View source for Isoxys

{{Short description|Genus of extinct arthropods}}
{{Italic title}}
{{Automatic taxobox
| image = Artistic reconstruction of Isoxys curvirostratus.jpg
| image_caption = ''Isoxys curvirostratus'' from the Chengjiang Biota
| fossil_range = {{Geological range|518|504|[[Atdabanian]] to [[Paibian]]}}
| taxon = Isoxys
| authority = [[Charles Doolittle Walcott|Walcott]], 1890
| subdivision_ranks = Species
| subdivision = See text
| image2 = Isoxys acutangulus 2.jpg
| image2_caption = Fossil of ''Isoxys acutangulus'' from the Burgess Shale
}}
'''''Isoxys''''' (meaning "equal surfaces") is a [[genus]] of [[extinct]] bivalved [[Cambrian]] [[arthropod]]; the various species of which are thought to have been freely swimming predators.<ref name='Vannier2009'>{{Cite journal| first1 = J. | first2 = C.| last1 = Vannier| last4 = Chen | first3 = X. | first4 = L.| title = Arthropod visual predators in the early pelagic ecosystem: evidence from the Burgess Shale and Chengjiang biotas| volume = 276| last3 = Hu| journal = Proceedings: Biological Sciences| issue = 1667| pages = 2567–2574| date=Jul 2009 | issn = 0962-8452| pmid = 19403536| pmc = 2686666| last2 = Garcia-Bellido| doi = 10.1098/rspb.2009.0361 }}</ref> It had a pair of large spherical eyes (which are the most commonly preserved feature of the soft-bodied anatomy),<ref name="Garcia2009">{{Cite journal | doi = 10.4202/app.2009.0024 | title = Soft-part preservation in two species of the arthropod Isoxys from the middle Cambrian Burgess Shale of British Columbia, Canada | year = 2009 | last1 = García-Bellido | first1 = D.| url = http://www.app.pan.pl/archive/published/app54/app20090024.pdf| journal = Acta Palaeontologica Polonica| volume = 54 | issue = 4 | pages = 699 | last2 = Vannier | first2 = J.| last3 = Collins | first3 = D. | s2cid = 53613716 | doi-access = free }}</ref> and two large frontal appendages used to grasp prey.<ref name=Vannier2009/>
== Description ==
[[File:Figure 3. Morphological reconstruction and phylogenetic position of Isoxys curvirostratus (cropped).jpg|thumb|Diagram of ''I. curvirostratus'' showing frontal (b) and [[biramous]] appendages (c,d) (c) and (d) represent first four limb pairs and remaining posterior limbs, respectively|left]]Species of ''Isoxys'' have roughly semicircular bivalved carapaces, which vary in morphology between species. The front and rear edges of the carapaces bear forward and posterior facing spines, respectively which in some species are greatly elongated''.<ref name=":1" />'' The carapaces of ''Isoxys'' are typically {{Convert|1.1 to 3.3|cm|in}} in length, excluding the spines, though some species are known to reach over {{Convert|6|cm|in}}. In long-spined species when including spine length, some specimens exceed {{Convert|10|cm|in}}.<ref>{{Cite journal |last1=García-Bellido |first1=Diego C. |last2=Paterson |first2=John R. |last3=Edgecombe |first3=Gregory D. |last4=Jago |first4=James B. |last5=Gehling |first5=James G. |last6=Lee |first6=Michael S. Y. |date=November 2009 |title=The bivalved arthropods Isoxys and Tuzoia with soft-part preservation from the Lower Cambrian Emu Bay Shale Lagerstätte (Kangaroo Island, Australia): Isoxys and Tuzoia From Emu Bay Shale (Australia) |journal=Palaeontology |language=en |volume=52 |issue=6 |pages=1221–1241 |doi=10.1111/j.1475-4983.2009.00914.x|s2cid=129374179 |doi-access=free }}</ref> The opening angle of the carapace was close to vertical, giving it a narrow profile when viewed from above.''<ref name=":1" />''
