Artiopoda is a

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 extinct

arthropods 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 ...

that includes

trilobite Trilobites (; meaning "three-lobed entities") are extinction, extinct marine arthropods that form the class (biology), class Trilobita. One of the earliest groups of arthropods to appear in the fossil record, trilobites were among the most succ ...

s and their close relatives. It was erected by Hou and Bergström in 1997 to encompass a wide diversity of arthropods that would traditionally have been assigned to the Trilobitomorpha. Trilobites, in part due to abundance of findings owing to their mineralized exoskeletons, are by far the best recorded, diverse, and long lived members of the clade. Other members, which lack mineralised exoskeletons, are known mostly from

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 ...

deposits.

Description

According to Stein and Selden (2012) artiopods are recognised by the possession of

filiform Filiform, thread or filament like, can refer to: * Filiform, a common term used in botany to describe a thread-like shape *Filiform, or filiform catheter, a medical device whose component parts or segments are all cylindrical and more or less un ...

antennulae, limbs with bilobate

exopod 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 (a ...

s (upper branches), with the proximal (closest to base of the limb) lobe being elongate and bearing a lamella, while the distal (further from the limb base) lobe is paddle-shaped and setiforous (bearing hair-or bristle like structures). The limb

endopod 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, : ...

(inner, leg-like branch) has seven

podomere 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, : ...

s/segments, with first four podomeres bearing inward facing (

endite 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, : ...

) structures, while podomeres five and six are stenopodous (cylindrical and stout). Common

plesiomorphies In phylogenetics, a plesiomorphy ("near form") and symplesiomorphy are synonyms for an ancestral character shared by all members of a clade, which does not distinguish the clade from other clades. Plesiomorphy, symplesiomorphy, apomorphy, an ...

also include the antennules and at least three sets of post-antennular limbs being attached to the head shield, the postantennular limbs having no or little differentiation into distinct morphologies, and broad paratergal folds which contribute to the

dorsoventrally Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position provi ...

(along the up-down axis) flattened look of artiopods. The limbs of artiopods have also been suggested to bear

exite 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, : ...

s, which were described as similar those of the

megacheira Megacheira ("great hands", also historically great appendage arthropods) is an extinct class of predatory arthropods defined by their possession of spined "great appendages". Their taxonomic position is controversial, with studies either consider ...

n ''

Leanchoilia ''Leanchoilia'' is a megacheiran marine arthropod known from Cambrian deposits of the Burgess Shale in Canada and the Chengjiang biota of China. Description ''L. superlata'' was about long and had long, whip-like flagellae extending from i ...

'' and probably not homologous to those present in crustaceans.

Taxonomy

Internal taxonomy

The Artiopoda have been considered by many studies to consist of two major clades; one reusing Trilobitomorpha to encompass trilobites, nektaspids, concilitergans and xandarellids, and the other called

Vicissicaudata Vicissicaudata is an unranked group of artiopodan arthropods, containing Cheloniellida, Aglaspidida and several other genera outside these groups like ''Sidneyia'' and ''Emeraldella.'' Description Vicissicaudatans are similar to most other ...

encompassing aglaspidids, xenopods and cheloniellids. There are some taxa, such as ''

Squamacula ''Squamacula'' is an extinct artiopodan arthropod from the Cambrian Series 2. The type species ''S. clypeata'' was described in 1997 from the Chengjiang biota of Yunnan, China. At the time of description there were only two known specimens of ''S ...

'' and the members of "Protosutura", which often are placed near the base of Artiopoda, outside the Vicissicaudata+Trilobitomorpha clade. These relationships are not always recovered.

Relationships with other arthropods

The relationship of Artiopoda with the two major clades of modern arthropods, the

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 ...

and the

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 ...

, are unresolved, with some phylogenies recovering Artiopoda as more closely to chelicerates, forming the clade

Arachnomorpha Arachnomorpha is a proposed subdivision or clade of Arthropoda, comprising the group formed by the trilobites and their close relatives (Artiopoda), Megacheira (which may be paraphyletic) and chelicerates. Under this proposed classification sche ...

, while others recover Artiopoda as more closely related to mandibulates, forming the clade

Antennulata Antennulata is a proposed clade of arthropods comprising Artiopoda (Trilobites and their close relatives), Marrellomorpha and Mandibulata (crustaceans, hexapods and myriapods). As its name suggests, a proposed unifying feature of the clade is th ...

. Some studies place them as stem-group euarthropods, with mandibulates and chelicerates more closely related to each other than either is to Artiopoda. Some studies have recovered a close relationship with

Marrellomorpha Marrellomorpha are an extinct group of arthropods known from the Cambrian to the Early Devonian. They lacked mineralised hard parts, so are only known from areas of exceptional preservation, limiting their fossil distribution. The best known mem ...

, with the proposed clade including Artiopoda and Marrellomorpha dubbed

Lamellipedia Lamellipedia is a proposed clade of arthropods, including trilobites and their relatives (Artiopoda) and Marrellomorpha Marrellomorpha are an extinct group of arthropods known from the Cambrian to the Early Devonian. They lacked mineralised har ...

, though this relationship is not found in other analyses. The enigmatic artiopodan-like arthropod ''

Kiisortoqia ''Kiisortoqia soperi'' is an extinct species of arthropod from the Early Cambrian Sirius Passet Lagerstätte in Greenland. While it had a superficially trilobite-like bodyform, it also possessed large frontal appendages similar to those of radiod ...

,'' which bears large "frontal appendages" has been suggested to be closely related to Artiopoda in some analyses.

Gallery

Phylogeny

After Jiao et al. 2021. Implied weights parsimony phylogeny after Berks et al. 2023.

References

{{Taxonbar, from=Q19842587 Arthropod taxa Taxa described in 1997 Cambrian first appearances Lopingian extinctions