The shell structure of Ptychodesma (Cyrtodontidae; Bivalvia) and its bearing on the evolution of the Pteriomorphia

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Abstract
Observations of the hinge structure of the Middle Devonian species Ptychodesma knappianum Hall and Whitfield 1872 confirm the cyrtodontid affinities of this once problematic genus. The shell microstructure of P. knappianum supports Douvillé’s (1913) hypothesis concerning the ancestral nature of nacreous shell microstructure in the Bivalvia, and suggests further that modern arcoids and pterioids evolved from nacro-prismatic cyrtodontid ancestors. The Arcoida generally retained a rigid sub-periostracal shell margin and consequently evolved rigid crossed microstructure and a strong dentition to effect proper guidance of the shell margins upon closure. In contrast, the early Pterioida evolved a more prominent flexible outer prismatic layer to assist effective closure along the shell margins, and they retained nacreous microstructure as an adaptation for shell durability.

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