For decades, mosasaurs have occupied a very specific mental real estate. They are the ocean’s final boss, the late-Cretaceous equivalent of a great white on steroids. Massive. Carnivorous. Fully marine. If dinosaurs were unlucky enough to end up in the water, that was their problem.
New research suggests not all of them stuck to the seas.
A study published in BMC Zoology argues that at least some mosasaurs were not strictly ocean dwellers. Toward the end of their reign, these animals appear to have spent real time in freshwater river systems, the same places where large dinosaurs gathered to drink, migrate, and occasionally drown. The idea comes from chemical analysis of mosasaur teeth found far inland in what is now North Dakota, alongside fossils from a Tyrannosaurus rex and a crocodylian.
That combination raised an obvious question. Did the tooth wash in from the sea, or did its owner actually live there?
This Prehistoric Sea Monster Didn’t Stay in the Ocean
To answer it, researchers from Sweden, the US, and the Netherlands analyzed isotopes preserved in the enamel of several mosasaur teeth. Isotopes are versions of elements that differ slightly in atomic makeup, and their ratios can act like environmental fingerprints. Oxygen isotopes are especially useful here because freshwater and seawater carry distinct signatures.
Freshwater environments contain more of the lighter oxygen isotope, oxygen-16, while oceans have relatively more oxygen-18. The mosasaur teeth from inland floodplains showed freshwater ratios, not marine ones.
“When we looked at two additional mosasaur teeth found at nearby, slightly older, sites in North Dakota, we saw similar freshwater signatures,” Melanie During, a vertebrate paleontologist at Uppsala University, told Science Alert. “These analyses show that mosasaurs lived in riverine environments in the final million years before going extinct.”
Carbon isotopes added another unsettling layer. Carbon ratios in teeth reflect diet, and these mosasaur samples stood out. According to During, the inland mosasaur tooth showed “a higher 13C value than all known mosasaurs, dinosaurs, and crocodiles,” which suggests the animal didn’t dive deep and may have fed on drowned dinosaurs.
One tooth found in the floodplain belonged to a group of mosasaurs estimated to reach around 11 meters, or about 36 feet. That puts it in the same size range as the largest killer whales.
“The size means that the animal would rival the largest killer whales, making it an extraordinary predator to encounter in riverine environments not previously associated with such giant marine reptiles,” said Per Ahlberg, a vertebrate palaeontologist at Uppsala University.
The researchers propose that this freshwater flexibility may have been a late adaptation, emerging during the final million years before the mass extinction that wiped out mosasaurs and dinosaurs alike. If so, the last days of the Cretaceous were even less forgiving than previously imagined.
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