The head had a pair of large spherical stalked eyes, as well as a pair of upward-curling frontal appendages, which have a varying number of podomeres/segments, depending on the species. Most podomeres on the frontal appendages have upward-facing [[endite]] spines, with the number and placement of spines varying between species. The last podomere of the frontal appendage is a curved terminal claw. The trunk lacks clear segmentation (arthrodization). Along the body are pairs of [[Biramous|biramous appendages]], the counts of which differ between species (''Isoxys curvirostratus'' has 14, while ''Isoxys auritus'' has 11). In ''Isoxys curvirostratus'' the [[endopod]]s have well defined segments/podomeres. The first four biramous limb pairs of ''Isoxys curvirostratus'' were shorter than the remaining pairs, with their endopods having borne well developed [[endite]] (inward facing) spines and ended with a curved subchela (claw), with these features absent in the endopods of the more posterior limbs. The [[exopod]]s of ''Isoxys curvirostratus'' had thick paddle-shaped lamellae which projected perpendicular to the limb axis.<ref name=":2" /> ''Isoxys volucris'' from Greenland had paddle-shaped exopods suggested to have been fringed with [[seta]]e.<ref>{{Cite journal |last1=Stein |first1=Martin |last2=Peel |first2=John S. |last3=Siveter |first3=David J. |last4=Williams |first4=Mark |date=2009-09-07 |title=Isoxys (Arthropoda) with preserved soft anatomy from the Sirius Passet Lagerstätte, lower Cambrian of North Greenland: Isoxys with soft anatomy from Greenland |url=https://onlinelibrary.wiley.com/doi/10.1111/j.1502-3931.2009.00189.x |journal=Lethaia |language=en |volume=43 |issue=2 |pages=258–265 |doi=10.1111/j.1502-3931.2009.00189.x}}</ref> The end of the trunk has paired [[telson]] flaps.<ref name=":2">{{Cite journal |last1=Zhang |first1=Caixia |last2=Liu |first2=Yu |last3=Ortega-Hernández |first3=Javier |last4=Wolfe |first4=Joanna |last5=Jin |first5=Changfei |last6=Mai |first6=Huijuan |last7=Hou |first7=Xian-guang |last8=Guo |first8=Jin |last9=Zhai |first9=Dayou |date=19 April 2023 |title=Three-dimensional morphology of the biramous appendages in Isoxys from the early Cambrian of South China, and its implications for early euarthropod evolution |url= |journal=Proceedings of the Royal Society B: Biological Sciences |volume=290 |issue=1997 |doi=10.1098/rspb.2023.0335 |pmid=37072042 |pmc=10113025 |s2cid=}}</ref>
== Ecology ==
Species of ''Isoxys'' are thought to have been actively swimming predators, using its frontal appendages to capture soft-bodied prey, with the frontalmost pairs of biramous limbs aiding in food processing.<ref name=":2" /> The various ''Isoxys'' species are thought to have occupied a variety of niches, from swimming just above the seafloor (nektobenthic) to open ocean swimmers ([[pelagic]]).<ref name=":1" /> Swimming was likely accomplished by rhythmic movement of the legs.<ref name=":2" /> Eyes of different specimens appear to have been adapted to different light intensities; one specimen of ''I. auritus'' was either crepuscular in shallow water, or lived in waters around 140&nbsp;m below the sea surface; whereas another was morphologically adapted to a diurnal light intensity in shallow waters.<ref name="Schoenemann2010">{{Cite journal| last1 = Schoenemann | first1 = B.| last2 = Clarkson | first2 = E. N. K.| title = Eyes and vision in the Chengjiang arthropod Isoxys indicating adaptation to habitat| journal = Lethaia| volume = 44| issue = 2| pages = no| year = 2010| doi = 10.1111/j.1502-3931.2010.00239.x}}</ref> ''Isoxys'' species with elongated carapace spines are likely to have engaged in [[Diel vertical migration|vertical migration]] up and down the water column, like many modern marine invertebrates.