Y. Kobayashi et al. / Scientific Reports
Carcass of Kamuysaurus, floating in the sea, with two mosasaurs (Mosasaurus hobetsuensis), two sea turtles (Mesodermochelys undulates) and four ammonoids (Pachydiscus japonicus).
A new study indicates that these large marine predators were also capable of living in freshwater rivers.
Os mosassaurosthe colossal marine reptiles that dominated the seas during the age of dinosaurs, were not confined to the oceans after all.
New research published in BMC Zoology suggests that some of these fearsome predators have also ventured out in freshwater riverswhere even dinosaurs may have been its victims.
An international team of researchers from Sweden, the United States and the Netherlands analyzed chemical signatures preserved in fossilized mosasaur teeth discovered in North Dakota. Their findings confirm that at least some mosasaurs were able to thrive in freshwater environments during the last million years before its extinction.
The study was prompted by an intriguing discovery in 2022: a large mosasaur tooth found in an alluvial plainalongside fossils of a Tyrannosaurus rex and a crocodilian. Until then, scientists debated whether the tooth had been taken inland by the sea or whether its owner actually lived in the river system, says .
To unravel the mystery, researchers carried out a isotope analysis of tooth enamelexamining the proportions of oxygen, strontium and carbon isotopes. These chemical markers can reveal where an animal lived and what it ate. Oxygen isotopes, in particular, are effective in distinguishing saltwater habitats from freshwater habitats, since freshwater contains more of the lighter oxygen isotope due to evaporation and rain cycles.
“The isotopic signatures clearly show that This mosasaur lived in fresh water” said vertebrate paleontologist Per Ahlberg of Uppsala University. Analysis of two additional mosasaur teeth from nearby sites showed similar results, reinforcing the conclusion that these giant reptiles inhabited riverine environments.
The size and shape of the tooth indicate that it belonged to a mosasaur that may have reached up to 11 meters in length. “This would make it an extraordinary predator to find in rivers, something not previously associated with giant marine reptiles like these,” Ahlberg noted.
The carbon isotope data added even more frightening information. Unlike mosasaurs that dived to great depths and generally have low carbon-13 values, this specimen had exceptionally high levels, suggesting that it was fed near the surface. According to researcher Melanie During, this raises the possibility that the animal preyed on dinosaurs that drowned when crossing rivers or gathering in waterholes.
The team believes this freshwater lifestyle may have been a late adaptation, as mosasaurs faced environmental changes in the final chapter of the Cretaceous period. Although this change did not save them from extinction, it drastically reshapes scientists’ understanding of prehistoric ecosystems.
