New data on molluscs and their shell microstructures from the Middle Cambrian Gowers Formation, Australia

Abstract

Numerous new cases of preserved shell micro- structure were discovered in molluscs from the Middle Cam- brian Gowers Formation (Ptychagnostus atavus ⁄Peronopsis opimus Zone, Floran Stage) in the Georgina Basin, Australia. The new data provide further evidence that, by the Middle Cambrian, molluscan shell microstructures were diverse, and many molluscs had a complex shell with multiple types of shell microstructure. In addition, many new occurrences of laminar microstructures are described herein. For many, the nature of these laminar microstructures is not known, but in three species the microstructure is foliated calcite, and in at least two the microstructure is more likely to have been cal- citic semi-nacre, a type of microstructure known in brachio- pods and bryozoans but unknown in modern molluscs. This commonality among these three closely related lophotroch- ozoans underscores a similar mechanism of biomineraliza- tion. Moreover, these observations suggest a prevalence of calcite-shelled lineages among molluscs from the Middle Cambrian, a time of calcite seas. In addition, the broad occurrence of laminar, nacre-like microstructures in many of these fossils reveals how widespread these strong (fracture- resistant) microstructures were in Middle Cambrian mol- luscs. Additionally, a few specimens of Yochelcionella preserve imprints of a bilaterally symmetrical pair of muscle scars. New taxa described here include Corystos thorntoniensis gen. et sp. nov., Yochelcionella snorkorum sp. nov., Yochelcionella saginata sp. nov., and Anhuiconus? agrenon sp. nov.