'Ghost' fossils of early coccolithophores point to a Triassic diversification of marine calcifying organisms.

Over geologic time, biocalcification - the process by which marine organisms make calcium carbonate (CaCO3) - has reshaped climates, ocean life, and seawater chemistry. In particular, the evolution of coccolithophores, the largest group of nannoplankton and today's most productive calcifiers, transformed ocean environments and the carbon cycle. Their origins, however, remain enigmatic. This is partly because studying coccolithophore fossils traditionally requires CaCO3 preservation. Here, we bypass this limitation, searching for their 'ghost' fossils -imprints on organic matter. We present coccolithophores from ~241-million-year-old (Triassic) rocks, predating previous records by ~26 million years (myrs). The >100 ghost fossils, exceptionally preserved within zooplankton faeces, show that coccolithophores, nannoplankton, 'modern' eukaryotic phytoplankton, and planktonic biocalcification evolved earlier than previously thought. Coccolithophores now first appear alongside stony corals and other unrelated calcifiers, suggesting a diversification of a range of marine calcifying organisms following Earth's deadliest mass extinction, the end-Permian event. These findings indicate that coccolithophore diversity remained remarkably low for ~50 myrs, until after the end-Triassic mass extinction, showing that both Triassic-bookending extinctions were critical in their evolution. Our discoveries elucidate the evolutionary origins of coccolithophores, but also highlight the role mass extinctions have played in shaping life on Earth.