One of the challenges in conservation biology is discovering ‘what was natural’ before human impacts. This problem is especially pressing in marine systems, where biological monitoring and other records are brief or lacking. Susan Kidwell has been tackling this problem in Southern California marine ecosystems by treating the shells acquired during marine surveys as a young fossil record to reveal the dramatic changes in species composition and abundance that have occurred across much of the region. This reconstructed history of the last few thousand years highlights the transformation of seafloor communities in response to approximately 300 years of shifting land use in the Los Angeles watershed, providing a tool that can help set priorities for restoration. Kidwell, a sedimentary geologist and paleoecologist, is the William Rainey Harper Professor in Geophysical Sciences at the University of Chicago.