Photos and Map The Paleocene-Eocene Thermal Maximum, or PETM, was a short interval of highly elevated global temperatures 56 million years ago that is frequently described as the best ancient analog for present-day climate warming. Fish are among the organisms thought to be most sensitive to warming climates, and tropical sea-surface temperatures during the PETM likely approached temperatures that are lethal to some modern marine fish species, according to some estimates. But newly discovered fish fossils from an eastern Egyptian desert site show that marine fishes thrived in at least some tropical areas during the PETM. The study, from a team of Egyptian scientists and a University of Michigan colleague, provides a snapshot of an ecosystem during an extreme warming event and may provide insights for the future.