The punctuation in the evolution of orbitoides in the campanian of South-West France

Abstract

For the Orbitoides assemblages analyzed from a number of closely sampled sections of Campanian calcarenites in south-west France there is a sudden change in the means of several parameters of the internal embryonicnepionic stage. The direction of the change is in accordance with the nepionic acceleration principle, known to be valid for all lineages of orbitoidallarger foraminifera. Below and above the level of the morphological break we seem to be dealing with two longer periods without any systematic change in the morphology. In two of the sections the abrupt shift could be narrowed down to a lithostratigraphic distance of about ten centimetres, an interval which is thought to correspond to a time duration of less than a few thousand years. In none of the sections could any evidence be found that might point to a hiatus in the sedimentation or to a notable change in the open-marine, shallow-water environment. Since the discontinuity was found in sections about 90 km apart (Aubeterre and Meschers) the change must have occurred simultaneously throughout the entire Aquitaine basin. If we take into account that bioturbation must have had an obliterating effect on our data, it seems safe to conclude that the modal morphological change was very large and geologically instantaneous. Because no comparable change in internal morphology could be detected in the accompanying Lepidorbitoides lineage it is thought likely that the fundamental change in the population composition of the Orbitoides was an autonomous happening fitting into the evolutional theory of homeostasis and punctuation. The pulsating pattern in the stasis parts of our data sets is rather weak; phyletic gradualism is still thought to be an acceptable theory to explain the gain of more advanced morphotypes and the loss of conservative ones in our Orbitoides sequence. At the end of our paper it is argued that the concept of sympatric speciation passing through low-frequency bottlenecks in the suites of populations is the one that would fit best to the evolutional history of the lineages of orbitoidal foraminifera. This model would combine the random character and the directional aspect of evolution, which are expressed in various ways in all better known lineages.