| Isoxyida Temporal range:
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| Restoration of Isoxys curvirostratus | |
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| Restoration of Surusicaris elegans | |
Scientific classification 
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| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Clade: | Deuteropoda |
| Order: | †Isoxyida Simonetta & Delle Cave, 1975 |
| Subgroups | |
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Isoxyids are members of the order Isoxyida and the family Isoxyidae, a group of basal arthropods that existed during the Cambrian and possibly early Ordovician periods. Its best known members are Isoxys, with 20 species found worldwide, and Surusicaris, known from a single species found in the Burgess Shale of Canada. Isoxyida is also sometimes taken to include the family Sunellidae, which comprises four genera: Sunella, Combinivalvula, Jinningella and Caudicaella, all of which are known from deposits in China,[1] with Caudicaella also known from the Heatherdale Shale of South Australia.[2] Isoxys and Surusicaris are distinguished by their bivalved carapaces and pair of upward curving grasping frontal appendages. Sunellids also had bivalved carapaces, but unlike other isoxyids these were distinguished by the presence of an anterodorsal sulcus on their carapaces. These arthropods are thought to have been predators, hunting soft-bodied prey in either the water column, or close to the seabed.[3][4][5]
Description
editIsoxys and Surusicaris have a combination of features seen in both stem-group arthropods, as well as more advanced taxa. They have semicircular bivalved carapaces, large, spherical 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 radiodonts, with a segmented and sclerotised (arthropodized) trunk being characteristic of most arthropods.[6][3] On the other hand, sunellids are known mostly from carapaces and their fossils do not preserve frontal appendages similar to those of Isoxys, but some preserve soft tissue, including an apparently segmented trunk, trunk appendages, large, spherical eyes and a possible median eye.[7] Their bivalved carapaces also have an anterodorsal sulcus, a character shared with bradoriids, another group of similar but unrelated arthropods.[1]
Distribution and chronology
editIsoxys has an almost cosmopolitan distribution, and is known from twenty species distributed in deposits of North America, Siberia, Australia, China and Europe. Surusicaris is limited to the Burgess shale site in British Columbia, and only contains a single species.[5] The four genera of Sunellidae are known from the Chengjiang biota and the Qingjiang biota in China, while Caudicaella is also known from the Heatherdale Shale in South Australia.[2] Isoxys? giganteus is known from the Early Ordovician (Tremadocian) Fezouata Formation of Morocco. If its identification as a species of Isoxys is correct, it would be the latest surviving species of the isoxyids by 20 million years. However, due to it only being known from the carapace and not soft tissues, its identity as an isoxyid is equivocal, and could potentially represent part of a radiodont instead.[8]
Taxonomy
editThis cladogram shows the relationships of the isoxyids compared to other arthropod groups, according to the results of Liu et al. 2026.[9]
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The true classification of the isoxyids has been somewhat controversial. Isoxyida is generally placed outside the crown group of Arthropoda, with both Mandibulata and Chelicerata being more closely related to each other than to isoxyids.[6][10] However, some studies alternatively place Isoxyida as more closely related to Mandibulata than to Chelicerata and therefore within the crown group.[11] Isoxyids are currently placed within Deuteropoda, a proposed clade of arthropods whose members distinguished from more basal stem group arthropods like radiodonts and "gilled lobopodians" by an anatomical reorganization of the head region, multisegmented head, a hypostome/labrum complex, and pairs of biramous (two branched) segmented (arthropodized) limbs running along the body.[12][13] 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 (part of Mandibulata), and that it was not closely related to the isoxyids.[14]
Ecology
editSpecies of the Isoxyida 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.[6] 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).[5] Swimming was likely accomplished by rhythmic movement of the legs.[6] Isoxyid species with elongated carapace spines are likely to have engaged in vertical migration up and down the water column, like many modern marine invertebrates.[5]
Gallery
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Diagram of Surusicaris -
Diagram of Isoxys curvirostratus -
The holotype, and only known fossil, of Surusicaris -
Fossil of Isoxys acutangulus
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See also
editReferences
edit- ^ a b c Collantes, Luis; Pates, Stephen. "Isoxys carbonelli and the palaeoenvironmental disparity of Isoxys during Cambrian Stage 3". Historical Biology. 0 (0): 1–9. doi:10.1080/08912963.2025.2529395. ISSN 0891-2963.
- ^ a b Chen, Fei-Yang; Betts, Marissa J.; Zhang, Zhi-Liang; Brock, Glenn A. (2025-06-01). "The bivalved arthropod Caudicaella aff. bispinata from the Heatherdale Shale (Cambrian Stage 3), South Australia". Palaeoworld. 34 (3): 100882. doi:10.1016/j.palwor.2024.09.006. ISSN 1871-174X.
