The Hell Creek Mosasaur, reconstruction
by Christopher DiPiazza. Credit: Christopher DiPiazza
Mosasaurs, giant marine reptiles that existed more than 66
million years ago, lived not only in the sea but also in rivers. This is shown
by new research based on analyses of a mosasaur tooth found in North Dakota and
believed to belong to an animal that could reach a length of 11 meters.
Conducted
by an international team of researchers led from Uppsala University, the study
shows that mosasaurs adapted to riverine environments in the final million
years before they became extinct.
The study is published in
the journal BMC Zoology.
In 2022, paleontologists found a large
tooth from a mosasaur in North Dakota. It was discovered in a fluvial deposit,
together with a tooth from a Tyrannosaurus rex, and a crocodylian jawbone in an
area known for remains of the duck-billed dinosaur Edmontosaurus.
The fact that land-dwelling dinosaurs,
river-dwelling crocodylians and giant marine reptiles were found together
raised the question: how did a mosasaur tooth end up in a river, when this
reptile was assumed to live in the sea?
An international team of researchers from the United States, Sweden and the Netherlands has now answered the question using isotope analyses of the mosasaur's tooth enamel.
Credit: Uppsala University
Isotopes show how they lived and what they ate
Since the mosasaur tooth, the T.
rex tooth and the crocodylian jawbone are of similar age, around 66 million
years old, the researchers were able to compare their chemical composition by
means of isotope analyses.
The analysis, carried out at the
Vrije Universiteit (VU) in Amsterdam, studied the ratio between different
isotopes of the elements oxygen, strontium and carbon. The mosasaur teeth
contained more of the lighter oxygen isotope (¹⁶O) than is usually seen in
marine mosasaurs, indicating that they lived in freshwater. The ratio between
different strontium isotopes also suggests a freshwater habitat.
"Carbon isotopes in teeth
generally reflect what the animal ate. Many mosasaurs have low ¹³C values
because they dive deep. The mosasaur tooth found with the T. rex tooth, on the
other hand, has a higher ¹³C value than all known mosasaurs, dinosaurs and
crocodiles, suggesting that it did not dive deep and may sometimes have fed on
drowned dinosaurs," says Melanie During, one of the study's corresponding
authors.
"The isotope signatures indicated that this mosasaur had inhabited this freshwater riverine environment. When we looked at two additional mosasaur teeth found at nearby, slightly older, sites in North Dakota, we saw similar freshwater signatures. These analyses show that mosasaurs lived in riverine environments in the final million years before going extinct," says During.
The mosasaur tooth that was found in
2022 in the Bismarck Area, North Dakota. Credit: Melanie During
When seas became rivers
The discovery sheds light on an
interesting chapter in Earth's history. The influx of freshwater into the
Western Interior Seaway, an inland sea that then stretched from north to south
across today's prairies and divided North America in two, increased over time.
This gradually transformed the sea water from saltwater to brackish water and
eventually to mostly freshwater, rather like in the Gulf of Bothnia.
The authors believe that this led
to the formation of a "halocline," in which a layer of freshwater sat
on top of heavier saltwater. This theory is borne out by isotope analyses.
"For comparison with the
mosasaur teeth, we also measured fossils from other marine animals and found a
clear difference. All gill-breathing animals had isotope signatures linking
them to brackish or salty water, while all lung-breathing animals lacked such
signatures. This shows that mosasaurs, which needed to come to the surface to
breathe, inhabited the upper freshwater layer and not the lower layer where the
water was more saline," says Per Ahlberg, co-author of the study and
promotor of Dr. During.
Adapted to new living conditions
The researchers argue that the
mosasaur teeth analyzed clearly came from individuals that were adapted to
these changing environments. Such a transition is not an unknown phenomenon
among large predators.
"Unlike the complex adaptation
required to move from freshwater to marine habitats, the reverse adaptation is
generally simpler," says During.
Modern examples of such adaptation
can be seen in river dolphins, which live in freshwater rivers despite being
descended from marine ancestors.
The estuarine crocodile, known in
Australia as the saltwater crocodile, is another example. It moves freely
between freshwater rivers and the open sea, hunting in both environments
depending on what prey is available.
Could be as big as a bus
Mosasaur fossils are abundant in
North American, European and African marine deposits that are 98–66 million
years old. However, they are only rarely found in North Dakota, which makes the
new discovery particularly noteworthy. The size of the tooth testifies to an
impressive creature that could grow up to 11 meters long, about the size of a
bus.
This estimate is corroborated by a
handful of mosasaur bones found earlier at a nearby site in North Dakota. The
tooth comes from a prognathodontine mosasaur, though the genus cannot be
determined with certainty.
Mosasaurs of the genus
Prognathodon, closely related to the animal that lost this tooth, had bulky
heads with sturdy jaws and teeth. They are widely regarded as opportunistic
predators that posed a significant threat to other large aquatic animals.
"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," says Ahlberg.
The study was conducted by
researchers from Uppsala University in collaboration with Eastern West Virginia
Community and Technical College, Moorefield, West Virginia, Vrije Universiteit
Amsterdam and the North Dakota Geological Survey.
The article is based on a chapter
of Melanie During's thesis, which she presented at Uppsala University in
November 2024.
Provided by Uppsala
University
Source: Sea reptile's tooth shows that mosasaurs could live in freshwater
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