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When dinosaurs roamed the land in the Mesozoic (251 to 65 million years ago), the top predators in the ocean were reptiles (1, 2). Three lineages of Mesozoic marine reptiles (plesiosaurs, ichthyosaurs, and mosasaurs) were especially successful (2) (see the first figure). They were similar to current marine mammals in many respects. They fed on fish, cephalopods, bivalves, and other air-breathing vertebrates (1). Ichthyosaurs evolved dolphin-like body plans. Plesiosaurs became underwater fliers, vaguely resembling sea lions (2, 3). It now appears that similarities to today's marine mammals extended further: On page 1379 of this issue, Bernard et al. (4) report that some ancient reptiles may have been able to sustain a constant body temperature (i.e., homeothermy).