Oxygen isotopic proxies of paleo-sea surface temperatures (SSTs) suggest that Maastrichtian (about 66 million years ago) tropical SSTs were lower than those of today, They also demonstrate that Maastrichtian latitudinal SST gradients were much lower than those of the present. The low Maastrichtian SST gradients indicate that meridional heat transport was much greater or latitudinal differences in the balance of radiation to and from the sea surface were much less extreme during the latest Cretaceous than they are today, or that both conditions were true, These findings challenge traditional interpretations of "greenhouse" Late Cretaceous climates.