Fibrous or Prismatic? A Comparison of the Lamello-Fibrillar Nacre in Early Cambrian and Modern Lophotrochozoans

SIMPLE SUMMARY: The current understanding of the origin and rapid diversification of biomineralized skeletal animals and their environmental interactions during the Cambrian Radiation is largely dependent on the phosphatized skeletal remains of early animals. However, many questions remain unresolved about this early Cambrian animal skeletonization bio-event, as identification of primary mineralogy and microstructural organization of these earliest skeletons is often difficult. The comparison of previously poorly understood fibrous microstructures (represented in this study by lamello-fibrillar nacre in early Cambrian mollusk and hyolith shells) with their modern counterparts (coleoid cuttlebones and serpulid tubes) shows key differences in shell microstructures of these animal groups. For example, the aragonitic lamello-fibrillar nacre in some Cambrian shells is herein demonstrated to represent a primitive calcitic prismatic microstructure. This also demonstrates the prevalence of calcitic shells in the Terreneuvian, a period of time when seawater chemistry was thought to predominantly facilitate the precipitation of aragonite skeletons, suggesting that biomineralization of prismatic calcite is strongly biologically controlled. ABSTRACT: 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.