2011" />

== Lifestyle ==
{{multiple image
| total_width = 450px
| image1 = Opabinia v2.png
| caption1 = Ecological reconstruction of ''Opabinia'' on the seafloor along with other [[Paleobiota of the Burgess Shale|fauna from the Burgess Shale]] 
| image2 = 20210812 Opabinia regalis proboscis frontal appendage mobility.gif
| caption2 = Suggested proboscis mobility of ''Opabinia''<ref name="Whittington1975" />
}}
The way in which the [[Burgess Shale]] animals were buried, by a mudslide or a sediment-laden current that acted as a sandstorm, suggests they lived on the surface of the seafloor.<ref name="Whittington1975" /> ''Opabinia'' probably used its proboscis to search the sediment for food particles and pass them to its mouth.<ref name="Whittington1975" /> Since there is no sign of anything that might function as jaws, its food was presumably small and soft.<ref name="Whittington1975" /> The paired gut [[diverticula]] may increase the efficiency of food digestion and intake of nutrition.<ref name=":7" /> Whittington (1975) believing that ''Opabinia'' had no legs, thought that it crawled on its lobes and that it could also have swum slowly by flapping the lobes, especially if it timed the movements to create a [[wave]] with the metachronal movement of its lobes.<ref name="Whittington1975" /> On the other hand, he thought the body was not flexible enough to allow fish-like undulations of the whole body.<ref name="Whittington1975"/>

==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}}

==Theoretical significance==
{{main|Cambrian explosion}}
[[File:Walcott_Cambrian_Geology_and_Paleontology_II_plate_28.jpg|thumb|Top left: retouched image of ''Opabinia'' (''Walcott Cambrian Geology and Paleontology II'' by [[Charles Doolittle Walcott]])]] ''Opabinia'' made it clear how little was known about soft-bodied animals, which do not usually leave fossils.<ref name="Whittington1975" /> When Whittington described it in the mid-1970s, there was already a vigorous debate about the early evolution of [[animal]]s. [[Preston Cloud]] argued in 1948 and 1968 that the process was "explosive",<ref>{{cite journal | author=Cloud, P. E. | year=1948
| title=Some problems and patterns of evolution exemplified by fossil invertebrates | journal=Evolution | volume=2 | issue=4 | pages=322–350 | doi=10.2307/2405523 | pmid=18122310 | jstor=2405523}} and {{Cite book
| author=Cloud, P. E. | year= 1968 | contribution=Pre-metazoan evolution and the origins of the Metazoa.
| pages=1–72 | editor=Drake, E. T. | title=Evolution and Environment | publisher=Yale University Press
| location=New Haven, Conn.
}}</ref> and in the early 1970s [[Niles Eldredge]] and [[Stephen Jay Gould]] developed their theory of [[punctuated equilibrium]], which views evolution as long intervals of near-stasis "punctuated" by short periods of rapid change.<ref>{{Cite book
| author1=Eldredge, N.
|contributor=Eldredge, N. |contributor2=Gould, S. J.
|contribution=APPENDIX: Punctuated Equilibria: An Alternative to Phyletic Gradualism | pages=193–224
| title=Time Frames: The Evolution of Punctuated Equilibria
|doi=10.1515/9781400860296.193
|isbn=9781400860296
|date=1989
}}</ref> On the other hand, around the same time Wyatt Durham and [[Martin Glaessner]] both argued that the animal kingdom had a long [[Proterozoic]] history that was hidden by the lack of fossils.<ref name="Bengtson2004">{{cite conference |last=Bengtson |first=Stefan |year=2004 |chapter=Early Skeletal Fossils |chapter-url=https://books.google.com/books?id=NNZZPwAACAAJ |editor1-last=Lipps |editor1-first=Jere H. |editor1-link=Jere H. Lipps |editor2-last=Waggoner |editor2-first=Benjamin M. |title=Neoproterozoic-Cambrian Biological Revolutions: Presented as a Paleontological Society Short Course at the Annual Meeting of the Geological Society of America, Denver, Colorado, November 6, 2004 |series=Paleontological Society Papers |volume=10 |location=New Haven, CT |publisher=Yale University Reprographics & Imaging Service; [[Paleontological Society]] |issn=1089-3326 |oclc=57481790 |access-date=2015-02-06}}</ref><ref>{{cite journal
|author=Durham, J. W. |year=1971 |title=The fossil record and the origin of the Deuterostomata |journal=Proceedings of the North American Paleontological Convention, Part H | pages=1104–1132 }} and {{Cite book |author=Glaessner, M. F. |year=1972 |contribution=Precambrian palaeozoology |pages=43–52 |editor=Jones, J. B. |editor2=McGowran, B. |title=Stratigraphic Problems of the Later Precambrian and Early Cambrian |volume=1 |publisher=University of Adelaide}}</ref> Whittington (1975) concluded that ''Opabinia'', and other [[taxa]] such as ''[[Marrella]]'' and ''[[Yohoia]]'', cannot be accommodated in modern groups. This was one of the primary reasons why Gould in his book on the [[Burgess Shale]], ''[[Wonderful Life (book)|Wonderful Life]]'', considered that Early Cambrian life was much more disparate and "experimental" than any later set of animals and that the [[Cambrian explosion]] was a truly dramatic event, possibly driven by unusual [[evolution]]ary mechanisms.<ref name=":0">{{cite book |author=Gould, S. J. |title=Wonderful Life |publisher=Hutchinson Radius |year=1989 |isbn=978-0-09-174271-3 |pages=124–136&nbsp;ff }}</ref> He regarded ''Opabinia'' as so important to understanding this phenomenon that he wanted to call his book ''Homage to Opabinia''.<ref>{{cite book |last=Knoll |first=A. H. |title=The First Three Billion Years of Evolution on Earth |publisher=Princeton University Press |year=2004 |page=192 |chapter=Cambrian Redux |chapter-url=https://books.google.com/books?id=Lq0S-C6zMHAC&q=%22burgess+shale%22&pg=PA192 |access-date=2009-04-22 |isbn=978-0-691-12029-4}}</ref>

