Deep origin of the crossed‐lamellar microstructure in early Cambrian molluscs

Aragonitic crossed-lamellar (CL) is one of themost commonly formed and extensively studied molluscanshell microstructures, yet its origin and early evolution within the Mollusca remains poorly understood. Here, a primitive CL microstructure from one of the oldest gastropods,Pela-giella madianensis, and the problematic hyolith Cupitheca sp. of the Cambrian Series 2 Xinji Formation on the North China Platform, was investigated. In P. madianensis, detailed characterization has revealed a typical four-ordered hierarchical orga-nization of aragonitic crystallites, and a thick layer of organic membranes surrounding its first-order lamellae. A transitional fibrous microstructure was observed between the outer CL and inner foliated aragonite structural layers. In Cupitheca sp.,only the first and second-order lamellae were visible due to preservation limitations, and the first-order lamellae were extremely irregular in shape and size, which is consistent with modern representatives. This study demonstrates that the capability to construct highly-mineralized intricate shells was acquired in early Cambrian stem-group gastropods. The CL microstructure first emerged in the early Cambrian and as a basal synapomorphic trait in total-group molluscs. Moreover, presence of the CL microstructure in problematic lophotrochozoans (i.e.hyoliths) is confirmed. This study contributes to a more complete picture of the evolutionary origin andarchitectural diversity of biomineralized mollusc shells during the Cambrian explosion, and strengthens the phylogenetic links between hyoliths and molluscs.This research was funded by the National Natural Science Foundation of China (Grant nos. 41930319, 41890845, 42102012, 41621003 and 42072003); Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB26000000), 111 Project (Grant No. D17013), Swedish Research Council (Grant nos. VR2016-04610 and VR2017-05183) a Young Thousand Talents Plan of China (Grant No. 41720104002) and China Postdoctoral Science Foundation (Grant No. 2022M712987).