Artiopoda

Artiopoda Temporal range:
Paradoxides a trilobite
Phytophilaspis, a Xandarellid
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
(unranked):	†Artiopoda Hou and Bergstrom, 1997
Subgroupings
†Squamacula Protosutura †Acanthomeridion †Zhiwenia †Australimicola †Campanamuta †Falcatamacaris †Molaria †Retifacies †Pygmaclypeatus †Bailongia Trilobitomorpha †Retifacies? †Pygmaclypeatus? †Campanamuta? †Kwanyinaspis †Arthroaspis †Nektaspida †Trilobita †Conciliterga †Haifengella †Helmetia †Kuamaia †Kwanyinaspis †Rhombicalvaria †Saperion †Skioldia †Tegopelte †Xandarellida (=Petalopleura) †Luohuilinella †Cindarella †Phytophilaspis †Sinoburius †Xandarella Vicissicaudata †Xenopoda (possibly paraphyletic) †Sidneyia †Emeraldella †Etania †Kodymirus †Eozetetes †Cheloniellida †Aglaspidida

The Artiopoda is a grouping 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. Hou and Bergström used the name Lamellipedia as a superclass to replace Trilobitomorpha that was originally erected at the subphylum level, which they considered inappropriate. Trilobites, in part due to their mineralising exoskeletons, are by far the most diverse and long lived members of the clade, with most records of other members, which lack mineralised exoskeletons, being from Cambrian deposits.

Description

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 antennulae, limbs with bilobate exopods, with the proximal lobe being elongate and bearing a lamella, while the distal lobe is paddle-shaped and setiforous (bearing hair-or bristle like structures). The limb endopod has seven podomeres, with first four podomeres bearing inward facing (endite) structures, while podomeres five and six are stenopodous (cylindrical and stout). Common plesiomorphies also include the antellules and at least three sets of post-antellulae limbs being incorporated into the head shield, the postantennular limbs having no or little differentiation into distinct morphologies, and broad paratergal folds which contribute to the dorsoventrally flattened look of artiopods. The limbs of artiopods also bear exites, which are similar to those of the megacheiran Leanchoilia and are likely not homologous to those present in crustaceans.

Taxonomy

Internal taxonomy

Leif Størmer recognised on the basis of limb types that a diverse group of Cambrian arthropods, known largely from the Burgess Shale, were likely to be relatives of the trilobites. Hou and Bergström formalised this grouping into the Lamellipedia, consisting of two clades: the Marrellomorpha and the Artiopoda. The Artiopoda have subsequently been considered 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.

Relationships with other arthropods

The relationship of Artiopoda with the two major clades of modern arthropods, the Chelicerata and the Mandibulata, 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.

Gallery

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  Restoration of the trilobite Triarthrus eatoni

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  Fossils of Naraoia compacta (Nektaspida)

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  Life restoration of Sidneyia (Vicissicaudata)

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  Diagram of Sinoburius (Xandarellida)

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  Diagram of Emeraldella brutoni (Vicissicaudata)

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