However, other discoveries and analyses soon followed, revealing similar-looking animals such as ''[[Anomalocaris]]'' from the Burgess Shale and ''[[Kerygmachela]]'' from [[Sirius Passet]].<ref name="Bergström1986 "/><ref name="Budd1993" /> Another Burgess Shale animal, ''Aysheaia'', was considered very similar to modern [[Onychophora]],<ref>{{cite journal |title=Affinities of ''Aysheaia'' (Onychophora), with Description of a New Cambrian Species |author=Robison, R. A. |journal=Journal of Paleontology |volume=59 |issue=1 |date=January 1985 |pages=226–235 |jstor=1304837}}</ref> which are regarded as close relatives of arthropods.<ref>{{cite journal |author1=Jacobs, D. K. |author2=Wray, C. G. |author3=Wedeen, C. J. |author4=Kostriken, R. |author5=DeSalle, R. |author6=Staton, J. L. |author7=Gates, R. D. |author8=Lindberg, D. R. |title=Molluscan engrailed Expression, Serial Organization, and Shell Evolution |year=2000 |journal=Evolution and Development |volume=2 |issue=6 |pages=340–347 |doi=10.1046/j.1525-142x.2000.00077.x |pmid=11256378|s2cid=25274057 }}</ref> Paleontologists defined a group called [[lobopodia]]ns to include fossil panarthropods that are thought to be close relatives of onychophorans, tardigrades and arthropods but lack jointed limbs. This group was later widely accepted as a paraphyletic grade that led to the origin of extant panarthropod phyla.

