Calcitic shells in the aragonite sea of the earliest Cambrian

Abstract The initial acquisition of calcium carbonate polymorphs (aragonite and calcite) at the onset skeletal biomineralization by disparate metazoans across Ediacaran-Cambrian transition is thought to be directly influenced Earth's seawater chemistry. It has been presumed that animal clades first acquired mineralized skeletons during so-called “aragonite sea” latest Ediacaran earliest Cambrian (Terreneuvian) possessed aragonite or high-Mg calcite skeletons, while arose in subsequent “calcite Series 2 low-Mg skeletons. Here, contrary previous expectations, we document shells one helcionelloid molluscs from basal southwestern Mongolia are composed entirely formed Terreneuvian sea. extraordinarily well-preserved Postacanthella have a simple prismatic microstructure identical their modern molluscan relatives. High-resolution scanning electron microscope observations show calcitic crystallites were originally encased within an intra- interprismatic organic matrix scaffold preserved aggregates apatite early diagenesis. This indicates not all taxa produced aragonitic shells, weakening direct link between mineralogy ambient chemistry evolution phylum. Rather, our study suggests was biologically controlled, possibly exerted associated matrix. presence different construction time when emerged may species dependent.