in shallow waters. Isoxys species with elongated carapace spines are likely to have engaged in vertical migration up and down the water column, like many modern marine invertebrates. Taxonomy Isoxys is thought to be one of the basalmost known arthropods, showing a combination of traits characteristic of more primitive stem-group arthropods like radiodonts, like lacking an arthrodized (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. A close relationship to the bivalved arthropod Tuzoia had historically been proposed based on the similarities of some aspects of their carapaces, but preserved soft tissues of Tuzoia described in 2022 suggest that they are not closely related. Species 20 species of Isoxys have been described, which have a global distribution, having been found in North America, Siberia, Australia, China and Europe, spanning from Cambrian Series 2 into the Miaolingian. Isoxys chilhoweanus Walcott, 1890 (type) Tennessee, USA, Cambrian Series 2 Isoxys acutangulus (Walcott, 1908) Balang Formation, Guizhou, China, Cambrian Stage 4, Burgess Shale, Canada, Miaolingian Isoxys auritus (Jiang, 1982) Chengjiang Biota, Yunnan, China, Cambrian Stage 3, Balang Formation, China, Cambrian Stage 4 Isoxys curvirostratus Vannier & Chen, 2000 Chengjiang Biota, China, Cambrian Stage 3 Isoxys paradoxus Hou, 1987 Chengjiang Biota, China, Cambrian Stage 3 Isoxys zhurensis Ivantsov, 1990 Sinsk Formation, Siberia, Cambrian Series 2 Isoxys bispinatus Cui, 1991 Shuijintuo Formation, Sichuan, China, Cambrian Series 2 Isoxys glaessneri García−Bellido, Paterson, Edgecombe, Jago, Gehling & Lee, 2009 Emu Bay Shale, Australia, Cambrian Stage 4 Isoxys communis Glaessner, 1979 Emu Bay Shale, Australia, Cambrian Stage 4 Isoxys guanduensis Wang et al., 2012 Guanshan Biota, Yunnan, China, Cambrian Stage 4 Isoxys minor Luo et al., 2008 Guanshan Biota, Yunnan, China, Cambrian Stage 4 Isoxys wudingensis Luo & Hu, 2006 Guanshan Biota, Yunnan, China, Cambrian Stage 4 Isoxys globulus Liu et al., 2018 Balang Formation, Guizhou, China, Cambrian Stage 4 Isoxys jianheensis Liu et al., 2018 Balang Formation, Guizhou, China, Cambrian Stage 4 Isoxys volucris Williams, Siveter & Peel, 1996, Sirius Passet, Greenland, Cambrian Stage 3 Isoxys mackenziensis Kimmig & Pratt 2015 Rockslide Formation, Canada, Miaolingian Isoxys longissimus Simonetta & Delle Cave, 1975 Burgess Shale, Canada, Miaolingian Isoxys carbonelli Richter & Richter, 1927, Pedroche Formation, Spain, Cambrian Series 2 Isoxys shandongensis Wang and Huang, 2010, Mantou Formation, Shandong, China, Miaolingian Indeterminate species are also known from the Spence