Mosasaurs, the fearsome rulers of ancient oceans, might have also stalked rivers. Recent research from scientists in Sweden, the U.S., and the Netherlands shows that these massive predators were not limited to saltwater. They could thrive in freshwater, too.
By studying isotopes in fossilized mosasaur teeth found in North Dakota, researchers discovered evidence that these reptiles could adapt to riverine environments. One tooth came from a mosasaur that might have grown to about 36 feet long. Imagine the terror of dinosaurs gathering at waterholes, not only wary of land predators but also these colossal creatures lurking below.
Per Ahlberg, a vertebrate paleontologist at Uppsala University, notes that encountering such a giant in a river was extraordinary, as it made the possibilities of encounters with apex predators much broader than previously thought. This adds a thrilling layer to our understanding of prehistoric ecosystems.
In 2022, paleontologists first uncovered this tooth alongside a Tyrannosaurus rex tooth and crocodile jawbone in an inland floodplain. Researchers needed to determine if the tooth’s owner lived there or if it had been swept in from the ocean. To solve this, they looked at isotopes in the tooth’s enamel, comparing it with other fossils to trace the mosasaur’s environment and diet.
Oxygen isotopes offer insight into habitats—light oxygen is more common in freshwater. The isotopic analysis showed that the mosasaur tooth contained signatures of freshwater living. “We found that other mosasaur teeth from nearby sites revealed similar freshwater patterns,” Melanie During, another paleontologist from Uppsala, explained. This indicates that mosasaurs were indeed living in rivers at the end of their reign.
Even more chilling, carbon isotopes in the tooth suggested that this mosasaur might have preyed on dinosaurs. Typically, many mosasaurs display low carbon values because they dove deep. However, this particular tooth had a higher value, suggesting it stayed near the surface and could have consumed fallen dinosaurs.
Experts think that this shift from saltwater to freshwater was a crucial adaptation for mosasaurs, likely occurring just before their extinction alongside the dinosaurs. The study highlights how adaptable and formidable these creatures were, showing that ancient life was even more complex than we once thought.
This research sheds light on an exciting aspect of our planet’s history. It was published in the journal BMC Zoology.
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