| Isoxyida | |
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| Restoration ofIsoxys curvirostratus | |
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| Restoration ofSurusicaris elegans | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Order: | †Isoxyida Simonetta & Delle Cave, 1975 |
| Family: | †Isoxyidae Vogdes, 1893 |
| Genera | |
| Synonyms | |
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Isoxyids are members of the orderIsoxyida and the familyIsoxyidae, a group of basalarthropods that existed during theCambrian period. It contains two genera,Isoxys, with 20 species found worldwide, andSurusicaris known from a single species found in theBurgess Shale of Canada. They are distinguished by their bivalved carapaces and pair of upward curving grasping frontal appendages.
Isoxyids have a combination of features seen in both stem-group arthropods, as well as more advanced taxa. Isoxyids have semicircular bivalvedcarapaces, large, sphericaleyes, a pair of large upward curling frontal appenages covered in spines, and pairs ofbiramous limbs running along the body. The trunk region appears to be unsegmented and lacks sclerotisation, similar toradiodonts, with a segmented and sclerotised (arthropodized) trunk being characteristic of most arthropods.[1][2] Currently only twogenera have been accepted to be members of this family. These beingIsoxys, which has an almostcosmopolitan distribution, and is known from twenty species, andSurusicaris, which is limited to theBurgess shale site inBritish 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]
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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 thatIsoxys was related to another bivalved arthropod from the Cambrian,Tuzoia, due to the similar aspects of their carapaces. However, a study in 2022 found thatTuzoia was a member ofHymenocarina, and that it was not closely related to the isoxyids. Currently the group has been placed into theDeuteropoda, a proposedclade 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 ahypostome/labrum complex,[5][6][7] being placed outside thecrown group of Arthropoda, with bothMandibulata andChelicerata 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 invertical migration up and down the water column, like many modern marine invertebrates.[4]
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