OVERVIEW OF THE FINE STRUCTURE OF MODERN “ALLOGROMIID” TESTS (FORAMINIFERA): ARE THERE MEANINGFUL CONSTRUCTIONAL THEMES?

“Allogromiids” (orders Allogromiida and Astrorhizida) are morphologically simple Foraminifera that are generally regarded as primitive. The test, if present, commonly consists of just a single organic or agglutinated chamber that ranges in shape from domed or spherical to tubular or stellate, and many species display extensive intraspecific variation. A few allogromiids are multi-chambered (e.g., Phthanotrochus) and some lack a test (e.g., the freshwater Reticulomyxa). Molecular studies (Pawlowski et al., 2002, 2003) demonstrate that these simplistic morphological features are inadequate for classification, and molecular clades bear little resemblance to morphology-based classifications. These simple traits, however, are underlain by intricate patterns at the ultrastructural level, and more refined studies should delineate broad constructional themes and provide additional insight into allogromiid evolution. Although very few species have been studied to date, we can nonetheless identify some preliminary themes in allogromiid test construction: (1a) “herringbone” wall in organic-walled taxa characterized by a finely fibrous wall with fibers oriented parallel to the cell surface (e.g., Allogromia); (1b) “herringbone” inner “theca” in taxa with an additional outer organic or agglutinated wall (e.g., Iridia, Myxotheca, Astrammina); (2) soft-shelled saccamminid construction with a prominent inner, fibrous organic lining overlain by an agglutinated layer with an organic cement (e.g., Cribrothalammina, Ovammina); (3) Crithionina construction characterized by an inner organic lining overlain by a thick layer of sediment bound by pseudopodia; and (4) hyperamminid construction characterized by a distinctive inner organic lining of crescent-shaped fibers oriented tangentially to the cell surface, which in turn is overlain by an agglutinated layer with organic cement. In general, the herringbone construction appears to be widely distributed across several of the molecular clades recognized in Pawlowski et al. (2002, 2003), and this construction may prove to be primitive among the basal foraminifera. Some of the differences in test construction have been attributed to motility or the mode of growth.