A HISTORY OF THE CLASSIFICATION OF FORAMINIFERA: CÉPHALOPODES TO RHIZARIA

The classification of Foraminifera, and our understanding of the clade’s position on the Tree of Life, has changed considerably over the past 200 years. Early scientists believed foraminiferans (forams) to be miniature versions of larger animals; for example., d’Orbigny (1826) classified forams together in a group of microscopic cephalopods, believing that he had seen their tiny heads, mouths and tentacles under the microscope. Dujardin (1835), unable to confirm the existence of organs inside of forams, proposed that their body was composed of a contractile “living jelly” which he named “sarcode” (protoplasm). Dujardin (1835) used the name Rhizopodes for these organisms, such as forams and Gromia, that moved using feet-like “glutinous filaments.” Schulze (1854) organized the testate Rhizopoda into the single-chambered Monothalamia and the multi-chambered Polythalamia, both of which contained forams. Throughout the 19^th century, ideas about how to classify foraminiferans shifted as scientists debated whether the cell bodies of single-celled organisms were equivalent to the cells that make up the tissues of plants and animals, and if so what place they occupied in the hierarchy of life. Siebold (1845), and later Bütschli (1880), classified forams and other rhizopods in the Protozoa a reflection of their belief that these motile organisms were more closely related to animals than to plants. Haeckel (1866) erected Protista, a third kingdom of single-celled organisms, to bridge the gap between the animal and plant kingdoms. Haeckel, a devotee of evolutionary theory, depicted these relationships on a literal diagram of a tree; Foraminifera (Polythalamia) is represented by a small twig branching off the branch Rhizopoda. Haeckel’s biogenetic law (ontogeny recapitulates phylogeny) influenced the paleontologists under whom Joseph Cushman would later study. In the mid-to-late 20^th century, characters based on cell ultrastructure were integrated into classification of single-celled eukaryotes. Our current understanding of the evolutionary relationships of Foraminifera is derived from molecular systematics. Foraminifera is now recognized as a monophyletic clade of single-celled eukaryotes that branches within the more inclusive clade Rhizaria as the probable sister group to Acantharia or Polycystinea.