Most of the major extinction events of the Phanerozoic are associated with climate change, in particular global warming. The fossil record thus provides a series of 'natural experiments' that enable us to examine the responses of marine ecosystems to global warming at a range of geographic, temporal and taxonomic scales that are far beyond the scope of modern, small-scale experiments. A key challenge is deciphering this record. Focusing on the critical Early Mesozoic interval, two questions will be addressed: how was marine ecosystem functioning affected by past global warming? and, which environmental factors were the most important in driving ecological change?