RAFINESQUINA ALTERNATA AS A PHOTOSYMBIONT HOST: SHEDDING NEW LIGHT ON OLD QUESTIONS

Discrepancies among hypotheses of life position and feeding habits of Rafinesquina spp. have left many questions unanswered. Examination of Rafinesquina alternata from the Cincinnatian of Ohio yields a new hypothesis for its paleobiology. R. alternata had several qualities similar to those of photosymbiont hosts, including: large shell size; shell morphology that would expose photosymbionts to sunlight; a high skeleton to biomass ratio; a simple alimentary system; rapid growth rate; and unusual population structure reflected in its taphonomic record.

Providing sunlight to symbionts without making itself vulnerable to predation was overcome by the direct passage of sunlight into the shell via laminated pseudopunctae with or without nonporous taleolae. Pseudopunctae and taleolae were homoplastic and served multiple functions in strophomenides (Williams 1997). Due to calcite’s optical properties, sunlight passing through taleolae may have provided sustainable energy for photosymbionts. R. alternata cross-sections show pseudopunctae extending through the shell layers (Pope 1967; Williams 1997). We found the average size of their pseudopunctae to be 0.039 mm, spaced from 0.187 mm to 0.91 mm apart. Shell penetration occurred only in pseudopunctae near the outer margin, with younger shell layers covering older pseudopunctae interiorad. If pseudopunctae served as lenses, the photosymbionts lived in the mantle near the shell margin.

Photosymbionts are suspected from the early Paleozoic and are the link between sunlight and high calcification rates (Wood 1993). Acquisition of symbionts either through vertical transfer or from the environment would have given R. alternata its ecological advantage, allowing unusually high populations. The episodic starvation model of deposition in the type area (Datillo et al. 2009) may explain conditions of water clarity and depositional stasis which permitted R. alternata, and perhaps other organisms, to host photosymbionts.

This hypothesis holds implications for life position, growth and epibiont relationships of R. alternata, and for its paleobiology and that of other strophomenides throughout the Paleozoic. Isotopic signatures may help verify the hypothesis. We anticipate that this study will put new light on old questions of several types.