{{Short description|Extinct order of artiopods}} {{Automatic taxobox | fossil_range = {{fossil range|Cambrian Stage 3|Cambrian Stage 4}} | image = Zhugeia acuticaudata.png | image_caption = Fossils of ''Zhugeia'' | display_parents = 2 | taxon = Xandarellida | authority = Chen, Ramsköld, Edgecombe & Zhou in Chen et al, 1996<ref name=cindarella>{{cite journal |last1=Ramsköld |first1=Lars |last2=Chen |first2=Junyuan |last3=Edgecombe |first3=Gregory D. |last4=Zhou |first4=Guiqing |title=Cindarella and the arachnate clade Xandarellida (Arthropoda, Early Cambrian) from China |journal=Transactions of the Royal Society of Edinburgh: Earth Sciences |date=1997 |volume=88 |issue=1 |pages=19–38 |doi=10.1017/S0263593300002297}}</ref> | type_species = †''Xandarella spectaculum'' | type_species_authority = Hou, Ramsköld & Bergström, 1991<ref name=xandarella>{{cite journal |last1=Hou |first1=Xianguang |last2=Ramsköld |first2=Lars |last3=Bergström |first3=Jan |title=Composition and preservation of the Chengjiang fauna –a Lower Cambrian soft-bodied biota |journal=Zoologica Scripta |date=October 1991 |volume=20 |issue=4 |pages=395–411 |doi=10.1111/j.1463-6409.1991.tb00303.x}}</ref> | subdivision_ranks = Genera | subdivision = *†''Xandarella'' *†''Luohuilinella'' *†''Cindarella'' *†''Sinoburius'' *†''Phytophilaspis'' *†''Zhugeia'' *†''Austroxandarella'' | synonyms = *Petalopleura <small> Hou and Bergström, 1997</small><ref name=HouBergstrom1997>{{cite book |last1=Hou |first1=Xianguang |last2=Bergström |first2=Jan |title=Arthropods of the Lower Cambrian Chengjiang fauna, southwest China |series=Fossils and Strata |date=22 December 1997 |volume=45 |pages=1–117 |doi=10.18261/8200376931-1997-01|isbn=82-00-37693-1 }}</ref> }}
'''Xandarellida''' is an extinct order of artiopod arthropods known from the Middle Cambrian of China, Australia<ref name=austro/> and Siberia. == Morphology == alt=Sinoburius_7|thumb|left|Diagrammatic reconstruction of ''Sinoburius'' 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.<ref name="sinoburius">{{cite journal |last1=Chen |first1=Xiaohan |last2=Ortega-Hernández |first2=Javier |last3=Wolfe |first3=Joanna M. |last4=Zhai |first4=Dayou |last5=Hou |first5=Xianguang |last6=Chen |first6=Ailin |last7=Mai |first7=Huijuan |last8=Liu |first8=Yu |date=December 2019 |title=The appendicular morphology of Sinoburius lunaris and the evolution of the artiopodan clade Xandarellida (Euarthropoda, early Cambrian) from South China |journal=BMC Evolutionary Biology |volume=19 |issue=1 |page=165 |bibcode=2019BMCEE..19..165C |doi=10.1186/s12862-019-1491-3 |pmc=6685191 |doi-access=free}}</ref> 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.<ref name=":0">{{cite journal |last1=Zhao |first1=Fangchen |last2=Bottjer |first2=David J. |last3=Hu |first3=Shixue |last4=Yin |first4=Zongjun |last5=Zhu |first5=Maoyan |date=25 September 2013 |title=Complexity and diversity of eyes in Early Cambrian ecosystems |journal=Scientific Reports |volume=3 |issue=1 |page=2751 |bibcode=2013NatSR...3.2751Z |doi=10.1038/srep02751 |pmc=3783037 |pmid=24067397}}</ref> ''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.<ref name="sinoburius" /> ''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.<ref name="phytophilaspis">{{cite journal |last=Ivantsov |first=A. Yu. |year=1999 |title=Trilobite-like arthropod from the Lower Cambrian of the Siberian Platform |url=http://www.app.pan.pl/archive/published/app44/app44-455.pdf |journal=Acta Palaeontologica Polonica |volume=44 |issue=4 |pages=455–466}}</ref> ''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.