A 66-million-year-old mosasaur tooth unearthed at Hell Creek is rewriting how we think about these giant predators. The fossil, found at one of the world’s most famous Late Cretaceous sites, suggests mosasaurs didn’t simply vanish into saltwater in their final chapters; some may have thrived in freshwater environments without losing their impressive size.
The full article is behind a paywall, so sign in or subscribe to read the entire piece.
In 2022, paleontologist Trissa Ford was digging in Hell Creek, North Dakota, when she spotted a Tyrannosaurus rex tooth. While exciting, the tooth was in poor condition, prompting field leader Dr. Clint Boyd of the North Dakota Geological Survey to carefully extract surrounding material. To everyone’s surprise, another tooth tumbled out.
Initially, the team suspected a T. rex dentition, but marine-reptile expert Dr. Nathan Van Vranken of Eastern West Virginia Community and Technical College identified the second tooth as coming from a mosasaur of the Prognathodontini subfamily. This finding challenges the popular image from films like Jurassic World, which often depict dramatic clashes between mosasaurs and tyrannosaurs—though the reality may be more nuanced.
The puzzle: Hell Creek lies roughly 1,200 miles from the nearest ocean, and the Cretaceous period was ending. How did a marine reptile tooth end up so far inland? To solve the mystery, the team enlisted Dr. Melanie During from Uppsala University, who has previously contributed insights into the asteroid’s timing and the dinosaurs’ demise. The researchers proposed that the tooth wasn’t moved by currents, and that Hell Creek wasn’t hosting a colossal salt lake. Instead, the mosasaur likely descended from river-adapted ancestors and occasionally preyed on unsuspecting dinosaurs.
During explains that North America’s Western Interior Seaway once separated eastern and western landmasses, with connections to both the Arctic Ocean and the Gulf of Mexico. Mosasaurs thrived in those waters, but the end of the dinosaur era brought dramatic geological changes. Mountain-building activity and a highly active San Andreas-type fault caused the continent to rise, gradually cutting off the Seaway from the Gulf and the Arctic. Rains then flushed salt from the region, transforming Hell Creek into a network of rivers interspersed with land.
Remarkably, mosasaurs appear to have endured these environmental upheavals and adapted to freshwater conditions. During notes that becoming a saltwater species is a drastic adaptation—requiring the organism to manage salt balance carefully—whereas returning to fresh water is comparatively easier. She points to examples like whales and seals that venture far upriver without apparent harm.
The discovery of mosasaur fossils in what is now Hungary showed that these reptiles could adapt to riverine habitats, though the Hungarian specimens were from a different, smaller genus (about 5 meters long). The Hell Creek tooth, by contrast, likely belonged to an 11-meter-long individual—“bigger than a killer whale,” During says, placing it well beyond the size of modern crocodiles or great white sharks.
Could freshwater adaptation extend beyond size? During compares the freshwater capability to Australia’s giant saltwater crocodiles, noting that some signs in the Hungarian mosasaur suggest partial land mobility. However, with only a tooth as evidence, it isn’t possible to confirm whether North American relatives followed the same terrestrial path.
When During first presented the idea of a giant freshwater mosasaur at conferences, she faced skepticism. The theory gained traction as she introduced additional lines of evidence—precise tooth location, oxygen and strontium isotope ratios in the enamel, and clues from earlier Western Inland Seaway mosasaurs living in brackish or partially freshwater conditions. She proposes that the Seaway once featured a layer of freshwater above saltier depths.
Marine mosasaurs are generally thought to feed on fish and turtles, but the tooth’s carbon isotope ratios are exceptional: they deviate significantly from typical mosasaur signatures and resemble dietary inputs more common in dinosaurs. This leads During to suggest that hadrosaurs, for example, could have formed part of this mosasaur’s diet.
If mosasaurs could thrive in freshwater, might legendary sea monsters like Nessie actually be mosasaurs rather than plesiosaurs? During responds that the possibility is open: Nessie could plausibly be either a mosasaur or a plesiosaur, and the idea invites further discussion.
The study detailing these findings was published in BMC Zoology.
