Test ultrastructure of fusulinid Foraminifera

Abstract

Foraminifera are separated systematically at the subordinal and suprafamilial level on the basis of their wall ultrastructure¹, yet those elements of the walls are near or at the resolution of the optical microscope². To delineate those elements we have initiated a comprehensive study of the test ultrastructure of calcareous Foraminifera, utilising high voltage transmission electron microscopy (HVTEM) of specimens thinned by ion bombardment^3,4. We find certain ultrastructural characteristics to be common both to all Foraminifera studied and to all other comparison biological carbonate: for example, organic inclusions are found within and on the boundaries between the crystals of miliolid, fusulinid and rotaliid foraminiferal tests, bivalve shells, coral skeletons and human teeth. Other characteristics are typical of a particular host mineral, but independent of phyletic affinities: for example, the size, shape and abundance of inclusions are different in aragonite crystals from calcite crystals. Still other characteristics are diagnostic of certain groups: for example, crystal sizes and shapes are definitely different for the various suborders of Foraminifera. These ultrastructural characteristics should lead to a better understanding of the relationships between groups of Foraminifera, and to a better understanding of the biological calcification process in general. But more immediately, they provide powerful criteria for the evaluation of ultrastructural modification during and after fossilisation, especially for extinct forms such as the fusulinid Foraminifera.