3D animation of Isoxys carinatus.
Animation by Phlesch Bubble © Royal Ontario Museum
Isoxys is included with Surusicaris in the family isoxyidae, based on the type of carapace, eyes, and frontal pair of raptorial appendages (Aria & Caron, 2015). Current evidence draws out a consensus among authors placing isoxyids as sister taxa to true arthropods (Edgecombe, 2020; Aria, 2022), although it is not clear whether Surusicaris and Isoxys are part of a single separate lineage (that is, form a monophyletic group). However, key characters such as the segmentation of the trunk and trunk appendages are still being investigated.
Isoxys – from the Greek isos, “equal,” and xystos, “smooth surface”; thus referring to the pair of smooth valves.
acutangulus – from the Latin acutus, “sharp, pointed,” and angulus, “angle”; thus referring to the acute angle of the cardinal spines.
Burgess Shale and vicinity: I. longissimus from Walcott, Raymond and Collins Quarries on Fossil Ridge.
Other deposits: I. chilhoweanus from the Chilhowee Group, Tennessee, USA; I. auritus, I. paradoxus and I. curvirostratus from the Maotianshan Shale of China; I. bispinatus from the Shuijingtuo Formation, Hubei, China; I. wudingensis from the Guanshan fauna of China; I. communis and I. glaessneri from the Emu Bay Shale of Australia; I. volucris from the Buen Formation, Sirius Passet in Greenland; I. carbonelli from the Sierro Morena of Spain, and I. zhurensis from the Profallotaspis jakutensis Zone of Western Siberia. Undescribed species from Canada; Mount Cap Formation in the Mackenzie Mountains, Northwest Territories and the Eager Formation near Cranbrook. Other undescribed species in the Kaili Formation, Guizhou Province, China and the Kinzers Formation, Pennsylvania, USA. See references in Briggs et al. (2008), García-Bellido et al. (2009b; 2009a), Stein et al. (2010), Vannier and Chen (2000).
The Walcott, Raymond and Collins Quarries on Fossil Ridge. Additional localities are known on Mount Field, Mount Stephen – the Tulip Beds (S7), the Trilobite Beds, near Stanley Glacier and across Marble Canyon localities, Kootenay National Park, British Columbia.
Walcott gave the name Isoxys to specimens from the lower Cambrian Chilhowee Group of Tennessee, USA, in 1890. He then later designated the first species from the Trilobite Beds on Mount Stephen, Anomalocaris? acutangulus (Walcott, 1908), erroneously placing it within the genus Anomalocaris. Simonetta and Delle Cave (1975) renamed it Isoxys acutangulus and discovered a second Burgess Shale species, I. longissimus. The original designations were based on carapaces only, making research on the ecology and affinity of Isoxys difficult. Since then, eyes, appendages and tailpiece have been described (García-Bellido et al., 2009b; Vannier et al., 2009; Fu, Zhang & Shu, 2011; Legg & Vannier, 2013; Fu et al., 2014). Trunk appendages seem atypical, with broad attachment of outer and inner branches—as in Surusicaris—and close contact of paired inner branches, which has been further illustrated in the related form Erratus sperare (Fu et al., 2022). Sexual dimorphism has also been proposed in Isoxys volucris based on the relative width of the doublure (Nielsen, Rasmussen & Harper, 2017).
The most prominent feature of Isoxys is the non-biomineralized carapace, which ranged in length from 1 cm to almost 4 cm, and covered most of the body. It was folded to give two equal hemispherical valves, and had pronounced spines at the front and back. A pair of bulbous, spherical eyes protrudes forward and laterally from under the carapace. They are attached to the head by very short stalks. A pair of frontal appendages that are segmented and non-branching (uniramous) is adjacent to the eyes. In I. acutangulus, the flexible appendages are curved with a serrated outline and five segments in total, including a basal part, three segments with stout outgrowths, and a pointed terminal segment. The trunk of the body has 13 pairs of evenly spaced appendages that are segmented and branch into two (biramous), with slender limbs and large, paddle-like flaps fringed with long setae. Outer and inner branches are broadly attached at their base, and pairs of inner branches also seem bound together. The tailpiece would be a tailfan, as in dinocaridids (Legg & Vannier, 2013). A cylindrical gut passes from the head to the ventral terminus of the telson, and is lined dorsally by large concatenate, triangular gut glands. I. longissimus is distinguished from I. acutangulus by the presence of extremely long spines and an elongate body shape.
Isoxys is known from hundreds of specimens collected on Fossil Ridge. In the Walcott Quarry, Isoxys acutangulus is relatively common and represents about 0.35% of the community whereas Isoxys longissimus is extremely rare (Caron and Jackson, 2008).
The streamlined body, thin carapace, the presence of paddle-shaped flaps in the appendages and a flap-bearing tailfan all suggest that Isoxys was a free-swimming animal. A predatory lifestyle is indicated by the large eyes, frontal appendage, and large gut glands. Isoxys would have swum just above the sea floor, seeking out prey in the water column and at the sediment-water interface. Isoxys is commonly found among benthic/nektobenthic assemblages (Caron & Jackson, 2008) and possess general morphological characteristics of other nektobenthic Cambrian arthropods.
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