<ref name="luohuilinella">{{cite journal |last1=Hou |first1=Xianguang |last2=Williams |first2=Mark |last3=Sansom |first3=Robert |last4=Siveter |first4=Derek J. |last5=Siveter |first5=David J. |last6=Gabbott |first6=Sarah |last7=Harvey |first7=Thomas H. P. |last8=Cong |first8=Peiyun |last9=Liu |first9=Yu |date=August 2019 |title=A new xandarellid euarthropod from the Cambrian Chengjiang biota, Yunnan Province, China |url=https://figshare.com/articles/journal_contribution/10219823 |journal=Geological Magazine |volume=156 |issue=8 |pages=1375–1384 |bibcode=2019GeoM..156.1375H |doi=10.1017/s0016756818000730 |hdl-access=free |hdl=2381/43477}}</ref> ''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.<ref name="sinoburius" /> ''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.<ref name="zhugeia">{{cite journal |last1=Zhu |first1=Yuyan |last2=Zeng |first2=Han |last3=Liu |first3=Yao |last4=Zhao |first4=Fangchen |date=2023 |title=New artiopodan euarthropods from the Chengjiang fauna (Cambrian Stage 3) at Malong, Yunnan, China |journal=Acta Palaeontologica Polonica |volume=68 |doi=10.4202/app.01080.2023 |doi-access=free}}</ref> ''Austroxandarella'' has an elongated pygidium, but otherwise resembles ''Xandarella'' closely.<ref name=austro>{{cite journal |last1=Edgecombe |first1=Gregory D. |last2=García-Bellido |first2=Diego C. |last3=Paterson |first3=John R. |title=A new xandarellid euarthropod from the lower Cambrian Emu Bay Shale, South Australia |journal=Journal of Paleontology |date=28 August 2025 |pages=1–9 |doi=10.1017/jpa.2025.10138}}</ref>
== Taxonomy and distribution == Xandarellida was originally defined by Chen, Ramsköld, Edgecombe & Zhou in 1996, including only the genera ''Xandarella'' and ''Cindarella''.<ref name=cindarella/> Hou and Bergström in 1997 grouped Xandarellida and ''Sinoburius'' together in a subclass called Petalopleura,<ref name=HouBergstrom1997/> but this grouping was later considered to be more poorly defined than Xandarellida, which is now used in preference to it.<ref name="sinoburius" /> 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.<ref name="sinoburius" /> Xandarellida contains numerous species, most from the Maotianshan Shales of China. The two exceptions to this are ''Phytophilaspis'' from the Sinsk Formation of Russian Siberia<ref name=":0" /><ref name="sinoburius" /> and ''Austroxandarella'' from the Emu Bay Shale.<ref name=austro/>
Phylogeny after Losso et al. 2025:<ref name=":3">{{Cite journal |last=Losso |first=Sarah R. |last2=Caron |first2=Jean-Bernard |last3=Ortega-Hernández |first3=Javier |date=2025-04-04 |title=Helmetia expansa Walcott, 1918 revisited – new insights into the internal anatomy, moulting and phylogeny of Conciliterga |url=https://www.tandfonline.com/doi/full/10.1080/14772019.2025.2468195 |journal=Journal of Systematic Palaeontology |volume=23 |issue=1 |article-number=2468195 |doi=10.1080/14772019.2025.2468195 |issn=1477-2019 |doi-access=free|url-access=subscription }}</ref>{{clade|{{clade |1=''Squamacula'' 80px |2={{clade |1={{clade |1=''Acanthomeridion'' 100px |2={{clade |1=''Zhiwenia'' 50px |2=''Australimicola'' }}}} |2={{clade |1=''Kwanyinaspis'' |2={{clade |1=Vicissicaudata 100px |label2=Trilobitomorpha |2={{clade |1={{clade |1={{clade |1=Nektaspida 100px |label2='''Xandarellida''' |2={{clade |1=''Cindarella'' 50px |2={{clade |1=''Luohuilinella'' 100px |2={{clade |1=''Sinoburius'' 100px |2={{clade |1=''Xandarella'' 50px |2=''Phytophilaspis'' 50px }}}}}}}}}} |2={{clade |1={{clade |1=''Retifacies'' 100px |2=''Pygmaclypeatus'' 100px }} |2={{clade |1=''Campanamuta'' |2=Trilobita 100px }}}}}} |2=Conciliterga 100px }}}}}}}}}}|label1=Artiopoda}}
== References == {{reflist}}
{{Artiopoda}} {{Taxonbar|from=Q124313314}}
Category:Artiopoda Category:Cambrian extinctions Category:Fossil taxa described in 1996 Category:Cambrian arthropods Category:Maotianshan shales fossils