The Cretaceous period is generally considered to have been a time of warm climate. Evidence for cooler episodes exists, particularly in the early Cretaceous period, but the timing and significance of these cool episodes are not well constrained. The seasonality of temperatures is important for constraining equator-to-pole temperature gradients and may indicate the presence of polar ice sheets; however, reconstructions of Cretaceous sea surface temperatures are predominantly based on the oxygen isotopic composition of planktonic foraminifera that do not provide information about such intra-annual variations. Here we present intra-shell variations in δ~(18)O values of rudist bivalves (Hippuritoidea) from palaeolatitudes between 8° and 31° N, which record the evolution of the seasonality of Cretaceous sea surface temperatures in detail. We find high maximum temperatures (～35 to 37 ℃) and relatively low seasonal variability (< 12℃) between 20° and 30° N during the warmer Cretaceous episodes. In contrast, during the cooler episodes our data show seasonal sea surface temperature variability of up to 18℃ near 25° N, comparable to the range found today. Such a large seasonal variability is compatible with the existence of polar ice sheets.
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