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

The Precambrian–Cambrian interval saw the first appearance of disparate modern metazoan phyla equipped with a wide array of mineralized exo- and endo-skeletons. However, the current knowledge of this remarkable metazoan skeletonization bio-event and its environmental interactions is limited because uncertainties have persisted in determining the mineralogy, microstructure, and hierarchical complexity of these earliest animal skeletons. This study characterizes in detail a previously poorly understood fibrous microstructure—the lamello-fibrillar (LF) nacre—in early Cambrian mollusk and hyolith shells and compares it with shell microstructures in modern counterparts (coleoid cuttlebones and serpulid tubes). This comparative study highlights key differences in the LF nacre amongst different lophotrochozoan groups in terms of mineralogical compositions and architectural organization of crystals. The results demonstrate that the LF nacre is a microstructural motif confined to the Mollusca. This study demonstrates that similar fibrous microstructure in Cambrian mollusks and hyoliths actually represent a primitive type of prismatic microstructure constituted of calcitic prisms. Revision of these fibrous microstructures in Cambrian fossils demonstrates that calcitic shells are prevalent in the so-called aragonite sea of the earliest Cambrian. This has important implications for understanding the relationship between seawater chemistry and skeletal mineralogy at the time when skeletons were first acquired by early lophotrochozoan biomineralizers.

The diversification of animals during the Cambrian Period is one of the most significant evolutionary events in Earth's history. However, the sequence of events leading to the origin of 'modern' ecosystems and the exact temporal relationship between Ediacaran and Cambrian faunas are uncertain, as identification of the Ediacaran-Cambrian boundary and global correlation through this interval remains problematic. Here we review the controversies surrounding global correlation of the base of the Cambrian and present new high-resolution biostratigraphic, lithostratigraphic and δ¹³C chemostratigraphic data for terminal Ediacaran to basal Cambrian strata in the Zavkhan Basin of Mongolia. This predominantly carbonate sequence, through the Zuun-Arts and Bayangol formations in southwestern Mongolia, captures a distinct, negative δ¹³C excursion close to the top of the Zuun-Arts Formation recognized as the BAsal Cambrian carbon isotope Excursion (BACE). In this location, the nadir of the BACE closely coincides with first occurrence of the characteristic early Cambrian protoconodont Protohertzina anabarica. Despite recent suggestions that there is an evolutionary continuum of biomineralizing animals across the Ediacaran-Cambrian transition, we suggest that this continuum is restricted to tubular forms, and that skeletal taxa such as Protohertzina depict 'true' Cambrian representatives that are restricted entirely to the Cambrian. Employing the first appearance of the trace fossil Treptichnus pedum to define the base of the Cambrian suffers significant drawbacks, particularly in carbonate settings where it is not commonly preserved. As T. pedum is the only proxy available to correlate the Cambrian Global boundary Stratotype Section and Point (GSSP) defined at Fortune Head, Newfoundland, we suggest that the GSSP be redefined elsewhere, in a new stratigraphic section that contains secondary markers that permit global correlation. We propose the nadir of the BACE as the favored candidate to define the base of the Cambrian. However, it is essential that the BACE be complemented with secondary markers. In many global sections the nadir of the BACE and the first occurrence of the genus Protohertzina are closely juxtaposed, as are the BACE and T. pedum. Hence these taxa provide essential biostratigraphic control on the BACE and increase potential for effective global correlation. We also recommend that an Auxiliary boundary Stratotype Section and Point (ASSP) be simultaneously established in order to incorporate additional markers that will aid global correlation of the Ediacaran-Cambrian boundary. The BAY4/5 section through the upper Zuun-Arts and Bayangol formations yields key shelly fossils and δ¹³C values and is therefore an ideal candidate for consideration as the GSSP for the Ediacaran-Cambrian boundary.