A TEST FOR THE POSSIBILITY OF PHOTOSYMBIOSIS IN FUSULINIDS: SIZE AND SHAPE TRENDS RELATED TO DEPTH OF HABITAT

Fusulinid foraminifera are extinct protists that are thought to have hosted photosymbionts, as in modern larger foraminifera, but the ancient host-symbiont relationship has never been demonstrated conclusively. Among modern larger foraminifera, deeper-dwelling species exhibit large surface-to-volume ratios (S/V) in order to maximize the amount of sunlight that can be captured for use by photosymbionts. Shallower-dwelling species exhibit small S/V in order to limit incoming sunlight, especially ultraviolet radiation. If modern symbiont-bearing foraminifera are useful analogues for fusulinids, then we predict that deeper-dwelling fusulinids ought to exhibit larger S/V than shallower-dwelling fusulinids. This prediction was tested by analyzing fusulinid shells from Virgilian (Upper Pennsylvanian) cyclothems in Kansas. Several hundred fusulinids from transgressive limestones and regressive limestones of the Oread, Lecompton and Deer Creek cycles were measured. Fusulinids from transgressive limestones exhibit larger S/V than those from regressive limestones, and specimens with the smallest S/V invariably occur in shoaling deposits at or near the tops of regressive limestones. Surprisingly, shell shape does not vary predictably with depth of habitat. Rather, changes in S/V were accomplished by changes in size, with larger shells always yielding small S/V values. The observed trends are significantly non-random with respect to depth of habitat (p = 0.037), a result that is consistent with the possibility of photosymbiosis in fusulinids.