<ref name=":1">{{Cite journal |last1=Pates |first1=Stephen |last2=Daley |first2=Allison C. |last3=Legg |first3=David A. |last4=Rahman |first4=Imran A. |date=2021-06-30 |title=Vertically migrating Isoxys and the early Cambrian biological pump |journal=Proceedings of the Royal Society B: Biological Sciences |volume=288 |issue=1953 |pages=20210464 |doi=10.1098/rspb.2021.0464 |pmc=8220267 |pmid=34157876}}</ref> Specimens of ''Isoxys minor'' have been found with eggs adhered to the inner surface of the carapace, indicating they engaged in brood care. The brood size was large, with approximately 300 eggs, each {{Convert|0.5|mm|in|frac=128}} across per (presumably female) individual. Egg bearing individuals were only around half the maximum size, suggesting that individuals continued to grow beyond sexual maturity.<ref>{{Cite journal |last1=Ma |first1=Jiaxin |last2=Pates |first2=Stephen |last3=Wu |first3=Yu |last4=Lin |first4=Weiliang |last5=Liu |first5=Cong |last6=Wu |first6=Yuheng |last7=Zhang |first7=Mingjing |last8=Fu |first8=Dongjing |date=2023-05-25 |title=Ontogeny and brooding strategy of the early Cambrian arthropod Isoxys minor from the Qingjiang biota |journal=Frontiers in Ecology and Evolution |volume=11 |doi=10.3389/fevo.2023.1174564 |issn=2296-701X |doi-access=free }}</ref>
== Taxonomy ==
[[File:Isoxys longissimus.jpg|thumb|Specimen of ''Isoxys longissimus'']]
''Isoxys'' is thought to be one of the [[Basal (phylogenetics)|basalmost]] known [[stem-group]] arthropods, showing a combination of traits characteristic of more [[Primitive (phylogenetics)|primitive]] [[stem-group]] arthropods like [[Radiodonta|radiodonts]], like lacking an arthrodized ([[Sclerotin|sclerotized]] and jointed) trunk exoskeleton, with those of modern arthropods, like possessing sclerotized and jointed (arthropodized) [[biramous]] limbs. It is one of two genera within the family [[Isoxyidae]], alongside ''[[Surusicaris]]''.<ref name=":2" /> A close relationship to the bivalved arthropod ''[[Tuzoia]]'' had historically been proposed based on the similarities of some aspects of their carapaces,<ref>{{Cite journal |last1=Ma |first1=Jiaxin |last2=Lin |first2=Weiliang |last3=Liu |first3=Cong |last4=Sun |first4=Ao |last5=Wu |first5=Yu |last6=Wu |first6=Yuheng |last7=Fu |first7=Dongjing |date=January 2022 |title=A new bivalved arthropod from the Cambrian (Stage 3) Qingjiang biota expands the palaeogeographical distribution and increases the diversity of Tuzoiidae |url=https://www.lyellcollection.org/doi/10.1144/jgs2020-229 |journal=Journal of the Geological Society |language=en |volume=179 |issue=1 |pages=jgs2020–229 |doi=10.1144/jgs2020-229 |bibcode=2022JGSoc.179..229M |s2cid=236289449 |issn=0016-7649}}</ref> but preserved soft tissues of ''Tuzoia'' described in 2022 suggest that they are not closely related.<ref name=":22">{{Cite journal |last1=Izquierdo-López |first1=Alejandro |last2=Caron |first2=Jean-Bernard |date=December 2022 |title=The problematic Cambrian arthropod Tuzoia and the origin of mandibulates revisited |journal=Royal Society Open Science |language=en |volume=9 |issue=12 |pages=220933 |doi=10.1098/rsos.220933 |pmid=36483757 |issn=2054-5703 |pmc=9727825|bibcode=2022RSOS....920933I }}</ref>
=== Species ===
20 species of ''Isoxys'' have been described,<ref name=":1" /> which have a global distribution, having been found in North America, Siberia, Australia, China and Europe, spanning from [[Cambrian Series 2]] into the [[Miaolingian]].<ref name=":0" />
* ''Isoxys chilhoweanus'' <small>Walcott, 1890 ([[Type species|type]])</small> [[Tennessee]], USA, [[Cambrian Series 2]]
