Identification of life-history stages in fusulinid foraminifera

The whorls of fusulinid foraminifera preserve a record of each individual's ontogeny, and therefore provide access to aspects of morphological life-history patterns. Outline-based morphometric analyses of representative fusulinid specimens assigned to Robustoschwagerina, Sphaeroschwagerina, and Schwagerina show that these genera exhibit characteristically different ontogenetic trajectories within a space defined by test size, test shape, and whorl number. In Robustoschwagerina and Sphaeroschwagerina the shape of the test altered during development, first exhibiting a spherical prolocular character, transforming to a fusiform shape, and then reverting to a secondary spherical form. In contrast, Schwagerina exhibits only the first two stages. Development-based morphological transitions also vary among these genera concerning the test size change. Moreover, these patterns appear to be taxon-specific, and so have potential utility for taxonomic identification as well as for understanding fusulinid life history. The distinct test inflation of the spherical pseudoschwagerines during their ontogeny raised the question whether there is a habitat shift in a certain developmental stage. Intriguingly, the patterns of their ontogenetic shape variation appear reminiscent of the morphological reversion that occurs in several small benthic genera such as Tretomphalus for which a late ontogenetic planktonic phase has been proposed. This similarity may have implications for the functional interpretation of spherical fusulinid tests.