| Isoxys | |
|---|---|
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| Isoxys curvirostratus from the Chengjiang Biota | |
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| Fossil ofIsoxys acutangulus from the Burgess Shale | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Order: | †Isoxyida |
| Family: | †Isoxyidae |
| Genus: | †Isoxys Walcott, 1890 |
| Type species | |
| Isoxys chilhoweanus Walcott, 1890 | |
| Species | |
See text | |
Isoxys (meaning "equal surfaces") is agenus ofextinct bivalvedCambrianarthropod; the various species of which are thought to have been freely swimming predators.[1] It had a pair of large spherical eyes (which are the most commonly preserved feature of the soft-bodied anatomy),[2] and two large frontal appendages used to grasp prey.[1]
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Species ofIsoxys have roughly semicircular bivalved carapaces, which vary in morphology between species. The front and rear edges of the carapaces bear forward and posterior facing spines, respectively which in some species are greatly elongated.[3] The carapaces ofIsoxys are typically 1.1 to 3.3 centimetres (0.43 to 1.30 in) in length (with the juveniles of some species being as small as 0.5 centimetres (0.20 in)[4]), excluding the spines, though some species are known to reach over 6 centimetres (2.4 in). In long-spined species when including spine length, some specimens exceed 10 centimetres (3.9 in).[5] The opening angle of the carapace was close to vertical, giving it a narrow profile when viewed from above.[3]
The head had a pair of large spherical stalked eyes, as well as a pair of upward-curling frontal appendages, which have a varying number of podomeres/segments, depending on the species. Most podomeres on the frontal appendages have upward-facingendite spines, with the number and placement of spines varying between species. The last podomere of the frontal appendage is a curved terminal claw. The trunk lacks clear segmentation (arthrodization). Along the body are pairs ofbiramous (divided into two parts) appendages, the counts of which differ between species (Isoxys curvirostratus has 14, whileIsoxys auritus has 11). InIsoxys curvirostratus theendopods (the inner, leg-like parts of the biramous limbs/appendages) have well defined segments/podomeres. The first four biramous limb pairs ofIsoxys curvirostratus were shorter than the remaining pairs, with their endopods having borne well developedendite (inward facing) spines and ended with a curved subchela (claw), with these features absent in the endopods of the more posterior limbs. Theexopods (outer part of the biramous limbs) ofIsoxys curvirostratus had thick paddle-shaped lamellae which projected perpendicular to the limb axis.[6]Isoxys volucris from Greenland had paddle-shaped exopods suggested to have been fringed withsetae.[7] The end of the trunk has pairedtelson flaps.[6]
Species ofIsoxys are thought to have been actively swimming predators, using its frontal appendages to capture soft-bodied prey, with the frontalmost pairs of biramous limbs aiding in food processing.[6] The variousIsoxys species are thought to have occupied a variety of niches, from swimming just above the seafloor (nektobenthic) to open ocean swimmers (pelagic).[3] Swimming was likely accomplished by rhythmic movement of the legs.[6] Eyes of different specimens appear to have been adapted to different light intensities; one specimen ofI. auritus was eithercrepuscular in shallow water, or lived in waters around 140 m below the sea surface; whereas another was morphologically adapted to a diurnal light intensity in shallow waters.[8]Isoxys species with elongated carapace spines are likely to have engaged invertical migration up and down the water column, like many modern marine invertebrates.[3] The elongate carapace spines, particularly the forward-facing ones, are thought to have improved hydrodynamic lift, as well as possibly serving as a defense against predators. Some pelagicIsoxys species qualified as beingzooplankton.[4] Specimens ofIsoxys minor have been found with eggs adhered to the inner surface of the carapace, indicating they engaged in brood care. The brood size was large, with approximately 300 eggs, each 0.5 millimetres (3⁄128 in) across per (presumably female) individual. Egg bearing individuals were only around half the maximum size, suggesting that individuals continued to grow beyond sexual maturity.[9]Isoxys species lacked a distinct planktic larval stage. At least someIsoxys species carapace morphology changed between juveniles and adults, which likely improved their hydrodynamic efficiency.[4]
Species ofIsoxys were preyed upon by other animals.[4] Carapaces of theSirius Passet speciesIsoxys volucris have been frequently found as stomach contents of the giant stem-chaetognath (arrow worm)Timorebestia,[10] with this species also found as stomach contents of aSidneyia-like arthropod known from the same deposit.[11] Likelycoprolites (fossilised feces) found inChengjiang biota deposits containingIsoxys carapaces are suggested to have been produced byradiodonts.[12]
Isoxys is thought to be one of thebasalmost knownstem-group arthropods, showing a combination of traits characteristic of moreprimitivestem-group arthropods likeradiodonts, 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 familyIsoxyidae, alongsideSurusicaris.[6]
Cladogram after Zhang et al. 2023:[6]
| Total group Arthropoda |
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A close relationship to the bivalved arthropodTuzoia had historically been proposed based on the similarities of some aspects of their carapaces,[13] but preserved soft tissues ofTuzoia described in 2022 suggest that they are not closely related.[14]
20 species ofIsoxys have been described,[3] which have a global distribution, having been found in North America, Siberia, Australia, China and Europe, spanning fromCambrian Series 2 into theMiaolingian.[15]
Indeterminate species are also known from theSpence Shale of Utah, dating to the Miaolingian, as well as theKaili Biota in Guizhou, China, which also dates to the Miaolingian.[15]
