The mineralogy of evaporite and other precipitated minerals, together with geochemical studies, has provided traditional sources of information about the major dissolved ion composition (solutes) of ancient lakes. The paleocompositional resolving power of these methods is generally greatest in high-salinity lakes that precipitate numerous solute-sensitive evaporite minerals as opposed to dilute saline lakes that precipitate only a few minerals. Ostracodes live in dilute saline lakes where a species occurrence is determined by the relative proportions of the lake's major dissolved anions, so that each species describes specific areas on an anion trilinear diagram. Moreover, the upper salinity tolerance of each species appears to depend upon the types of major anions in solution and is therefore anion-specific. Knowledge about both anion and anion-salinity tolerances of an ostracode may ultimately provide a means of estimating absolute anion concentrations in paleolakes. Because ostracodes are common fossils in lake sediments, they provide an important new source of original paleocompositional information suitable for many geologic, climatic, geochemical, and paleontologic studies.