* ''Isoxys acutangulus'' <small>(Walcott, 1908)</small> [[Balang Formation]], [[Guizhou]], China, [[Cambrian Stage 4]], [[Burgess Shale]], Canada, [[Miaolingian]]
* ''Isoxys auritus'' <small>(Jiang, 1982)</small> [[Chengjiang Biota]], [[Yunnan]], China, [[Cambrian Stage 3]],  Balang Formation, China, Cambrian Stage 4
* ''Isoxys'' ''curvirostratus'' <small>Vannier & Chen, 2000</small> Chengjiang Biota, China, Cambrian Stage 3
* ''Isoxys'' ''paradoxus'' <small>Hou, 1987</small> Chengjiang Biota, China, Cambrian Stage 3
* ''Isoxys'' ''zhurensis'' <small>Ivantsov, 1990</small> [[Sinsk Formation]], Siberia, [[Cambrian Series 2]]
* ''Isoxys bispinatus'' <small>Cui, 1991</small> [[Shuijintuo Formation]], [[Sichuan]], China, Cambrian Series 2
* ''Isoxys glaessneri'' <small>García−Bellido, Paterson, Edgecombe, Jago, Gehling & Lee, 2009</small> [[Emu Bay Shale]], Australia, Cambrian Stage 4
* ''Isoxys'' ''communis'' <small>Glaessner, 1979</small> [[Emu Bay Shale]], Australia, Cambrian Stage 4
* ''Isoxys guanduensis'' <small>Wang et al., 2012</small> [[Guanshan Biota]], Yunnan, China, Cambrian Stage 4
* ''Isoxys'' ''minor'' <small>Luo et al., 2008</small> Guanshan Biota, Yunnan, China, Cambrian Stage 4
* ''Isoxys'' ''wudingensis'' <small>Luo & Hu, 2006</small> Guanshan Biota, Yunnan, China, Cambrian Stage 4
* ''Isoxys globulus'' <small>Liu et al., 2018</small> Balang Formation, Guizhou, China, Cambrian Stage 4
* ''Isoxys jianheensis'' <small>Liu et al., 2018</small> Balang Formation,  Guizhou, China, Cambrian Stage 4
* ''Isoxys volucris'' <small>Williams, Siveter & Peel, 1996</small>, [[Sirius Passet]], Greenland, Cambrian Stage 3
* ''Isoxys mackenziensis'' <small>Kimmig & Pratt 2015</small> [[Rockslide Formation]], Canada, Miaolingian
* ''Isoxys longissimus'' <small>Simonetta & Delle Cave, 1975</small> Burgess Shale, Canada, Miaolingian
* ''Isoxys'' ''carbonelli'' <small>Richter & Richter, 1927</small>, [[Pedroche Formation]], Spain, Cambrian Series 2
* ''Isoxys shandongensis'' <small>Wang and Huang, 2010</small>, [[Mantou Formation]], Shandong, China, Miaolingian
Indeterminate species are also known from the [[Spence Shale]] of Utah, dating to the Miaolingian, as well as the [[Kaili Formation|Kaili Biota]] in Guizhou, China, which also dates to the Miaolingian.<ref name=":0">{{Cite journal |last1=Liu |first1=S. |last2=Peng |first2=J. |last3=Wen |first3=R. |last4=Liang |first4=B. |date=2018-06-30 |title=New data for Isoxys of the Balang Fauna (Cambrian Stage 4), South China |url=http://www.geology.cz/bulletin/contents/art1673 |journal=Bulletin of Geosciences |language=en |pages=147–162 |doi=10.3140/bull.geosci.1673 |s2cid=134294843 |issn=1802-8225|doi-access=free }}</ref>
==See also==
{{Portal|paleontology|Arthropods}}
* [[Arthropod]]
* [[Cambrian explosion]]
* [[Chengjiang biota]]
** [[List of Chengjiang Biota species by phylum]]
*[[Paleobiota of the Burgess Shale]]
==References==
{{Reflist}}
== External links ==
* {{Cite web|date=2011|title=''Isoxys acutangulus''|work=Burgess Shale Fossil Gallery|publisher=Virtual Museum of Canada|url=http://burgess-shale.rom.on.ca/en/fossil-gallery/view-species.php?id=69|archive-url=https://web.archive.org/web/20201112025257/http://burgess-shale.rom.on.ca/en/fossil-gallery/view-species.php?id=69|archive-date=2020-11-12|url-status=dead|access-date=2023-01-21}}
{{Taxonbar|from=Q143944}}
[[Category:Burgess Shale fossils]]
[[Category:Cambrian arthropods]]
[[Category:Burgess Shale animals]]
[[Category:Maotianshan shales fossils]]
[[Category:Paibian]]
[[Category:Cambrian genus extinctions]]
[[Category:Wheeler Shale]]
[[Category:Paleozoic life of the Northwest Territories]]