In 1922, Trueman stated that an evolutionary series from Ostrea (now Liostrea) liassica to Gryphaea incurva existed in the basal zones of the lower Jurassic. Using bivariate and size-frequency analysis, the degree of coiling and size increase are studied, and the progressive increase claimed by Trueman has not been proven. It is likely that Gryphaea evolved from an Ostrea-like ancestor, but as yet a convincing series has not been established in any of the zones examined in England or Germany.

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