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

Abstract

Chancelloriids are a group of ‘problematic’ fossils characterized by bag-shaped body equipped with mineralized sclerites on the external surface. Among the chancelloriid taxa, the genus Dimidia was known only by isolated sclerites from small shelly faunas and once regarded as a junior synonym of Allonnia. The complete body of Dimidia simplex Jiang is described herein for the first time, based on well-preserved specimens from the Chengjiang biota (Cambrian Stage 3) of South China. The name Dimidia is resurrected since the characteristic sclerites in the scleritome are distinctive within all known chancelloriid genera with complete bodies. The sclerites of Dimidia are 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 of Dimidia were 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 of Dimidia contributes 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.