ALGAL SYMBIOSIS IN FUSULINOIDEANS (PALEOZOIC LARGER FORAMINIFERA)?

The Plattsmouth Limestone of the Oread Formation (Upper Pennsylvanian) in Iowa contains a distinctive chert lithology known to local rockhounds as “rice agate,” because it contains silicified, white fusulinoideans in a dark matrix. Petrographic analysis of the “rice agate” reveals the presence of two populations of spherical, siliceous bodies within the fusulinoidean shells. The two populations differ in size, with one group of spheres being ~40 µm in diameter and the other group being ~15 µm in diameter. The larger spheres are abiotic in origin, as they occur not only in fusulinoideans but also in the chert matrix. The smaller spheres, in contrast, might be the silicified remains of algal symbionts. They occur only inside fusulinoidean shells, and they are roughly the same size as algal symbionts found in modern, larger foraminifera. Algal symbiosis in fusulinoideans has been suspected since the 1970s, just a few years after recognition of widespread photosymbiosis in modern, larger foraminifers. Until now, however, putative evidence of preserved algal symbionts in fusulinoideans was limited to a single specimen from the Phosphoria Formation (Permian). If algal symbiosis was common in fusulinoideans, then it might explain the dramatic evolutionary diversification of fusulinoideans beginning in Early Pennsylvanian time. Algal symbiosis could have been a catalyst for morphologic experimentation as fusulinoideans vertically subdivided the oligotrophic, clear-water environments to which they were adapted. By analogy, among extant symbiont-bearing forams, shallow-water taxa possess relatively opaque shells with low surface area-to-volume ratios in order to limit the amount of light entering the test and to protect against ultraviolet radiation. Deeper-dwelling taxa possess more transparent shells with higher surface area-to-volume ratios. Early fusulinoideans were mostly discoidal to nautiloid in shape, but they quickly diversified into subspherical, subquadratic and fusiform shapes with a variety of wall structures and secondary secreted deposits that facilitated varying opacity of the tests in different taxa. Compartmentalization of shells by septal folding and transverse septula might have been further adaptations for housing symbionts.