Artiopoda

Artiopoda
Temporal range: Early Cambrian - Late Permian
Misszhouia, a nektaspid
Cheloniellon, a cheloniellid
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Arthropoda
Clade: Deuteropoda
Clade: †Artiopoda
Hou and Bergstrom, 1997
Subgroups

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

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.

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 (which like artiopods and mandibulates, have single unbranched antennae as their first head appendages), with the proposed clade including Artiopoda and Marrellomorpha dubbed Lamellipedia, though this relationship is not consistently recovered in 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

Bailongia longicaudata

Trilobitomorpha
Vicissicaudata

Implied weights parsimony phylogeny after Berks et al. 2023.

External phylogeny

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

References

  1. ↑ Liu, Cong; Fu, Dongjing; Wu, Yu; Zhang, Xingliang (July 2024). "Cambrian euarthropod Urokodia aequalis sheds light on the origin of Artiopoda body plan" (PDF). iScience. 27 (8) 110443. Bibcode:2024iSci...27k0443L. doi:10.1016/j.isci.2024.110443. PMC 11325232. PMID 39148713.
  2. ↑ Lerosey-Aubril, Rudy (March 2015). "Notchia weugi gen. et sp. nov.: a new short-headed arthropod from the Weeks Formation Konservat-Lagerstätte (Cambrian; Utah)". Geological Magazine. 152 (2): 351–357. Bibcode:2015GeoM..152..351L. doi:10.1017/S0016756814000375.
  3. ↑ Zhu, Y.; Zeng, H.; Liu, Y.; Zhao, F. (2023). "New artiopodan euarthropods from the Chengjiang fauna (Cambrian, Stage 3) at Malong, Yunnan, China". Acta Palaeontologica Polonica. 68. doi:10.4202/app.01080.2023.
  4. ↑ Hou, X. & Bergström, J. 1997. Arthropods of the Lower Cambrian Chengjiang fauna, southwest China. Fossils and Strata, No. 45. Scandinavian University Press, Oslo, 22 Dec 1997: 116 pp.
  5. ↑ Schmidt, Michel; Hou, Xianguang; Zhai, Dayou; Mai, Huijuan; Belojević, Jelena; Chen, Xiaohan; Melzer, Roland R.; Ortega-Hernández, Javier; Liu, Yu (2022). "Before trilobite legs: Pygmaclypeatus daziensis reconsidered and the ancestral appendicular organization of Cambrian artiopods". Philosophical Transactions of the Royal Society B: Biological Sciences. 377 (1847) 20210030. bioRxiv 10.1101/2021.08.18.456779. doi:10.1098/rstb.2021.0030. PMID 35125003.
  6. ↑ Stein, Martin; Selden, Paul A. (June 2012). "A restudy of the Burgess Shale (Cambrian) arthropod Emeraldella brocki and reassessment of its affinities". Journal of Systematic Palaeontology. 10 (2): 361–383. Bibcode:2012JSPal..10..361S. doi:10.1080/14772019.2011.566634. ISSN 1477-2019. S2CID 55018927.
  7. ↑ Liu, Yu; Edgecombe, Gregory D.; Schmidt, Michel; Bond, Andrew D.; Melzer, Roland R.; Zhai, Dayou; Mai, Huijuan; Zhang, Maoyin; Hou, Xianguang (2021-07-30). "Exites in Cambrian arthropods and homology of arthropod limb branches". Nature Communications. 12 (1): 4619. Bibcode:2021NatCo..12.4619L. doi:10.1038/s41467-021-24918-8. ISSN 2041-1723. PMC 8324779. PMID 34330912.
  8. 1 2 Ortega-Hernández, Javier; Legg, David A.; Braddy, Simon J. (February 2013). "The phylogeny of aglaspidid arthropods and the internal relationships within Artiopoda". Cladistics. 29 (1): 15–45. Bibcode:2013Cladi..29...15O. doi:10.1111/j.1096-0031.2012.00413.x. PMID 34814371. S2CID 85744103.
