Evolution of European lower JurassicGryphaea (Gryphaea)and contemporaneous bivalves

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Abstract
Standardised data, representing ‘adulthood’, reveal that in characters of gross shape, shell thickness and size G. (Gryphaea) exhibits marked changes through the European Lower Jurassic. Evolution, evidently anagenetic, was not certainly unidirectional and even‐paced (truly gradualistic) in any character — for some characters brief periods of stasis may have occurred. However, few, if any, intervals were marked by stasis in all characters, and no horizons by especially conspicuous change. Punctuated equilibrium is thus not displayed. Parallel patterns of gross shape, thickness and size change can be readily integrated in an ‘adaptational’ model of evolution upon appreciation that each variable is likely to affect shell stability, a factor influencing survival in reclining bivalves. Thus periods of evolution towards a more stable gross shape (broader, flatter) are considered to be a selectively‐driven response to coeval size and thickness changes (reductions) tending to diminish stability in a shell poorly adapted at the outset for reclining on typical early Jurassic sea‐floors. While size must be reckoned to affect physical stability, adaptation to a stable food supply was probably the main reason for overall size increase in G. (Gryphaea) a trend shared with an ecologically‐diverse suite of L. Jurassic bivalves. The secondary effect of improving shell stability (in all but earliest Jurassic forms) probably accounts for the long delay in achievement of a paradigmatic gross shape in G. (Gryphaea). A lack of significant shape evolution in other L. Jurassic bivalves may reflect their comparatively well‐adapted form, but deficiency of heritable variation could also have been a factor. Testable explanations for slow rates of change in the common trait of size increase include very weak selection and slow improvement in stability of food supply.