Body reconstruction, taxonomy, and biostratigraphy of a ‘problematic’ chancelloriid

AbstractChancelloriids are a group of ‘problematic’ fossils characterized by bag-shaped body equipped with mineralized sclerites on the external surface. Among the chancelloriid taxa, the genusDimidiawas known only by isolated sclerites from small shelly faunas and once regarded as a junior synonym ofAllonnia. The complete body ofDimidia simplexJiang is described herein for the first time, based on well-preserved specimens from the Chengjiang biota (Cambrian Stage 3) of South China. The nameDimidiais resurrected since the characteristic sclerites in the scleritome are distinctive within all known chancelloriid genera with complete bodies. The sclerites ofDimidiaare densely arranged, each composed of two symmetrical, acute rays that pointed out with their long axes nearly vertical to the body surface, structurally representing an intermediate type between the single-rayed and the common rosette-like composite sclerites of chancelloriids. The remains ofDimidiawere previously found across South China, Gondwana, and Laurentia, and stratigraphically ranging from the upper Stage 2 to Wuliuan Stage of the Cambrian. The discovery of complete bodies ofDimidiacontributes to revealing the diversity and clarifying the ‘problematic’ taxonomy of chancelloriids, and emphasizes the necessity to scrutinize more scleritome fossils to interpret the taxonomy and phylogenetic affinity of other small shelly fossils.