How did terrestrial dinosaurs, river-dwelling crocodylians, and gigantic marine reptiles end up in the same fluvial deposit? The answer came from the isotope laboratories at the Ä¢¹½ÊÓƵ, of among others Earth scientists Melanie During, Jeroen van der Lubbe and where independent analysis of the tooth enamel revealed isotopic values so distinct from typical marine signatures that it’s conclusive: this mosasaur spent significant time in freshwater environments. The tooth recorded freshwater conditions across three isotope systems, which would only be possible from freshwater habitation, as seen in this .
The findings are published in . VU Amsterdam worked together with Nathan van Vranken (Eastern West Virginia Community and Technical College, USA), Clint Boyd (Uppsala University, Sweden) and Per Ahlberg (North Dakota Geological Survey, USA).
Not just a visitor
The isotope signatures indicate that this mosasaur was not simply a lost vagrant or incidental visitor to freshwater. Analysis of two additional mosasaur teeth from nearby North Dakota localities revealed similar freshwater signatures. While each tooth was sampled only once to preserve its integrity for exhibition, the consistency of freshwater indicators across three mosasaur teeth of different ages strongly suggests these animals weren’t seasonal visitors. Instead, the evidence points to a gradual shift in the shallow sea they inhabited, with salinity decreasing over time, leading to more freshwater conditions.
Large Needles in the Haystack
Mosasaur fossils are abundant across various North American, European, and African marine deposits dating from 98 to 66 million years ago. They are, however, only rarely found in North Dakota, which makes the new discovery particularly noteworthy. ''The size of the tooth suggests a truly impressive creature; one that could reach up to 11 meters in length. This estimate is corroborated by a handful of mosasaur bones found earlier in North Dakota. At this size, the animal would rival the largest killer whales, making it an extraordinary predator to encounter in riverine environments, which were thus far not typically associated with such giant marine reptiles'', During explains.
Built for the Hunt
The tooth belongs to a prognathodontid mosasaur, though it cannot be confidently identified down to genus level. Prognathodontids featured bulky heads with sturdy jaws and teeth and are generally seen as opportunistic predators that would have posed a significant threat to other large aquatic animals. These robust animals were, however, typically slightly to much smaller than Mosasaurus; the genus that was originally discovered near the Dutch city of Maastricht and likely inspired the colossal, exaggerated, crocodile-like appearance, featured in the Jurassic Park franchise. Prognathodontids were equipped to subdue swimming reptiles and fishes as well as hard-shelled prey. Their dietary preferences overlap with those of Mosasaurus, although the latter is considered an absolute top predator capable of predating on the largest sea-living animals of its time.
When Seas Became Rivers
The discovery illuminates a remarkable chapter in Earth's history. Freshwater influx into the Western Interior Seaway, which once divided present-day North America, increased over time. This gradually transformed the seaway from saltwater to brackish and eventually to largely freshwater. A halocline, where freshwater sat atop denser seawater, likely formed along the way.
Van der Lubbe: ''The mosasaur teeth analysed in this study clearly come from individuals that adapted to these changing environments. Such a transition is not unprecedented among large predators. Unlike the complex adaptations required for moving from freshwater to marine habitats, the reverse journey is generally easier. Modern examples include river dolphins, which thrive in freshwater rivers despite being descended from marine ancestors. Or the Saltwater Crocodile, who move freely between freshwater rivers and the open sea.'' The shift to freshwater could have been driven by ecological pressures such as the availability of new resources or changing habitat conditions. The evidence now shows these apex predators weren't just the ocean's top predators: they expanded their reign into freshwater habitats as well!
The Power of Isotopic Analysis
Mosasaurs shed thousands of teeth throughout their lifetimes, and these teeth serve as reliable indicators of their presence in ancient ecosystems. However, pinpointing where these mosasaurs lived requires the precision instruments and isotope expertise available at the Amsterdam laboratories. The VU Earth Sciences department laboratory’s capabilities have allowed researchers to explain an interesting fossil find as conclusive evidence of mosasaurs inhabiting river environments in the final million years before their extinction, 66 million years ago.