Biology:Artiopoda

Artiopoda is a clade of extinct arthropods that includes trilobites 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 deposits.

Description

File:Retifacies diagram.webp

Morphology of Cambrian artiopod Retifacies, showing the morphology of the limbs (E, F) Key for limb morphology: exopod (exo) exite (exi) protopodite (pt) podomere (pd) spine/setae (s) terminal claw (tc)

According to Stein and Selden (2012) artiopods are recognised by the possession of filiform (elongate and relatively thin) antennae, limbs with bilobate exopods (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 (inner, leg-like branch) has seven podomeres/segments, with first four podomeres bearing inward facing (endite) structures, while podomeres five and six are stenopodous (cylindrical and stout). Common plesiomorphies (ancestral traits) 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 (along the up-down axis) flattened look of artiopods. The limbs of artiopods have also been suggested to bear exites, which were described as similar those of the megacheiran Leanchoilia and probably not homologous to those present in crustaceans.

• 
  Restoration of the trilobite Triarthrus eatoni
• 
  Restoration of Naraoia magna (Nektaspida)
• 
  Life restoration of Sidneyia (Vicissicaudata)
• 
  Diagram of Sinoburius (Xandarellida)
• Diagram of Olenoides serratus (Trilobita)
  Diagram of Olenoides serratus (Trilobita)
• 
  Life restoration of Aglaspis (Aglaspidida)
• 
  Life restoration of Tegopelte (Conciliterga)
• 
  Life restoration of Pleuroctenium (Agnostida)

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 encompassing aglaspidids, xenopods and cheloniellids. Trilobitomorpha is united by the fusing of the posterior sections of the trunk into a pygidium, while Vicissicaudata is united by a differentiated limbless posterior section of the trunk, often with the presence of a tailspine.

A small group, Protosutura, comprising Zhiwenia, Acanthomeridion and Australimicola, has been recognised based on the shared presence of simple sutures on the top (dorsal surface) of the carapace where it splits during ecdysis (moulting), and a short tailspine. This group is often placed basal to the Trilobitomorpha-Vicissicaudata split, alongside other taxa like Squamacula. These relationships are not always recovered. Some studies have recovered Artiopoda as non-monophyletic.

Relationships with other arthropods

The relationship of Artiopoda with the two major clades of modern arthropods, Chelicerata (the group of sea spiders, horseshoe crabs and arachnids such as spiders and scorpions) and Mandibulata (which contains insects, crustaceans, centipedes and millipedes, among others), are unresolved, with some phylogenies recovering Artiopoda as more closely to chelicerates, forming the clade Arachnomorpha, while others recover Artiopoda as more closely related to mandibulates, forming the clade Antennulata. Supporters of a relationship with chelicerates have highlighted shared features of the trunk section, while supporters of the Antennulata hypothesis have noted that the first head appendages of both Artiopoda and Mandibulata are a pair of unbranched antennae (the first head appendages of chelicerates by contrast are the claw or pincer-like chelicerae), which these authors propose is a shared ancestral trait (synapomorphy) of Antennulata.

Some studies place artiopods as stem-group euarthropods, with mandibulates and chelicerates more closely related to each other than either is to Artiopoda, though a majority of studies favour a position within the arthropod crown group for Artiopoda.

Some studies have recovered a close relationship with Marrellomorpha, with the proposed clade including Artiopoda and Marrellomorpha dubbed Lamellipedia, though this relationship is not found in other analyses. The enigmatic artiopodan-like arthropod Kiisortoqia, which bears large "frontal appendages" has been suggested to be closely related to Artiopoda in some analyses.

Phylogeny

Internal phylogeny

After Jiao et al. 2021.

Artiopoda

Squamacula clypeata     Protosutura     Zhiwenia coronata     Australimicola spriggi         Acanthomeridion serratum     Acanthomeridion anacanthus           Bailongia longicaudata   Trilobitomorpha   Retifacies abnormalis           Kwanyinaspis maotiashanensis   Trilobita   Olenoides serratus     Eoredlichia intermedia       Conciliterga   Haifengella corona         Kuamaia lata     Helmetia expansa         Tegopelte gigas     Skioldia aldna     Saperion glumaceum             Xandarellida   Cindarella eucalla       Luohuilinella deletres     Luohuilinella rarus       Sinoburius lunaris       Xandarella spectaculum     Phytophilaspis pergamena           Nektaspida   Buenaspis forteyi       Tariccoia arrusensis     Soomaspis splendida     Liwia convexa         Naraoia spinosa       Naraoia compacta     Misszhouia longicaudata               Vicissicaudata   Sidneyia inexpectans       Emeraldella brocki   Cheloniellida   Cheloniellon calmani       Triopus draboviensis     Duslia insignis           Kodymirus vagans       Eozetetes gemmelli   Aglaspidida   Beckwithia typa         Chlupacaris dubia       Quasimodaspis brentsae       Tremaglaspis unite       Tremaglaspis vanroyi     Brachyaglaspis singularis             Cyclopites vulgaris     Australaglaspis stonyensis       Uarthrus instabilis     Flobertia kochi       Aglaspella granulifera       Gogglops ensifer       Glypharthrus simplex     Aglaspis spinifer           Chraspedops modesta       Glypharthrus thomasi       Glypharthrus trispinicaudatus     Glypharthrus magnoculus

Implied weights parsimony phylogeny after Berks et al. 2023.

Artiopoda

Thulaspis 100px     Squamacula 80px       Protosutura   Zhiwenia 50px     Australimicola 35px     Acanthomeridion 100px       Vicissicaudata   Sidneyia 100px       Cheloniellida 70px       Emeraldella 70px       Kodymirus 70px       Eozetetes     Aglaspidida 100px             Trilobitomorpha     Retifacies 100px     Pygmaclypeatus 100px         Xandarellida 70px       Nektaspida 70px       Conciliterga 100px       Kwanyinaspis 70px       Phytophilaspis 70px     Trilobita 100px

External phylogeny

Cladogram supporting the Antennulata hypothesis after Liu et al; 2026:

Panarthropoda

† "Lobopodia" 80 px 90 px 50 px (paraphyletic, ancestral to tardigrades, onychophorans and arthropods)   Total group Euarthropoda   †Megadictyon 80 px         †Kerygmachela 80 px       †Pambdelurion 80 px       †Opabiniidae 70 px       † Radiodonta (e.g Anomalocaris) 70 px   Deuteropoda   †Erratus 70 px       †Sunella 70px       †Isoxyidae 70px + "Great appendage bivalved forms" 70px         †Fengzhengia 80 px     †Kylinxia 70px         †Megacheira 80px     Pan-Chelicerata 70px 70px         †Kiisortoqia 70px     †Bushizheia 70px   Antennulata   †Artiopoda 70px   Mandibulata   †Fuxianhuiida 70px       †Hymenocarina 70px       Myriapoda 70px     Pancrustacea 70px 70px

References

Template:Artiopoda Wikidata ☰ Q19842587 entry