Isoxyids are members of the order Isoxyida and the family Isoxyidae, a group of basal

arthropod Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), mineralised with calcium carbonate, a body with differentiated (Metam ...

s that existed during the

Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...

period. It contains two genera, ''

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 preserve ...

'', with 20 species found worldwide, and ''

Surusicaris ''Surusicaris'' is an extinct genus of bivalved arthropod, known from the Cambrian Burgess Shale of British Columbia, Canada. It is considered to be closely related to ''Isoxys,'' and like it has spined grasping frontal appendages. Description ...

'' known from a single species found in the

Burgess Shale The Burgess Shale is a fossil-bearing deposit exposed in the Canadian Rockies of British Columbia, Canada. It is famous for the exceptional preservation of the soft parts of its fossils. At old (middle Cambrian), it is one of the earliest fos ...

of Canada. They are distinguished by their bivalved carapaces and pair of upward curving grasping frontal appendages.

Description

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 An eye is a sensory organ that allows an organism to perceive visual information. It detects light and converts it into electro-chemical impulses in neurons (neurones). It is part of an organism's visual system. In higher organisms, the ey ...

, a pair of large upward curling frontal appenages covered in spines, and pairs of

biramous The arthropod leg is a form of jointed appendage of arthropods, usually used for walking. Many of the terms used for arthropod leg segments (called podomeres) are of Latin origin, and may be confused with terms for bones: ''coxa'' (meaning hip, ...

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. Currently only two

genera Genus (; : genera ) is a taxonomic rank above species and below family as used in the biological classification of living and fossil organisms as well as viruses. In binomial nomenclature, the genus name forms the first part of the binomial s ...

have been accepted to be members of this family. These being ''

'', which has an almost

cosmopolitan Cosmopolitan may refer to: Internationalism * World citizen, one who eschews traditional geopolitical divisions derived from national citizenship * Cosmopolitanism, the idea that all of humanity belongs to a single moral community * Cosmopolitan ...

distribution, and is known from twenty species, and ''

'', which is limited to the

Burgess shale The Burgess Shale is a fossil-bearing deposit exposed in the Canadian Rockies of British Columbia, Canada. It is famous for the exceptional preservation of the soft parts of its fossils. At old (middle Cambrian), it is one of the earliest fos ...

site in

British Columbia British Columbia is the westernmost Provinces and territories of Canada, province of Canada. Situated in the Pacific Northwest between the Pacific Ocean and the Rocky Mountains, the province has a diverse geography, with rugged landscapes that ...

, 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.

Taxonomy

This cladogram shows the relationships of the isoxyids compared to other arthropod groups, according to the results of Zhang et al. 2023. 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 Hymenocarina is an Order (biology), order of extinct marine arthropods known from the Cambrian. They possess bivalved carapaces, typically with exposed posteriors. Members of the group are morphologically diverse and had a variety of ecologies, i ...

, and that it was not closely related to the isoxyids. Currently the group has been placed into the

Deuteropoda Deuteropoda is a proposed clade of Arthropoda, arthropods whose members are distinguished from more Basal (phylogenetics), basal stem-group arthropods like Radiodonta, radiodonts by an anatomical reorganization of the head region, namely the appe ...

, a proposed

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

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, being placed outside the

crown group In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor ...

of Arthropoda, with both

Mandibulata The clade Mandibulata constitutes one of the major subdivisions of the phylum Arthropoda, alongside Chelicerata. Mandibulates include the crustaceans, myriapods (centipedes and millipedes, among others), and all true insects. The name "Mandibul ...

and

Chelicerata The subphylum Chelicerata (from Neo-Latin, , ) constitutes one of the major subdivisions of the phylum Arthropoda. Chelicerates include the sea spiders, horseshoe crabs, and arachnids (including harvestmen, scorpions, spiders, solifuges, tic ...

being more closely related to each other than to isoxyids.

Ecology

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. 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 The pelagic zone consists of the water column of the open ocean and can be further divided into regions by depth. The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or water column between the sur ...

). Swimming was likely accomplished by rhythmic movement of the legs. 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.

Gallery

References

{{Taxonbar, from1=Q117853288 Arthropod taxa Extinct arthropods

Description

Taxonomy

Ecology

Gallery

See also

References