ScienceDaily has a story about mosasaurs. It seems that giant mosasaurs didn’t just live in the oceans… they also prowled rivers. A large tooth found in a North Dakota deposit along with a Tyrannosaurus rex tooth and a jawbone from a crocodylian revealed an isotope signature consistent with a live in fresh water. See the paper “King of the Riverside” in the journal BMC Zoology.

The tooth was uncovered in 2022 from a river deposit in North Dakota. It was found alongside a tooth from a Tyrannosaurus rex and a jawbone from a crocodylian, in a region already known for fossils of the duck-billed dinosaur Edmontosaurus. The unusual mix of land dinosaurs, river-dwelling crocodiles, and a giant marine reptile immediately stood out. If mosasaurs were ocean animals, how did one of their teeth end up preserved in a river?

To solve this puzzle, researchers from the United States, Sweden, and the Netherlands examined the chemical makeup of the mosasaur tooth enamel using isotope analysis.

Because the mosasaur tooth, the T. rex tooth, and the crocodylian jawbone all date to roughly the same time, about 66 million years ago, the scientists could directly compare their chemistry. The work was carried out at the Vrije Universiteit (VU) in Amsterdam and focused on isotopes of oxygen, strontium, and carbon. The mosasaur tooth contained unusually high levels of the lighter oxygen isotope (¹⁶O), which is typical of freshwater environments rather than marine ones. Strontium isotope ratios also pointed to a freshwater habitat.