Late Paleozoic fusuline foraminifera are thought to have hosted photosymbionts, as do 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 in order to maximize the amount of sunlight that can be captured for use by photosymbionts. Shallower-dwelling species exhibit smaller surface-to-volume ratios in order to limit incoming sunlight, especially ultraviolet radiation. If modern symbiont-bearing foraminifera are appropriate analogues for fusulines, then deeper-dwelling fusulines ought to exhibit larger surface-to-volume ratios than shallower-dwelling ones. This prediction was tested by analyzing fusuline shells from the Virgilian (Upper Pennsylvanian) Oread, Lecompton and Deer Creek cyclothems in Kansas. Specimens from deeper-water “middle” limestones exhibit significantly larger surface-to-volume ratios than those from regressive “upper” limestones, and specimens with the smallest surface-to-volume ratios occur in shoaling deposits at or near the tops of regressive limestones. Shell shape does not vary predictably with depth of habitat. Rather, changes in surface-to-volume ratio were accomplished mainly by changes in size, with larger shells always characterized by smaller ratios. The observed trend is significantly nonrandom with respect to depth of habitat (p  =  0.012). The trend is not likely the result of hydrodynamic adaptation, postmortem size sorting or size decrease along a bottom oxygen gradient. It most likely reflects geometric optimization for photosymbiosis.

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