Imagine a terrifying prehistoric sea creature, one that not only ruled the oceans but also ventured into freshwater environments. Recent studies suggest that mosasaurs, the top predators of marine ecosystems during the age of dinosaurs, didn’t limit themselves to saltwater; they were capable of living in rivers as well. This revelation comes from a collaborative research effort by scientists in Sweden, the United States, and the Netherlands, who examined isotopic compositions in several mosasaur teeth discovered across North Dakota.
An artist's rendition of a freshwater mosasaur illustrates how these formidable creatures might have appeared. The findings indicate that some of these ancient reptiles could reach lengths of approximately 11 meters (36 feet), making them a fearsome presence not just in oceanic depths, but also in river systems.
This revelation adds a chilling twist to the idea of dinosaurs gathering at watering holes. Imagine the scene: thirsty dinosaurs, already on the lookout for terrestrial predators, now had to be wary of gigantic, bus-sized mosasaurs lurking beneath the surface, ready to strike. "The size indicates that this creature would compete with the largest killer whales, posing an extraordinary threat in riverine habitats previously thought to be safe from such massive marine reptiles," notes Per Ahlberg, a vertebrate paleontologist at Uppsala University in Sweden.
Mosasaurs were carnivorous aquatic reptiles prominent in the late Cretaceous period. While certain species remained small, most were colossal, enabling them to dominate the oceanic landscape for millions of years. Thus, it raised eyebrows when paleontologists unearthed a mosasaur tooth alongside a Tyrannosaurus rex tooth and a crocodilian jawbone in an inland floodplain back in 2022. The question arose: Did this mosasaur inhabit the freshwater environment, or had it been washed in from the sea?
To unravel this mystery, researchers embarked on an analysis of the tooth's enamel, utilizing isotope ratios found in other fossil specimens, including teeth from sharks and ammonites. Isotopes are variants of elements distinguished by differing numbers of neutrons within their atoms. By examining these ratios, scientists can glean insights into an animal's dietary habits and habitat preferences. In this study, the focus was on the isotopes of oxygen, strontium, and carbon.
Oxygen isotopes, in particular, are instrumental in distinguishing between saltwater and freshwater habitats. The lighter isotope, 16O, tends to evaporate from the ocean and subsequently fall as precipitation, resulting in freshwater bodies containing more 16O and significantly less of its heavier counterpart, 18O. The analysis revealed that the oxygen and strontium isotopic signatures in the mosasaur tooth confirmed its adaptation to life in a freshwater environment.
"Upon examining two additional mosasaur teeth from nearby, slightly older sites in North Dakota, we found similar signatures indicating a freshwater lifestyle," says Melanie During, another vertebrate paleontologist at Uppsala University. "These findings demonstrate that mosasaurs thrived in riverine settings during the final million years leading up to their extinction."
The carbon isotope ratio further supported this narrative, adding an unsettling detail: this river-dwelling creature may have consumed dinosaurs as part of its diet. "Typically, carbon isotopes in teeth reflect an animal's diet," explains During. "Many mosasaurs show low values of 13C due to deep diving, but the tooth found alongside the T. rex tooth exhibited a higher 13C value than any known mosasaurs, dinosaurs, or crocodiles, suggesting it might not have dived deeply and could have occasionally fed on drowned dinosaurs."
Researchers propose that the ability of mosasaurs to transition from saltwater to freshwater habitats may represent a late evolutionary adaptation that occurred in the last million years before their extinction event, which coincided with the demise of the dinosaurs. This intriguing study has been documented in the journal BMC Zoology, shedding light on the complex lifestyles of these ancient reptiles.
