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Xandarellida
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<h2 id="morphology">Morphology</h2> <p>In terms of anatomy, xandarellids share numerous characteristics such as a <a href="/facts/Hypostome_(trilobite)/XhgnW03e">natant hypostome</a>, alongside a unique feature known as “segment decoupling” where the amount of limbs exceeds the amount of tergites.<a class="footnote-ref" id="fnref:1" href="#fn:1"><sup>1</sup></a> They also share a posteriorly extended semicircular head shield and ventral stalked eyes. The genus <i><a href="/facts/Cindarella/Nw44dAD3">Cindarella</a></i> in particular has complex eyes, with over 2000 <a href="/facts/Ommatidia/T2jummw4">ommatidia</a> in each.<a class="footnote-ref" id="fnref:2" href="#fn:2"><sup>2</sup></a> <i>Cindarella</i> also has a large anterior spine on the fifteenth tergite. <i><a href="/facts/Xandarella/D8PUwWdx">Xandarella</a></i> has an unusual tripartite head shield, with ventral eye slits likely retained from the ancestral stalked eyes, alongside possibly lacking the anterior spine.<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a> <i><a href="/facts/Phytophilaspis/XIXPt59c">Phytophilaspis</a></i> has an especially intricate head shield, sharing its tripartite nature with <i>Xandarella</i> alongside bearing several sutures all over its body and seemingly having almost completely fused tergites.<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a> <i><a href="/facts/Luohuilinella/e8aInjdv">Luohuilinella</a></i> has a large amount of tergites (with the type species having 27) but otherwise resembles other xandarellids save for an unfused three-segmented pygidium.<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a> <i><a href="/facts/Sinoburius/hVmS0PoJ">Sinoburius</a></i> resembles <i>Xandarella</i> in having an elongate posterior spine and <i>Phytophilaspis</i> in having all but a few tergites fused, although it exhibits secondary loss of eye slits. Unusually, <i>Sinoburius</i> also has two small lateral spines near its posterior.<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a> <i>Zhugeia</i> is relatively usual for the clade, however its median spine is very long and needle-like, in addition to having a relatively large pygidium.<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a> </p> <h2 id="taxonomy-and-distribution">Taxonomy and distribution</h2> <p>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.<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a> Xandarellida is classified as a member of <a href="/facts/Artiopoda/6l0KmnYa">Artiopoda</a>, the clade containing the trilobites and their close relatives, typically as members of <a href="/facts/Trilobitomorpha/Tk8ppaKc">Trilobitomorpha</a>, which contains the closest relatives of trilobites within Artiopoda.<a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a> Xandarellida contains numerous species, most from the <a href="/facts/Maotianshan_Shales/EUIL90sp">Maotianshan Shales</a> of China. The only exception to this is <i><a href="/facts/Phytophilaspis/XIXPt59c">Phytophilaspis</a></i>, which is instead from the <a href="/facts/Sinsk_Formation/cB77kpMH">Sinsk Formation</a> of Russian Siberia.<a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a><a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a> </p><p> Phylogeny after Losso et al. 2025:<a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a></p> <table><tbody><tr><td><a href="/facts/Artiopoda/6l0KmnYa">Artiopoda</a></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p><i><a href="/facts/Squamacula/ImaAkLdw">Squamacula</a></i> </p></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p><i><a href="/facts/Acanthomeridion/VQQuuxub">Acanthomeridion</a></i> </p></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p><i><a href="/facts/Zhiwenia/RKF8LGIu">Zhiwenia</a></i></p></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><p><i>Australimicola</i></p></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p><i><a href="/facts/Kwanyinaspis/vvwTNH0a">Kwanyinaspis</a></i></p></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p><a href="/facts/Vicissicaudata/6l0KmnYa">Vicissicaudata</a> </p></td></tr><tr><td></td></tr><tr><td><a href="/facts/Trilobitomorpha/Tk8ppaKc">Trilobitomorpha</a></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p><a href="/facts/Nektaspida/0jJWNB37">Nektaspida</a> </p></td></tr><tr><td></td></tr><tr><td>Xandarellida</td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p><i><a href="/facts/Cindarella/Nw44dAD3">Cindarella</a></i> </p></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p><i><a href="/facts/Luohuilinella/e8aInjdv">Luohuilinella</a></i> </p></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p><i><a href="/facts/Sinoburius/hVmS0PoJ">Sinoburius</a></i> </p></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p><i><a href="/facts/Xandarella/D8PUwWdx">Xandarella</a></i> </p></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><p><i><a href="/facts/Phytophilaspis/XIXPt59c">Phytophilaspis</a></i> </p></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p><i><a href="/facts/Retifacies/Nyz8ILHp">Retifacies</a></i> </p></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><p><i><a href="/facts/Pygmaclypeatus/UPJ3ftBj">Pygmaclypeatus</a></i> </p></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p><i>Campanamuta</i></p></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><p><a href="/facts/Trilobita/Tk8ppaKc">Trilobita</a> </p></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><p><a href="/facts/Conciliterga/WMpkTCjj">Conciliterga</a> </p></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>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. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6685191" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6685191</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>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. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3783037" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3783037</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>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. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6685191" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6685191</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>Ivantsov, A. Yu. (1999). "Trilobite-like arthropod from the Lower Cambrian of the Siberian Platform" (PDF). Acta Palaeontologica Polonica. 44 (4): 455–466. <a href="http://www.app.pan.pl/archive/published/app44/app44-455.pdf" target="_blank">http://www.app.pan.pl/archive/published/app44/app44-455.pdf</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>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. <a href="https://figshare.com/articles/journal_contribution/10219823" target="_blank">https://figshare.com/articles/journal_contribution/10219823</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>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. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6685191" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6685191</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>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. <a href="https://doi.org/10.4202%2Fapp.01080.2023" target="_blank">https://doi.org/10.4202%2Fapp.01080.2023</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>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. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6685191" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6685191</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> <li id="fn:9"><p>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. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6685191" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6685191</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li> <li id="fn:10"><p>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. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3783037" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3783037</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></p></li> <li id="fn:11"><p>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. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6685191" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6685191</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></p></li> <li id="fn:12"><p>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. <a href="https://www.tandfonline.com/doi/full/10.1080/14772019.2025.2468195" target="_blank">https://www.tandfonline.com/doi/full/10.1080/14772019.2025.2468195</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></p></li> </ol>