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During the first two years of the grant my undergraduate student assistants and I created composite standard databases for assessing diversity, frequencies of origination and extinction, and secular changes in provincialism among foraminifers during the Late Paleozoic ice age (LPIA). The data showed that rates of foraminiferal evolution accelerated during the LPIA, and that increases in diversity and frequencies of origination and extinction were positively correlated with increases in provincialism. We demonstrated also that fusulinoidean foraminifers, in particular, dominated overall foraminiferal associations during most of the LPIA, so that overall patterns in the evolutionary history of LPIA foraminifers can be understood only by understanding patterns in the evolutionary history of fusulinoideans. We hypothesized that the dramatic increase in fusulinoidean diversity was fueled by the advent of algal symbiosis, an evolutionary novelty that allowed fusulinoideans to occupy previously uninhabitable, oligotrophic environments. Once fusulinoideans acquired algal symbionts and invaded oligotrophic environments, they diversified as a result of morphologic and physiologic adaptations for further partitioning the low-nutrient settings to which they were uniquely suited.

The third and final year of the grant was devoted to assembling and evaluating evidence in support of the algal symbiont hypothesis. We discovered what appear to be preserved remains of unicellular algae inside the shells of exceptionally well-preserved, silicified fusulinoideans. It is difficult, however, to distinguish these putative algae from silica lepispheres, which form through entirely inorganic processes. A more conclusive line of evidence for algal symbiosis in fusulinoideans might be found in the carbon isotopic composition of their shells. It is well known, for example, that certain modern, symbiont-bearing foraminifers draw carbon for their shells from a photosynthetically fixed, internal carbon pool. The calcite of their shells is depleted in ¹³C relative to the ocean water in which they live because of carbon fractionation associated with photosynthesis. It follows that fusulinoidean shells also should be relatively depleted in ¹³C if, in fact, fusulinoideans hosted photosymbionts. Following this line of reasoning, we sampled shell calcite from 22 non-silicified fusulinoideans for stable carbon isotope analysis. All but one of the shells yielded δ¹³C values lower than the independently established range of values for marine calcite of the same age: δ¹³C in fusulinoidean shell calcite ranges from -6.01 ‰ to 4.16 ‰ (mean = 1.40 ‰); in contrast, δ¹³C in Permian marine calcite ranges from 4.0 ‰ to 5.8 ‰. This is the first compelling evidence for the existence of photosymbiosis in fusulinoideans, although the phenomenon has been suspected ever since the early 1970s.