  9. 1 2 3 4 5 Ortega-Hernández, Javier (2026-01-02). "A decade after Deuteropoda – insights into the systematics, evolution and dynamics of Cambrian euarthropods". Journal of Systematic Palaeontology. 24 (1). doi:10.1080/14772019.2026.2663907. ISSN 1477-2019.
  10. ↑ Du, Kun-sheng; Ortega-Hernández, Javier; Yang, Jie; Zhang, Xi-guang (June 2019). "A soft-bodied euarthropod from the early Cambrian Xiaoshiba Lagerstätte of China supports a new clade of basal artiopodans with dorsal ecdysial sutures". Cladistics. 35 (3): 269–281. doi:10.1111/cla.12344. PMID 34622993.
  11. 1 2 3 Berks, H. O.; Lunde Nielsen, M.; Flannery-Sutherland, J.; Thorshøj Nielsen, A.; Park, T.-Y. S.; Vinther, J. (2023). "A possibly deep branching artiopodan arthropod from the lower Cambrian Sirius Passet Lagerstätte (North Greenland)". Papers in Palaeontology. 9 (3) e1495. Bibcode:2023PPal....9E1495B. doi:10.1002/spp2.1495. hdl:1983/09d0bc6d-6cdf-42fb-83da-093437420fe6.
  12. ↑ Aria, Cédric (2022-04-26). "The origin and early evolution of arthropods". Biological Reviews. 97 (5): 1786–1809. Bibcode:2022BioRv..97.1786A. doi:10.1111/brv.12864. ISSN 1464-7931. PMID 35475316. S2CID 243269510.
  13. ↑ Liu, Yao; Zeng, Han; Zhao, Fangchen; Zhu, Yuyan; Li, Yimeng; Yin, Zongjun; Zhu, Maoyan (31 May 2025). "A tiny Cambrian stem-mandibulate reveals independent evolution of limb tagmatization and specialization in early euarthropods". Scientific Reports. 15 (1). doi:10.1038/s41598-025-03544-0. PMC 12126567.
  14. ↑ Moysiuk, Joseph; Izquierdo-López, Alejandro; Kampouris, George E.; Caron, Jean-Bernard (July 2022). "A new marrellomorph arthropod from southern Ontario: a rare case of soft-tissue preservation on a Late Ordovician open marine shelf". Journal of Paleontology. 96 (4): 859–874. Bibcode:2022JPal...96..859M. doi:10.1017/jpa.2022.11. ISSN 0022-3360.
  15. ↑ Stein, Martin (November 2013). "Cephalic and appendage morphology of the Cambrian arthropod Sidneyia inexpectans". Zoologischer Anzeiger - A Journal of Comparative Zoology. 253 (2): 164–178. doi:10.1016/j.jcz.2013.05.001.
  16. ↑ Jiao, De-Guang; Du, Kun-Sheng; Zhang, Xi-Guang; Yang, Jie; Eggink, Daniel (May 2022). "A new small soft-bodied non-trilobite artiopod from the Cambrian Stage 4 Guanshan Biota". Geological Magazine. 159 (5): 730–734. Bibcode:2022GeoM..159..730J. doi:10.1017/S0016756821001254. ISSN 0016-7568.
  17. ↑ Berks, H. O.; Nielsen, M. L.; Flannery-Sutherland, J.; Nielsen, A. T.; Park, T.-Y. S.; Vinther, J. (2023). "A possibly deep branching artiopodan arthropod from the lower Cambrian Sirius Passet Lagerstätte (North Greenland)". Papers in Palaeontology. 9 (3). e1495. Bibcode:2023PPal....9E1495B. doi:10.1002/spp2.1495. hdl:1983/09d0bc6d-6cdf-42fb-83da-093437420fe6. S2CID 259253098.
  18. ↑ Liu, Cong; Pates, Stephen; Zhang, Mingjing; Wu, Yu; Ma, Jiaxin; Fu, Dongjing; Zhang, Xingliang (2026-03-21). "3D morphology of the Cambrian bivalved arthropod Sunella informs about head segmentation, arthrodization, and arthropodization". Communications Biology. doi:10.1038/s42003-026-09909-z. ISSN 2399-3642.