Habitat specific growth rate adaptation in oysters during early Bartonian: effects of high temperature and low seasonality in the Indian palaeotropics

Seasonally resolved paleoclimate reconstructions of extreme climatic events, coupled with responses of contemporaneous organisms, offer valuable information for mitigating the future. The tropical position of the Indian subcontinent during the Middle Eocene Climatic Optimum (MECO ~41Ma), interrupting a general cooling trend, provides an ideal setting to investigate low-latitude seasonality under hot climate conditions, considering the modern minimal seasonality in the tropics. To better understand the seasonality and its impact on shallow marine fauna, a high-resolution multiproxy study is conducted on oyster specimens of the species Flemingostrea pseudoflemingi from the early Bartonian palaeo-tropical Kutch Basin. Clumped isotope (Δ47) thermometry combined with δ18O analyses from excellently preserved shell specimens suggest low seasonal temperature fluctuations (~3°C) with a mean temperature of 34.4±2.0°C (95% CI) in the warmer season and 31.4±2.4°C in the months of highest rainfall. Oyster populations from distinct regions within the basin display varying sizes. Smaller variants, inhabiting a zone influenced by marine conditions (δ18Ow: -0.2±0.5‰ to 0.4±1.0‰) experienced slower growth rates (~18 µm/day) only during the warmer months. In contrast, larger variants, probably residing in a shallower environment prone to larger impact of evaporation and fluvial influence (δ18Ow: -1.0±0.8‰ to 1±0.7‰), maintained a steady growth rate (~58 µm/day) throughout the year. Although the observed low seasonality is similar to modern tropical settings of the Indian and Pacific Oceans, the significantly warmer temperature range necessitated growth adaptations in oyster populations, as nutrient availability in diverse habitats were critical to their growth.