A 66-million-year-old tooth indicates that this ocean predator could also hunt in rivers.
A 66-million-year-old tooth discovered in an ancient riverine area in the United States suggests that some mosasaurs, giant marine reptiles from the age of dinosaurs, may have lived and hunted in freshwater environments in the last moments before extinction.
The tooth in question was discovered in 2022 in the Hell Creek Formation, in North Dakota (USA), in a riverine area that was once connected to an ancient sea known as the Western Interior Sea.
The mosasaur tooth discovered in Hell Creek.
Trissa Shaw.
This belongs to a mosassauro Prognathodontinia group of extinct lizard-like reptiles that could reach 12 meters in length, based on similarities between the textured patterns on their surface and the teeth of other members of this group.
One and led by Uppsala University (Sweden) shows that Mosasaurs adapted to riverine environments in the last million years before their extinctionthe Efe agency reported on Friday.
Mosasaurus fed on freshwater animals
The tooth was discovered in a river deposit, along with a Tyrannosaurus rex tooth and a crocodile jawin an area known for the remains of the duck-billed dinosaur Edmontosaurus.
This led researchers to wonder how a mosasaur tooth ended up in a river, given that this reptile was believed to live in the sea.
The tooth showed no signs of transportation, suggesting that it lived and died in Hell Creek, a region where no mosasaur teeth from the same period have been found.
The authors analyzed the isotopes in the tooth enamel to deduce the conditions in which it lived and found oxygen and strontium isotopic signatures associated with freshwater environments.
The team believes this may be because the mosasaur fed on freshwater animals, indicating that it was capable of living and hunting far from the sea.
Additional analysis of teeth from older mosasaurs and other animals from the Western Interior Sea revealed isotopic concentrations more consistent with a freshwater habitat than a saltwater one.
The authors propose that Prognathodontini may have been opportunistic predators adapted to a freshwater environment.
An enlarged image of the Brachychampsa teeth collected in this study.
Melanie During.
This discovery sheds light on a chapter in Earth’s history: the influx of freshwater into the Western Interior Sea, an inland sea that once divided North America from north to south, increased over time and gradually transformed saltwater into brackish water and then into freshwater.
The authors believe this led to the formation of a halocline, in which a layer of freshwater overlaid the denser layer of saltwater – a theory supported by isotopic analyses.
The analysis of several marine fossils revealed a fundamental difference, that animals that breathed through gills had isotopic signatures that linked them to brackish or salt water, while those that breathed through lungs did not show such signatures.
This demonstrates that the mosasaurs, which needed to come to the surface to breathe, inhabited the upper layer of fresh water, and not the lower, more saline layer, explained Per Ahlberg, co-author of the study, cited by Uppsala University.
