The Giant Marine Gastropod Campanile Giganteum (Lamarck, 1804) as a High-Resolution Archive of Seasonality in the Eocene Greenhouse World

Giant gastropods are among the largest mollusks in the fossil record, but their potential as paleoseasonality archives has received little attention. Here, we combine stable isotope and trace element analyses with microscopic observations and growth modeling on shells of two species of the gastropod genus Campanile: the extinct Campanile giganteum from Lutetian (~45 Ma) deposits in the Paris Basin (France), the longest gastropod known from the fossil record, and its modern relative Campanile symbolicum from southwestern Australia. The C. giganteum shells contain original aragonite and have pristine nacre in their apertures. We show that these gastropods attained growth rates exceeding 600 mm/year along their helix, depositing over 300 cm³ aragonite per year. High growth rates and excellent preservation make C. giganteum excellent archives for reconstructing environmental change at high (potentially daily) temporal resolution, while providing enough material for methods such as clumped isotope analysis. Growth models show that Campanile gastropods grew nearly year-round, albeit slower in winter. Stable oxygen isotope ratios in modern C. symbolicum faithfully record a seasonal variability of 18–25°C in sea surface temperature, only failing to record the coolest winter temperatures (down to ~16°C). Similarly, C. giganteum specimens likely record a nearly complete seasonal temperature range. Assuming constant sea water isotope composition, their oxygen isotope seasonality of up to 2.5‰ would translate to a Lutetian temperature range of 21–32°C in the Paris Basin. We hypothesize that these high and seasonally variable temperatures formed the breeding ground for the Lutetian shallow marine biodiversity hotspot in the Paris Basin.