Overview
- Peer‑reviewed research published December 12 in BMC Zoology reports oxygen, strontium, and carbon isotope signatures in a Hell Creek mosasaur tooth that match freshwater conditions.
- Two slightly older mosasaur teeth from nearby North Dakota sites show the same freshwater chemistry, indicating this behavior was not an isolated occurrence.
- Comparisons with coeval fossils show gill‑breathing animals retained brackish or marine signals while lung‑breathers, including mosasaurs, tracked freshwater, implying use of the surface layer.
- Authors propose that increasing freshwater input created a stratified Western Interior Seaway with a freshwater cap, allowing mosasaurs to occupy rivers during their final million years.
- The Hell Creek tooth, attributed to a prognathodontine, indicates an individual up to about 11 meters long and carbon values consistent with shallow foraging that may have included drowned dinosaurs.