Article: Evolution of gryphaeate oysters in the mid-Jurassic of western Europe

European Callovian (and later) forms of Gryphaea (Bilobissa) arose not from earlier representatives of the subgenus but from Catinula, a much smaller, frequently ribbed form, here regarded as a subgenus of Gryphaea. Evolution was essentially gradualistic. G. (Catinula) itself arose from an early G. (Bilobissa) species at the Toarcian/Aalenian boundary. In this case evolution was rapid (and apparently restricted to a small geographical area) but there is little evidence of stasis before and afterwards. The earlier G. (Bilobissa) lineage became extinct in the late Bajocian or early Bathonian. The morphologies of G. (Bilobissa) and G. (Catinula) may represent alternative adaptations for reclining in similar, low-energy environments, respectively favoured under conditions of high and low potential for shell growth. Such potential may have been controlled by ocean temperature and/or salinity. Most of the change between G. (Bilobissa) and G. (Catinula) probably resulted from alteration of growth rates. This almost certainly involved genetic change, although ecophenotypic variation may have been a precursor.