DIFFERENCES BETWEEN EVOLUTION OF MEAN FORM AND EVOLUTION OF NEW MORPHOTYPES - AN EXAMPLE FROM LATE CRETACEOUS PLANKTONIC-FORAMINIFERA

Morphological evolution in the Late Cretaceous (Maastrichtian) Contuso truncana lineage of planktonic foraminifera was studied at DSDP Sites 525 (South Atlantic) and 384 (North Atlantic). A multivariable approac h was used to separate aspects of form controlled by geographical vari ation (size, spiral roundness of the test, percentage of kummerform sp ecimens) from those due to changes that occurred simultaneously in geo graphically distant populations of the lineage (shell conicity, number of chambers in the last whorl). A gradual increase in mean shell coni city was observed over the last 3 million years of the Cretaceous. It arose from the combination of a rapid development of highly conical sh ells after 68.5 Ma and a long-term trend of progressive disappearance of the ancestral morphotype. Therefore, despite the gradual change in ''mean form,'' the morphological evolution in the Contusotruncana line age differs from the classical image of phyletic gradualism. The gradu al increase in mean shell conicity in the lineage was accompanied by a remarkable decrease in its absolute abundance (shell accumulation rat e), suggesting that the changes in shell morphology might not have bee n neutral with respect to natural selection. Apparently, gradual chang e in ''mean form'' of fossil lineages does not require an equally grad ual development of morphological novelties. It may be caused by natura l selection operating on a constant range of variation in populations living in environments without geographical barriers.