The Evolution of Gryphaea

Abstract

The coiling of the Liassic oyster Gryphaea is discussed and the assessment of tightness of coiling investigated. Hence Hallam's (1960a, b) statistical work on Gryphaea is interpreted and from his original measurements a highly significant increase in thetightness of coiling is demonstrated in the Gryphaeas of the higher Sinemurian horizons of Britain. The presence of a strong natural selection pressure favourable to more tightly coiled individuals of the communities is indicated, and the nature of the lineage discussed. It is suggested that recent criticisms of Trueman's work lack foundation.