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| Erratus sperare Fu, Legg, Daley, Budd, Wu & Zhang, 2022 |
Abstract
Biramous appendages are a common feature among modern marine arthropods that evolved deep in arthropod phylogeny. The branched appendage of Cambrian arthropods has long been considered as the ancient biramous limb, sparking numerous investigations on its origin and evolution. Here, we report a new arthropod, Erratus sperare gen. et sp. nov., from the Lower Cambrian (Stage 3, 520 Ma) Chengjiang biota of Yunnan, China, with unique trunk appendages formed of lateral anomalocaridid-type flaps and ventral subconical endopods. These appendages represent an intermediate stage of biramous limb evolution, i.e. from ‘two pairs of flap appendages' in radiodonts to ‘flap + endopod’ in Erratus, to ‘exopod + endopod’ in the rest of carapace-bearing arthropods that populate the basal region of the upper-stem lineage arthropods (deuteropods). The new species occupies a phylogenetic position at the first node closer to deuteropods than to radiodonts, and therefore pinpoints the earliest occurrence of the endopod within Deuteropoda. The primitive endopod is weakly sclerotized, and has unspecialized segments without endites or claw. The findings might support previous claims that the outer branch of the biramous limb of fossil marine arthropods, such as trilobites, is not a true exopod, but is instead a modified exite.
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| The ecological reconstruction of Erratus sperare, showing the nektonic life style. The initial situation of trunk limbs, as flaps associated with weakly sclerotized endopods indicates a swimming rather than fully benthic habit. |
Phylum Arthropoda
Erratus sperare gen. et sp. nov.
Diagnosis for genus and species. Arthropod bearing a reticulate carapace with an anterior cardinal spine and a rounded posterior margin, body composed of at least 11 flap-bearing segments, the anterior seven of which possess subconical endopods; the wrinkle-like lineations more prominent on the four posterior flaps.
Holotype. XDBZ101, part and counterpart of an individual preserved in ventral aspect; XDBZ102 (figure 2c), an isolated carapace preserved in dorsal aspect showing the new taxon without the posterior cardinal spine. Specimens stored in the Museum of Northwest University, Xi'an, China.
Locality and horizon. Lower Cambrian (Series 2, Stage 3, Eoredlichia–Wutingaspis trilobite Assemblage Zone) Jianshan section of the Yu'anshan Member (Helinpu Formation), situated in Haikou, eastern Yunnan, China.
Etymology. From Erratus (Latin), ‘roaming’, referring to the potentially nektonic habit of this taxon, and sperare (Latin), meaning ‘hope’.
Dongjing Fu, David A. Legg, Allison C. Daley, Graham E. Budd, Yu Wu and Xingliang Zhang. 2022. The Evolution of Biramous Appendages revealed by A Carapace-bearing Cambrian Arthropod. Philosophical Transactions of the Royal Society B: Biological Sciences. 377(1847); ‘The impact of Chinese palaeontology on evolutionary research’. compiled and edited by Xiaoya Ma, Guangxu Wang and Min Wang. DOI: 10.1098/rstb.2021.0034
New fossil reveals origin of arthropod breathing system