Periods of high atmospheric CO 2 levels during the Cretaceous-early Paleogene (∼ 140 to 34 Myr ago) were marked by very high polar temperatures and reduced latitudinal gradients relative to the Holocene. These features represent a challenge for most climate models, implying either higher-than-predicted climate sensitivity to atmospheric CO 2 or systematic biases or misinterpretations in proxy data. Here, we present a reconstruction of marine temperatures at polar (> 80 •) and middle (∼ 40 •) paleolatitudes during the Early Jurassic (∼ 180 Myr ago) based on the clumped isotope (47) and oxygen isotope (δ 18 O c) analyses of shallow buried pristine mollusc shells. Reconstructed calcification temperatures range from ∼ 8 to ∼ 18 • C in the Toarcian Arctic and from ∼ 24 to ∼ 28 • C in Pliensbachian midpaleolatitudes. These polar temperatures were ∼ 10-20 • C higher than present along with reduced latitudinal gradients. Reconstructed seawater oxygen isotope values (δ 18 O w) of −1.5 ‰ to 0.5 ‰ VSMOW and of −5 ‰ to −2.5 ‰ VS-MOW at middle and polar paleolatitudes, respectively, point to a significant freshwater contribution in Arctic regions. These data highlight the risk of assuming the same δ 18 O sw value for δ 18 O-derived temperature from different oceanic regions. These findings provide critical new constraints for model simulations of Jurassic temperatures and δ 18 O sw values and suggest that high climate sensitivity has been a hallmark of greenhouse climates for at least 180 Myr.