[[File:Crown n Stem Groups.svg|thumb|Concept of [[stem group]]s<ref name="CraskeJefferies1989" />{{colbegin}}{{ubl
|{{legend2|text={{resize|120%|'''—'''}}||{{nowrap|{{=}} Lines of descent}}}}
|{{legend2|black|border=1px solid gray|{{=}} Basal node}}
|{{legend2|white|border=1px solid gray|{{=}} Crown node}}
|{{legend2|#8080ff|border=1px solid gray|{{=}} Total group}}
|{{legend2|#faada7|border=1px solid gray|{{=}} Crown group}}
|{{legend2|#fdefa4|border=1px solid gray|{{=}} Stem group}}
}}{{colend}}]]
While this discussion about specific fossils such as ''Opabinia'' and ''Anomalocaris'' was going on in the late 20th century, the concept of [[stem group]]s was introduced to cover evolutionary "aunts" and "cousins". A [[crown group]] is a group of closely related living animals plus their last common ancestor plus all its descendants. A stem group contains offshoots from members of the lineage earlier than the last common ancestor of the crown group; it is a ''relative'' concept, for example [[tardigrade]]s are living animals that form a crown group in their own right, but Budd (1996) regarded them also as being a stem group relative to the arthropods.<ref name="Budd1996" /><ref name="CraskeJefferies1989">{{cite journal |author1=Craske, A. J. |author2=Jefferies, R. P. S. |year=1989 |title=A new mitrate from the Upper Ordovician of Norway, and a new approach to subdividing a plesion |journal=Palaeontology |volume=32 |pages=69–99
}}</ref> Viewing strange-looking organisms like ''Opabinia'' in this way makes it possible to see that, while the Cambrian explosion was unusual, it can be understood in terms of normal evolutionary processes.<ref name=Budd2003>{{cite journal |author=Budd, G. E. |year=2003 |title=The Cambrian Fossil Record and the Origin of the Phyla |journal=Integrative and Comparative Biology |volume=43 |issue=1 |pages=157–165 |doi=10.1093/icb/43.1.157 |doi-access=free |pmid=21680420}}</ref>

{{Clear}}

==See also==
{{Portal|Paleontology}}
* {{annotated link|Body plan}}
* {{annotated link|Radiodonta}}
* [[Paleobiota of the Burgess Shale]]

==References==
{{Reflist|2}}

==Further reading==
* {{cite journal
| author=Bergström, J. | year=1987| title=The Cambrian ''Opabinia'' and ''Anomalocaris''
| journal=Lethaia | volume=20 | issue=2 | pages=187–188 | doi=10.1111/j.1502-3931.1987.tb02037.x
| bibcode=1987Letha..20..187B}}
* {{cite journal | author1=Briggs, D. E. G. |author2=Whittington, H. B. | year=1987
| title=The affinities of the Cambrian animals ''Anomalocaris'' and ''Opabinia''
| journal=Lethaia | volume=20 | issue=2 | pages=185–186 | doi=10.1111/j.1502-3931.1987.tb02036.x |bibcode=1987Letha..20..185B }}

==External links==
{{Commons category}}
{{Wikispecies}}
* {{Cite web|date=2011|title=''Opabinia regalis''|work=Burgess Shale Fossil Gallery|publisher=Virtual Museum of Canada|url=http://burgess-shale.rom.on.ca/en/fossil-gallery/view-species.php?id=93|archive-url=https://web.archive.org/web/20201112025257/http://burgess-shale.rom.on.ca/en/fossil-gallery/view-species.php?id=93|archive-date=2020-11-12|url-status=dead|access-date=2023-01-21}}
* [http://paleobiology.si.edu/burgess/opabinia.html Smithsonian page on ''Opabinia'', with photo of Burgess Shale fossil]

{{Dinocaridida}}
{{Taxonbar|from=Q50570}}

[[Category:Dinocaridida]]
[[Category:Cambrian arthropods]]
[[Category:Burgess Shale fossils]]
[[Category:Taxa named by Charles Doolittle Walcott]]
[[Category:Fossil taxa described in 1912]]
[[Category:Cambrian genus extinctions]]