Exceptional sub-micrometer details of shell microstructure are preserved in phosphatic micro-steinkerns representing several phyla from shell beds of the Upper Ordovician of the Cincinnati Arch region, USA. These fossils provide the most detailed record of Ordovician mollusk shell microstructures, as well as exceptional details on the earliest cases of undisputed nacre. The trend towards nacre in the Mollusca is one aspect of the surge in escalation between mollusks and their predators during the Great Ordovician Biodiversification Event.

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