Brachiopod shell spiral deviations (SSD): Implications for trace element proxies

The importance of brachiopod shells as archives of past environmental changes, via chemical proxies (isotopes and trace elements), will increase significantly if the age of specimens can be determined accurately. Here, we evaluate the interpretation of spiral shell deviations throughout the ontogeny of the modern brachiopod species Laqueus rubellus collected from Sagami Bay (Japan), in combination with high-resolution biogeochemistry. The determination of spiral maxima and minima indicates that there is a different growth rate for ventral and dorsal valves, as well as for juvenile and adult specimens, and that the anterior shell regions provide a more accurate record of shell growth. The combination of trace element proxies (P/Ca and Mg/Ca) with shell spiral deviations indicates that the maxima represent prominent periods of shell growth that occurs during the beginning of the autumn. By counting the maxima on dorsal valves of fully mature specimens, these specimens can record shell growth up to 6-7 years, which is confirmed by independent morphological and isotopic studies in the same brachiopod species. The integration of statistical measurements of shell morphologies with chemical measurements represents an innovative methodology for specimen-age determinations in brachiopods, and provides a better understanding of trace element proxies. Overall, the application of this approach to fossil brachiopods may open a new avenue for paleoseasonality and paleoceanography studies.