# Isoxys

Isoxys (meaning "equal surfaces") is a genus of extinct bivalved Cambrian arthropod; the various species of which are thought to have been freely swimming predators. It had a pair of large spherical eyes (which are the most commonly preserved feature of the soft-bodied anatomy), and two large frontal appendages used to grasp prey.

## Description

Species of Isoxys 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. The carapaces of Isoxys 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)), 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). The opening angle of the carapace was close to vertical, giving it a narrow profile when viewed from above.
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-facing endite 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 of biramous (divided into two parts) appendages, the counts of which differ between species (Isoxys curvirostratus has 14, while Isoxys auritus has 11). In Isoxys curvirostratus the endopods (the inner, leg-like parts of the biramous limbs/appendages) have well defined segments/podomeres. The first four biramous limb pairs of Isoxys curvirostratus were shorter than the remaining pairs, with their endopods having borne well developed endite (inward facing) spines and ended with a curved subchela (claw), with these features absent in the endopods of the more posterior limbs. The exopods (outer part of the biramous limbs) of Isoxys curvirostratus had thick paddle-shaped lamellae which projected perpendicular to the limb axis. Isoxys volucris from Greenland had paddle-shaped exopods suggested to have been fringed with setae. The end of the trunk has paired telson flaps.

## Ecology

Species of Isoxys 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. The various Isoxys species are thought to have occupied a variety of niches, from swimming just above the seafloor (nektobenthic) to open ocean swimmers (pelagic). Swimming was likely accomplished by rhythmic movement of the legs. Eyes of different specimens appear to have been adapted to different light intensities; one specimen of I. auritus was either crepuscular 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. 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. 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 pelagic Isoxys species qualified as being zooplankton. Specimens of Isoxys 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. Isoxys species lacked a distinct planktic larval stage. At least some Isoxys species carapace morphology changed between juveniles and adults, which likely improved their hydrodynamic efficiency.
Species of Isoxys were preyed upon by other animals. Carapaces of the Sirius Passet species Isoxys volucris have been frequently found as stomach contents of the giant stem-chaetognath (arrow worm) Timorebestia, with this species also found as stomach contents of a Sidneyia-like arthropod known from the same deposit. Likely coprolites (fossilised feces) found in Chengjiang biota deposits containing Isoxys carapaces are suggested to have been produced by radiodonts.

## Taxonomy

Isoxys is thought to be one of the basalmost known stem-group 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.
Cladogram after Zhang et al. 2023:
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 Shale of Utah, dating to the Miaolingian, as well as the Kaili Biota in Guizhou, China, which also dates to the Miaolingian.