The ichnogenus Teredolites and the Teredolites Ichnofacies is an accepted proxy for marine influence in paralic to open marine depositional environments. Actualistic approaches and the process-ichnologic framework allow independent analysis of both quantitative and semiquantitative data to interpret the physicochemical conditions when Teredolites are present. Measurements collected in modern environments and Jurassic–Eocene successions produce a spatially and temporally robust dataset of 14,137 borings from 17 tracemaking genera. Life history strategies of extant tracemakers are reflected in Mesozoic borings, which allow reconstruction of paleosalinity. Trends extrapolated indicate four ecotones in modern paralic to nearshore settings, which can be identified into the Late Cretaceous. These zones display variation in boring metrics (length [L], width [W], L:W ratio, size diversity index), densities, and relative composition of Teredolites. Zone 1 represents upper estuary to tidally modified fluvial successions with salinities from 0.5–10 ppt (oligohaline to alpha-mesohaline). Zone 2 represents upper to central estuary successions with salinities from 10–19 ppt (beta-mesohaline to lower polyhaline). Zone 3 represents central to lower estuary successions with salinities from 15–30 ppt (beta-mesohaline to euhaline). Zone 4 represents backshore to open marine successions with salinities from 20–30+ ppt (polyhaline to euhaline). Boring metrics compared against identified fossil genera suggest high species richness in the Western Interior Seaway coincided with physical adaptations to compensate for interspecific competition—differences in reproduction style and media preferences at the time of settlement—in the Late Cretaceous.