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Xandarellida
Extinct order of artiopods

Xandarellida is an extinct order of artiopod arthropods known exclusively from the Middle Cambrian of China and Siberia.

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Morphology

In terms of anatomy, xandarellids share numerous characteristics such as a natant hypostome, alongside a unique feature known as “segment decoupling” where the amount of limbs exceeds the amount of tergites.1 They also share a posteriorly extended semicircular head shield and ventral stalked eyes. The genus Cindarella in particular has complex eyes, with over 2000 ommatidia in each.2 Cindarella also has a large anterior spine on the fifteenth tergite. Xandarella has an unusual tripartite head shield, with ventral eye slits likely retained from the ancestral stalked eyes, alongside possibly lacking the anterior spine.3 Phytophilaspis has an especially intricate head shield, sharing its tripartite nature with Xandarella alongside bearing several sutures all over its body and seemingly having almost completely fused tergites.4 Luohuilinella has a large amount of tergites (with the type species having 27) but otherwise resembles other xandarellids save for an unfused three-segmented pygidium.5 Sinoburius resembles Xandarella in having an elongate posterior spine and Phytophilaspis in having all but a few tergites fused, although it exhibits secondary loss of eye slits. Unusually, Sinoburius also has two small lateral spines near its posterior.6 Zhugeia is relatively usual for the clade, however its median spine is very long and needle-like, in addition to having a relatively large pygidium.7

Taxonomy and distribution

Xandarellida was defined by Chen, Ramsköld, Edgecombe & Zhou in 1996. It was often referred to by the name Petalopleura in historical publications, but this grouping was later considered to be more poorly defined than Xandarella, which is now used in preference to it.8 Xandarellida is classified as a member of Artiopoda, the clade containing the trilobites and their close relatives, typically as members of Trilobitomorpha, which contains the closest relatives of trilobites within Artiopoda.9 Xandarellida contains numerous species, most from the Maotianshan Shales of China. The only exception to this is Phytophilaspis, which is instead from the Sinsk Formation of Russian Siberia.1011

Phylogeny after Losso et al. 2025:12

Artiopoda

Squamacula

Acanthomeridion

Zhiwenia

Australimicola

Kwanyinaspis

Vicissicaudata

Trilobitomorpha

Nektaspida

Xandarellida

Cindarella

Luohuilinella

Sinoburius

Xandarella

Phytophilaspis

Retifacies

Pygmaclypeatus

Campanamuta

Trilobita

Conciliterga

References

  1. Chen, Xiaohan; Ortega-Hernández, Javier; Wolfe, Joanna M.; Zhai, Dayou; Hou, Xianguang; Chen, Ailin; Mai, Huijuan; Liu, Yu (December 2019). "The appendicular morphology of Sinoburius lunaris and the evolution of the artiopodan clade Xandarellida (Euarthropoda, early Cambrian) from South China". BMC Evolutionary Biology. 19 (1): 165. Bibcode:2019BMCEE..19..165C. doi:10.1186/s12862-019-1491-3. PMC 6685191. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6685191 ↩

  2. Zhao, Fangchen; Bottjer, David J.; Hu, Shixue; Yin, Zongjun; Zhu, Maoyan (25 September 2013). "Complexity and diversity of eyes in Early Cambrian ecosystems". Scientific Reports. 3 (1): 2751. Bibcode:2013NatSR...3.2751Z. doi:10.1038/srep02751. PMC 3783037. PMID 24067397. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3783037 ↩

  3. Chen, Xiaohan; Ortega-Hernández, Javier; Wolfe, Joanna M.; Zhai, Dayou; Hou, Xianguang; Chen, Ailin; Mai, Huijuan; Liu, Yu (December 2019). "The appendicular morphology of Sinoburius lunaris and the evolution of the artiopodan clade Xandarellida (Euarthropoda, early Cambrian) from South China". BMC Evolutionary Biology. 19 (1): 165. Bibcode:2019BMCEE..19..165C. doi:10.1186/s12862-019-1491-3. PMC 6685191. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6685191 ↩

