Complex hierarchical microstructures of Cambrian mollusk Pelagiella: insight into early biomineralization and evolution.

Shell microstructure and mineralogy of Pelagiella madianensis Zhou & Xiao, 1984, a globally distributed Cambrian micromollusk, are investigated based on abundant and extraordinarily well-preserved specimens from Xinji Formation, Longxian, Shaanxi, North China. Five types of aragonitic microstructures have been recognized. The lamello-fibrillar microstructure, previously known from Pelagiella, constructs the outermost shell layer, while the remaining four types are reported here for the first time in this genus. They include fibrous foliated, foliated aragonite, crossed foliated lamellar and isolated tablets. The animal constructs these five types of microstructures to build its shell in a complex hierarchical pattern with four orders: crystallite columns, laths, folia and lamellae. These findings demonstrate that the capability of building complex shell microstructures had already evolved by the Cambrian explosion. In addition, this work shows that early aragonitic shells were constructed with fibers, laths, folia and isolated tablets, indicating increased controls over biomineralization by the animal.