Robert M. Finks

Natural selection in a Cretaceous oyster

A population of shells of the Cretaceous oyster Agerostrea mnesenterica was ob- tained from a single locality. The age at death for each shell could be determined, and the population was deemed to be essentially undisturbed and capable of providing a mortality record of the local population averaged over a span of time. We measured four morphometric characters which, on the basis of a functional morphologic model, could be expected to have adaptive value. The frequency distribution of each character was analyzed separately for the three year old and for the six year and older specimens in order to determine which indi- viduals died young and which survived to old age. Ontogenetic effects were separated from those of differential mortality by independent means. The results indicated that there was (1) directed selection for maximum curvature, (2) centripetal selection for a plica number of about 8, and (3) a net centripetal effect on both arc length and plica height, composed of selection against largest values coupled with a possible selection against smaller values that cannot be separated from ontogenetic effects. Only the selection for maximum curvature and optimal plica number can be reconciled with the predictions of the model.