  4. Ivantsov, A. Yu. (1999). "Trilobite-like arthropod from the Lower Cambrian of the Siberian Platform" (PDF). Acta Palaeontologica Polonica. 44 (4): 455–466. http://www.app.pan.pl/archive/published/app44/app44-455.pdf ↩

  5. Hou, Xianguang; Williams, Mark; Sansom, Robert; Siveter, Derek J.; Siveter, David J.; Gabbott, Sarah; Harvey, Thomas H. P.; Cong, Peiyun; Liu, Yu (August 2019). "A new xandarellid euarthropod from the Cambrian Chengjiang biota, Yunnan Province, China". Geological Magazine. 156 (8): 1375–1384. Bibcode:2019GeoM..156.1375H. doi:10.1017/s0016756818000730. hdl:2381/43477. https://figshare.com/articles/journal_contribution/10219823 ↩

  6. Chen, Xiaohan; Ortega-Hernández, Javier; Wolfe, Joanna M.; Zhai, Dayou; Hou, Xianguang; Chen, Ailin; Mai, Huijuan; Liu, Yu (December 2019). "The appendicular morphology of Sinoburius lunaris and the evolution of the artiopodan clade Xandarellida (Euarthropoda, early Cambrian) from South China". BMC Evolutionary Biology. 19 (1): 165. Bibcode:2019BMCEE..19..165C. doi:10.1186/s12862-019-1491-3. PMC 6685191. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6685191 ↩

  7. Zhu, Yuyan; Zeng, Han; Liu, Yao; Zhao, Fangchen (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. https://doi.org/10.4202%2Fapp.01080.2023 ↩

  8. Chen, Xiaohan; Ortega-Hernández, Javier; Wolfe, Joanna M.; Zhai, Dayou; Hou, Xianguang; Chen, Ailin; Mai, Huijuan; Liu, Yu (December 2019). "The appendicular morphology of Sinoburius lunaris and the evolution of the artiopodan clade Xandarellida (Euarthropoda, early Cambrian) from South China". BMC Evolutionary Biology. 19 (1): 165. Bibcode:2019BMCEE..19..165C. doi:10.1186/s12862-019-1491-3. PMC 6685191. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6685191 ↩

  9. Chen, Xiaohan; Ortega-Hernández, Javier; Wolfe, Joanna M.; Zhai, Dayou; Hou, Xianguang; Chen, Ailin; Mai, Huijuan; Liu, Yu (December 2019). "The appendicular morphology of Sinoburius lunaris and the evolution of the artiopodan clade Xandarellida (Euarthropoda, early Cambrian) from South China". BMC Evolutionary Biology. 19 (1): 165. Bibcode:2019BMCEE..19..165C. doi:10.1186/s12862-019-1491-3. PMC 6685191. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6685191 ↩

  10. Zhao, Fangchen; Bottjer, David J.; Hu, Shixue; Yin, Zongjun; Zhu, Maoyan (25 September 2013). "Complexity and diversity of eyes in Early Cambrian ecosystems". Scientific Reports. 3 (1): 2751. Bibcode:2013NatSR...3.2751Z. doi:10.1038/srep02751. PMC 3783037. PMID 24067397. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3783037 ↩

  11. Chen, Xiaohan; Ortega-Hernández, Javier; Wolfe, Joanna M.; Zhai, Dayou; Hou, Xianguang; Chen, Ailin; Mai, Huijuan; Liu, Yu (December 2019). "The appendicular morphology of Sinoburius lunaris and the evolution of the artiopodan clade Xandarellida (Euarthropoda, early Cambrian) from South China". BMC Evolutionary Biology. 19 (1): 165. Bibcode:2019BMCEE..19..165C. doi:10.1186/s12862-019-1491-3. PMC 6685191. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6685191 ↩

  12. Losso, Sarah R.; Caron, Jean-Bernard; Ortega-Hernández, Javier (2025-04-04). "Helmetia expansa Walcott, 1918 revisited – new insights into the internal anatomy, moulting and phylogeny of Conciliterga". Journal of Systematic Palaeontology. 23 (1): 2468195. doi:10.1080/14772019.2025.2468195. ISSN 1477-2019. https://www.tandfonline.com/doi/full/10.1080/14772019.2025.2468195 ↩