- ^ a b Aria, Cédric; Caron, Jean-Bernard (2015-06-03). Friedman, Matt (ed.). "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". PLOS ONE. 10 (6) e0124979. Bibcode:2015PLoSO..1024979A. doi:10.1371/journal.pone.0124979. ISSN 1932-6203. PMC 4454494. PMID 26038846.
- ^ Vannier, J.; Garcia-Bellido, C.; Hu, X.; Chen, L. (Jul 2009). "Arthropod visual predators in the early pelagic ecosystem: evidence from the Burgess Shale and Chengjiang biotas". Proceedings: Biological Sciences. 276 (1667): 2567–2574. doi:10.1098/rspb.2009.0361. ISSN 0962-8452. PMC 2686666. PMID 19403536.
- ^ a b c d Pates, Stephen; Daley, Allison C.; Legg, David A.; Rahman, Imran A. (2021-06-30). "Vertically migrating Isoxys and the early Cambrian biological pump". Proceedings of the Royal Society B: Biological Sciences. 288 (1953) 20210464. doi:10.1098/rspb.2021.0464. PMC 8220267. PMID 34157876.
- ^ a b c d Zhang, Caixia; Liu, Yu; Ortega-Hernández, Javier; Wolfe, Joanna; Jin, Changfei; Mai, Huijuan; Hou, Xian-guang; Guo, Jin; Zhai, Dayou (19 April 2023). "Three-dimensional morphology of the biramous appendages in Isoxys from the early Cambrian of South China, and its implications for early euarthropod evolution". Proceedings of the Royal Society B: Biological Sciences. doi:10.1098/rspb.2023.0335. PMC 10113025.
- ^ Zhang, Xingliang; Shu, Degan (2007). "Soft anatomy of sunellid arthropods from the Chengjiang Lagerstutte, Lower Cambrian of southwest China". Journal of Paleontology. 81 (6): 1412–1422. doi:10.1666/06-031R.1. ISSN 0022-3360.
- ^ Yoshino, Saraku; Oji, Tatsuo (2026-05-27). "A bivalved arthropod from the lower Ordovician Fezouata Shale suggests the post Cambrian survival and gigantism of Isoxys (Isoxyda, Isoxyidae)". Historical Biology: 1–12. doi:10.1080/08912963.2026.2663158. ISSN 0891-2963.
- ^ Liu, Cong; Pates, Stephen; Zhang, Mingjing; Wu, Yu; Ma, Jiaxin; Fu, Dongjing; Zhang, Xingliang (2026-03-21). "3D morphology of the Cambrian bivalved arthropod Sunella informs about head segmentation, arthrodization, and arthropodization". Communications Biology. doi:10.1038/s42003-026-09909-z. ISSN 2399-3642.
- ^ Aria, Cédric (26 April 2022). "The origin and early evolution of arthropods". Biological Reviews. 97 (5): 1786–1809. doi:10.1111/brv.12864. ISSN 1464-7931.
- ^ O’Flynn, Robert J.; Liu, Yu; Hou, Xianguang; Mai, Huijuan; Yu, Mengxiao; Zhuang, Songling; Williams, Mark; Guo, Jin; Edgecombe, Gregory D. (2023-09-25). "The early Cambrian Kylinxia zhangi and evolution of the arthropod head". Current Biology. 33 (18): 4006–4013.e2. doi:10.1016/j.cub.2023.08.022. hdl:10141/623137. ISSN 0960-9822. PMID 37643622.
- ^ Ortega-Hernández, Javier (2016), "Making sense of 'lower' and 'upper' stem-group Euarthropoda, with comments on the strict use of the name Arthropoda von Siebold, 1848", Biol. Rev., 91 (1): 255–273, doi:10.1111/brv.12168, PMID 25528950, S2CID 7751936
- ^ Ma, Jiaxin; Lin, Weiliang; Liu, Cong; Sun, Ao; Wu, Yu; Wu, Yuheng; Fu, Dongjing (January 2022). "A new bivalved arthropod from the Cambrian (Stage 3) Qingjiang biota expands the palaeogeographical distribution and increases the diversity of Tuzoiidae". Journal of the Geological Society. 179 (1): jgs2020–229. Bibcode:2022JGSoc.179..229M. doi:10.1144/jgs2020-229. ISSN 0016-7649. S2CID 236289449.
- ^ Izquierdo-López, Alejandro; Caron, Jean-Bernard (December 2022). "The problematic Cambrian arthropod Tuzoia and the origin of mandibulates revisited". Royal Society Open Science. 9 (12) 220933. Bibcode:2022RSOS....920933I. doi:10.1098/rsos.220933. ISSN 2054-5703. PMC 9727825. PMID 36483757.
