The ecology of modern and fossil vertebrates revisited by lithium isotopes

•δ7Li values in modern vertebrates' bones/teeth reflect their aquatic environments.•δ7Li values are also valuable for tracking the feeding habits of mammals.•δ7Li values in fossil mineralized tissues are consistent with modern values. The vertebrate fossil record documents a plethora of transitions between aquatic and terrestrial environments but their causes are still debated. Quantifying the salinity of living environments is therefore crucial for precising the sequence of ecological transitions. Here, we measured lithium stable isotope composition of mineralized tissues (δ7Limt) of extant and extinct vertebrates from various aquatic environments: seawater, freshwater/terrestrial, and “transitional environments” (i.e. brackish waters, or seasonal access to freshwater and seawater). We report statistically higher δ7Limt values for seawater vertebrates than freshwater ones, taxonomic groups considered separately. Moreover, vertebrates living in transitional environments have intermediate δ7Limt values. Therefore, we show that δ7Limt values of both extant and extinct vertebrates can discriminate their aquatic habitat.