Abstract
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 (δ 7 Limt) 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 δ 7 Limt values for seawater vertebrates than freshwater ones, taxonomic groups considered separately. Moreover, vertebrates living in transitional environments have intermediate δ 7 Limt values. Therefore, we show that δ 7 Limt values of both extant and extinct vertebrates can discriminate their aquatic habitat.