ABSTRACT

Two transgressions of the Cretaceous interior sea, the Claggett and Bearpaw, greatly reduced the quantity of terrestrial habitats. Dinosaur lineages of the terrestrial Two Medicine and Judith River Formations of central Montana show evolutionary changes associated with at least the more extensive Bearpaw transgression. A lacustrine varve-like sequence assodated with a pulmonate-dominated gastropod fauna is reported from the uppermost Two Medidne Formation and together with abundant caliche horizons suggests sporadic or possibly seasonal precipitation. This together with a review of paleodimatic data and models as well as comparisons with geographically similar modern localities indicate that the more extensive Bearpaw transgression had a climatic impact which also qualitatively affected terrestrial ecosystems. This elimination of a wetter coastal region would have been an evolutionary barrier mainly for lower coastal plain dinosaurs.