Isoxyida

Isoxyids have a combination of features seen in both stem-group arthropods, as well as more advanced taxa. Isoxyids 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.[1][2] Currently only two genera have been accepted to be members of this family. These being Isoxys, which has an almost cosmopolitan distribution, and is known from twenty species, and Surusicaris, which is limited to the 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.[2][3][4]

This cladogram shows the relationships of the isoxyids compared to other arthropod groups, according to the results of Zhang et al. 2023.[1]

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 'deutocerebral' pair), a multisegmented head, and a hypostome/labrum complex,[5][6][7] being placed outside the crown group of Arthropoda, with both Mandibulata and Chelicerata being more closely related to each other than to isoxyids.[1]

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.[1] 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).[4] Swimming was likely accomplished by rhythmic movement of the legs.[1] 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.[4]