THE MAIN MORPHOLOGICAL TRENDS IN THE DEVELOPMENT OF THE FORAMINIFERAL APERTURE AND THEIR TAXONOMIC SIGNIFICANCE

Abstract

As a result of the intensive movement of the cytoplasm through the aperture when communication with the environment is required, this area has an important and variable functional burden. Additional skeletal structures have a fundamental supporting function along the course of this strong cytoplasmic stream and may be related to the compartmentalization and differentiation of the cytoplasm. As a result of these important functional roles, the structure of the aperture is one of the basic diagnostic features in foraminiferal taxonomy. The simplest and least diverse apertural types are found in the most ancient unilocular or pseudochambered representatives of the classes Lagynata and Astrorhizata. Their development from simple to complicated ones in the different classes (following the new foraminiferal macrosystem proposed by Mikhalevich) shows a significant number of parallelisms and convergences. In both the lower agglutinated groups and the higher calcareous members of the classes Spirillinata, Miliolata, Nodosariata and Rotaliata, the evolutionary trends of the apertures are similar within the same class, even if those of the lower groups are always structurally simpler and less diversified. These trends continue until all the possibilities of the pre-existing structures are exhausted. Then, new structures, sometimes affecting the whole cell organization arise and the possibilities of morphological changes multiply, leading to evolutionary divergence. J. Micropalaeontol. 20(1): 13-28, July 2001.