The early Cambrian Kylinxia zhangi and evolution of the arthropod head

The early Cambrian Kylinxia zhangi occupies a pivotal position in arthropod evolution, branching from the euarthropod stem lineage between radiodonts (Anomalocaris and relatives) and “great-appendage” arthropods. 1 Zeng H. Zhao F. Niu K. Zhu M. Huang D. An early Cambrian euarthropod with radiodont-like raptorial appendages. Nature. 2020; 588: 101-105https://doi.org/10.1038/s41586-020-2883-7. 10.1144/jgs2021-164 Crossref PubMed Scopus (0) Google Scholar ,2 O’Flynn R.J. Williams M. Yu M. Harvey T.H.P. Liu Y. A new euarthropod with large frontal appendages from the early Cambrian Chengjiang biota. Palaeontol. Electron. 2022; 25: a6https://doi.org/10.26879/1167 Crossref Scopus (3) Google Scholar Its combination of appendage and exoskeletal features is viewed as uniquely bridging the morphologies of so-called “lower” and “upper” stem-group euarthropods. 3 Ortega-Hernández J. Making sense of ‘lower’ and ‘upper’ stem-group Euarthropoda, with comments on the strict use of the name Arthropoda von Siebold, 1848. Biol. Rev. 2016; 91: 255-273https://doi.org/10.1111/brv.12168 Crossref PubMed Scopus (94) Google Scholar ,4 Budd G.E. The origin and evolution of the euarthropod labrum. Arthropod Struct. Dev. 2021; 62101048https://doi.org/10.1016/j.asd.2021.101048 Crossref Google Scholar Microtomographic study of new specimens of Kylinxia refines and corrects previous interpretation of head structures in this species. Phylogenetic analyses incorporating new data reinforce the placement of Kylinxia in the euarthropod stem group but support new hypotheses of head evolution. The head of Kylinxia is composed of six segments, as in extant mandibulates, e.g., insects. 5 Fusco G. Minelli A. Arthropod segmentation and tagmosis. in: Minelli A. Boxshall G. Fusco G. Arthropod Biology and Evolution. Springer, 2013: 197-221 Crossref Google Scholar In Kylinxia, these are an anterior sclerite associated with an unpaired median eye and paired lateral eyes (thus three rather than five eyes as was previously described 1 Zeng H. Zhao F. Niu K. Zhu M. Huang D. An early Cambrian euarthropod with radiodont-like raptorial appendages. Nature. 2020; 588: 101-105https://doi.org/10.1038/s41586-020-2883-7. 10.1144/jgs2021-164 Crossref PubMed Scopus (0) Google Scholar ), deutocerebral frontal-most appendages, and four pairs of biramous appendages (rather than two pairs of uniramous appendages). Phylogenetic trees suggest that a six-segmented head in the euarthropod crown group was already acquired by a common ancestor with Kylinxia. The segmental alignment and homology of spinose frontal-most appendages between radiodonts and upper stem-group euarthropods 6 Moysiuk J. Caron J.-B. A three-eyed radiodont with fossilized neuroanatomy informs the origin of the arthropod head and segmentation. Curr. Biol. 2022; 32: 3302-3316.e2https://doi.org/10.1016/j.cub.2022.06.027 Abstract Full Text Full Text PDF PubMed Scopus (14) Google Scholar ,7 Zeng H. Zhao F. Zhu M. Innovatiocaris, a complete radiodont from the early Cambrian Chengjiang Lagerstätte and its implications for the phylogeny of Radiodonta. J. Geol. Soc. London. 2022; 180https://doi.org/10.1144/jgs2021-164 Crossref Scopus (5) Google Scholar ,8 Aria C. The origin and early evolution of arthropods. Biol. Rev. Camb. Philos. Soc. 2022; 97: 1786-1809https://doi.org/10.1016/j.asd.2021.101048 Crossref PubMed Scopus (24) Google Scholar ,9 Aria C. Zhao F. Zeng H. Guo J. Zhu M. Fossils from South China redefine the ancestral euarthropod body plan. BMC Evol. Biol. 2020; 20: 4https://doi.org/10.1186/s12862-019-1560-7 Crossref PubMed Scopus (29) Google Scholar ,10 Chen J. Waloszek D. Maas A. A new “great-appendage” arthropod from the Lower Cambrian of China and homology of chelicerate chelicerae and raptorial anteroventral appendages. Lethaia. 2004; 37: 3-20https://doi.org/10.1080/00241160410004764 Crossref Scopus (70) Google Scholar is bolstered by morphological similarities and inferred phylogenetic continuity between Kylinxia and other stem-group euarthropods.