Sclerochronology of the large scallops Gigantopecten latissimus and Pecten jacobaeus in a Pliocene warmer Mediterranean Sea

Abstract

Here we apply a sclerochronological approach to reconstruct the life-history of two stenohaline bivalves of the family Pectinidae, Gigantopecten latissimus and Pecten jacobaeus from the Pliocene of Italy. The specimens come from the locality of Torrita di Siena (Siena-Radicofani Basin, Tuscany), dated to the late Zanclean-Piacenzian with nannoplankton biostratigraphy and Sr-isotope stratigraphy. After measuring the width of micro-growth increments and verifying that the shells were not diagenetically altered, we sampled them at high resolution for carbon and oxygen stable isotopes. δ13C and δ18O shell values allowed to distinguish between specimens that lived above or below the thermocline. Those influenced by surface waters indicate temperatures (for δ18Oseawater = 1.5‰) with winter minima of 16–18 °C and summer values up to 28–29 °C, close to present temperature conditions in the tropical west-African climate belt. In line with this, we found that the two species had similar seasonal growth patterns, with faster growth during colder months and summer slowdown, a typical adaptation of bivalves of tropical affinity suffering from summer temperature extremes. Despite this similar adaptation, G. latissimus, with large and heavy shells (length up to 30 cm) became extinct around 3.0 Ma, while the smaller P. jacobaeus survived Plio-Pleistocene cooling. Different growth rates between the two species and, therefore, different metabolic costs, might explain such differential response, together with differences in reproduction strategies. Habitat loss and fragmentation, due to the decrease of shelf margins and organogenic substrates caused by cooling and sea level fall, are abiotic factors that could have also contributed to the extinction of G. latissimus. P. jacobaeus adaptation to live across a larger bathymetric range, which indicates the ability to thrive in a wider range of temperatures, most likely played a role in its survival. Further studies, including more specimens across multiple localities, will help verifying these hypotheses.