SIZE AND SHAPE TRENDS IN FUSULINOIDEANS (LATE PALEOZOIC FORAMINIFERA) RELATED TO PHOTOSYMBIOSIS

Modern larger foraminifera contain photosymbionts that provide energy to their hosts and indirectly assist the hosts in calcification. Photosymbiosis has been a driving force in larger foraminiferal evolution because symbiont-bearing taxa experimented with morphologic adaptations for partitioning habitats within the photic zone. In particular, certain deeper-dwelling taxa acquired transparent shell walls and flattened shapes in order to maximize the amount of light that could enter the shell. Certain shallow-dwelling taxa, in contrast, acquired spherical shapes and relatively opaque walls in order to protect against ultraviolet radiation and limit the amount of light entering the shell. Fusulinoideans are extinct forams, which, on the basis of ecologic and morphologic similarities to modern larger forams, are thought to have hosted photosymbionts. If so, then one can hypothesize that fusulinoidean evolutionary history should have been characterized by increasing morphologic disparity as a progressively greater variety of shell types were produced to exploit narrow depth ranges within the photic zone. This hypothesis is tested against a null expectation by determining variances in shell size, shape, surface area, and volume for 30 to 40 representative species per geologic epoch throughout Late Paleozoic time. The dominant trend in fusulinoidean evolution was size increase. Early species were mostly small, but they were joined in later times by progressively larger species so that variances in size, surface area and volume increased through time, as predicted. Variance in the ratio of surface area to volume decreased with increasing size, however, because this ratio is always low for large shells, regardless of shell shape. All larger fusulinoideans exhibited very low surface-to-volume ratios, suggesting that they were optimally adapted for the intense light conditions of the shallow photic zone. Optimization for very shallow habitats might account for the very high rates of evolutionary turnover exhibited by fusulinoideans, and it might also have contributed to the total demise of fusulinoideans during the end-Permian mass extinction.