Jurassic soft–bottom oyster Crassostrea from Japan

Abstract

A new species of oyster, Crassostrea tetoriensis, is described from the lower part of the Middle Jurassic–Cretaceous Tetori Group. This is the oldest record of the genus and one of the earliest muddy–bottom–dwelling oysters. Characteristics of the species include an elongate spatulate outline and narrow attachment area, an elongate hinge area with deep umbonal cavity, a reniform adductor muscle scar, and a smooth commissural margin without chomata. The oysters are cemented to each other and constitute bouquet–like aggregates or colonies of shells in muddy deposits of inferred brackish–water origin. The shells contain lenticular or blister–like chambers filled with a chalky deposit. These ecological and structural properties are characteristic of Crassostrea, and may be regarded as the result of adaptation of the sessile, suspension feeding animals to soft muddy substrates. Oysters are a group of bivalves that have lost mobility and live cemented mostly to hard substrates. Crassostrea and its allies returned secondarily to soft bottoms. The find of C. tetoriensis indicates that the return occurred early in the history of oysters, 60 myr after the first appearance of ostreid oysters in Late Triassic.