Article: Variation in growth-rate and form of a Bathonian (Middle Jurassic) oyster in England, and its environmental implications

We review controls on extensional growth rate and shell thickness in the extant oyster Crassostrea. Data on these shell parameters for the ecologically similar Bathonian oyster Praeexogyra hebridica, sampled at carefully selected sites, are then used to test a hypothesis that small size in contemporaneous marine bivalves of north-west Europe was the product of reduced salinity. The hypothesis is refuted, at least to the extent that some additional factor must be involved. The relatively low extensional growth rate and shell thickness at the highest-salinity site, together with the elongate ('etiolated') shape there and the low extensional growth rate at all sites in comparison with Crassostrea, suggests that this factor is reduced primary productivity. Other faunal, sedimentological and diagenetic evidence is consistent with low productivity. We point to other possible instances of reduced productivity during the Middle Jurassic and discuss the possible role of this factor in encouraging the widespread colonization of brackish-water environments by bivalves in the Bathonian Stage.