View source for Isoxyida

{{short description|Extinct order of arthropods}}
{{Automatic taxobox
| fossil_range = {{fossil range|Cambrian Stage 3|Miaolingian}}
| image = Artistic reconstruction of Isoxys curvirostratus.jpg
| image_caption = Restoration of ''[[Isoxys|Isoxys curvirostratus]]''
| image2 = Surusicaris reconstruction.png
| image2_caption = Restoration of ''[[Surusicaris|Surusicaris elegans]]''
| taxon = Isoxyidae
| authority = Vogdes, 1893
| parent_authority = Simonetta & Delle Cave, 1975
| synonyms = *Isoxysidae {{au|Brooks & Caster, 1956}}
| subdivision_ranks = Genera
| subdivision = * {{extinct}}''[[Isoxys]]''
* {{extinct}}''[[Surusicaris]]''
}}
'''Isoxyids''' are members of the order '''Isoxyida''' and the family '''Isoxyidae''', a group of basal [[arthropod]]s that existed during the [[Cambrian]] period. It contains two genera, ''[[Isoxys]]'', with 20 species found worldwide, and ''[[Surusicaris]]'' known from a single species found in the [[Burgess Shale]] of Canada. They are distinguished by their bivalved carapaces and pair of upward curving grasping frontal appendages.
== Description ==
Isoxyids have a combination of features seen in both stem-group arthropods, as well as more advanced taxa. Isoxyids have semicircular bivalved [[Carapace|carapaces]], large, spherical [[Arthropod eye|eyes]], a pair of large upward curling frontal appenages covered in spines, and pairs of [[biramous]] limbs running along the body. The trunk region appears to be unsegmented and lacks sclerotisation, similar to [[Radiodonta|radiodonts]], with a segmented and sclerotised (arthropodized) trunk being characteristic of most arthropods.<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 |doi=10.1098/rspb.2023.0335 |s2cid=|pmc=10113025 }}</ref><ref name=":0" /> Currently only two [[genera]] have been accepted to be members of this family. These being ''[[Isoxys]]'', which has an almost [[Cosmopolitan distribution|cosmopolitan]] distribution, and is known from twenty species, and ''[[Surusicaris]]'', which is limited to the [[Burgess Shale|Burgess shale]] site in [[British Columbia]], and only contains a single species. These arthropods are thought to have been predators, hunting soft-bodied prey in either the water column, or close to the seabed.<ref name=":0">{{Cite journal |last1=Aria |first1=Cédric |last2=Caron |first2=Jean-Bernard |date=2015-06-03 |editor-last=Friedman |editor-first=Matt |title=Cephalic and Limb Anatomy of a New Isoxyid from the Burgess Shale and the Role of "Stem Bivalved Arthropods" in the Disparity of the Frontalmost Appendage |journal=PLOS ONE |language=en |volume=10 |issue=6 |pages=e0124979 |bibcode=2015PLoSO..1024979A |doi=10.1371/journal.pone.0124979 |issn=1932-6203 |pmc=4454494 |pmid=26038846 |doi-access=free}}</ref><ref name="Vannier2009">{{Cite journal |last1=Vannier |first1=J. |last2=Garcia-Bellido |first2=C. |last3=Hu |first3=X. |last4=Chen |first4=L. |date=Jul 2009 |title=Arthropod visual predators in the early pelagic ecosystem: evidence from the Burgess Shale and Chengjiang biotas |journal=Proceedings: Biological Sciences |volume=276 |issue=1667 |pages=2567–2574 |doi=10.1098/rspb.2009.0361 |issn=0962-8452 |pmc=2686666 |pmid=19403536}}</ref><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>
== Taxonomy ==
{{clade|{{clade
|label2=[[Deuteropoda]]
|1={{clade
|1=[[Radiodonta]]
}}
|2={{clade
|1={{clade
|1='''Isoxyida'''
|2=
}}
|label2=
|2={{clade
|1=[[Fuxianhuiida]]<br>[[Megacheira]]<br>[[Artiopoda]]<br>crown group arthropods ([[Mandibulata]], [[Chelicerata]])
}}
}}
}}|label1=}}
This cladogram shows the relationships of the isoxyids compared to other arthropod groups, according to the results of Zhang et al. 2023.<ref name=":2" />
The true classification of the isoxyids has been somewhat controversial. Although they are often regarded as basal arthropods, where they actually fit into the arthropod family tree has been contested. Originally, it was thought that ''Isoxys'' was related to another bivalved arthropod from the Cambrian, ''[[Tuzoia]]'', due to the similar aspects of their carapaces. However, a study in 2022 found that ''Tuzoia'' was a member of [[Hymenocarina]], and that it was not closely related to the isoxyids. Currently the group has been placed into the [[Deuteropoda]], a proposed [[clade]] of arthropods whose members are distinguished by an anatomical reorganization of the head region, including the appearance of a differentiated first appendage pair (the '[[Deutocerebrum|deutocerebral]]' pair), a multisegmented head, and a [[Hypostome (trilobite)|hypostome]]/[[Labrum (arthropod mouthpart)|labrum]] complex,<ref name="Ortega-Hernández">{{Citation |last1=Ortega-Hernández |first1=Javier |title=Making sense of 'lower' and 'upper' stem-group Euarthropoda, with comments on the strict use of the name Arthropoda von Siebold, 1848 |journal=Biol. Rev. |volume=91 |issue=1 |pages=255–273 |year=2016 |doi=10.1111/brv.12168 |pmid=25528950 |s2cid=7751936}}</ref><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 |bibcode=2022JGSoc.179..229M |doi=10.1144/jgs2020-229 |issn=0016-7649 |s2cid=236289449}}</ref><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 |bibcode=2022RSOS....920933I |doi=10.1098/rsos.220933 |issn=2054-5703 |pmc=9727825 |pmid=36483757}}</ref> being placed outside the [[crown group]] of Arthropoda, with both [[Mandibulata]] and [[Chelicerata]] being more closely related to each other than to isoxyids.<ref name=":2" />
== Ecology ==
Species of the Isoxydia are thought to have been actively swimming predators, using their frontal appendages to capture soft-bodied prey, with the frontalmost pairs of biramous limbs aiding in food processing.<ref name=":2" /> The various  species of the order 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" /> Isoxyid 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" />
== Gallery ==
{{Gallery |mode=packed |align=center
|File:Surusicaris restoration.png|Diagram of ''Surusicaris''
|File:Figure 3. Morphological reconstruction and phylogenetic position of Isoxys curvirostratus (cropped).jpg|Diagram of ''Isoxys curvirostratus''
|File:Surusicaris (cropped).png|The [[holotype]], and only known fossil, of ''Surusicaris''
|File:Isoxys acutangulus 2.jpg|Fossil of ''Isoxys acutangulus''
}}
==References==
{{Reflist}}
{{Taxonbar|from1=Q117853288}}
[[Category:Arthropod taxa]]
[[Category:Extinct arthropods]]