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Scientists discover ancient river-dwelling mosasaur in North Dakota

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Scientists discover ancient river-dwelling mosasaur in North Dakota


Some 66 million years ago, a city bus-sized terrifying predator prowled a prehistoric river in what is now North Dakota. 

This finding is based on the analysis of a single mosasaur tooth conducted by an international team of researchers from the United States, Sweden, and the Netherlands. 

The tooth came from a prognathodontine mosasaur — a reptile reaching up to 11 meters long. This makes it an apex predator on par with the largest killer whales.

It shows that massive mosasaurs successfully adapted to life in rivers right up until their extinction.

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The mosasaur tooth was found in 2022 in the Bismarck Area, North Dakota. Credit: Melanie During 

Isotope analysis

Dating from 98 to 66 million years ago, abundant mosasaur fossils have been uncovered in marine deposits across North America, Europe, and Africa.

However, these marine reptile fossils have been rarely found in North Dakota before. 

In this new study, the large mosasaur tooth was unearthed in a fluvial deposit (river sediment) in North Dakota. 

Its neighbors in the dirt were just as compelling: a tooth from a Tyrannosaurus rex and a crocodylian jawbone. Interestingly, all these fossilized remains came from a similar age, around 66 million years old. 

This unusual gathering — sea monster, land dinosaur, and river croc — raised an intriguing question: If the mosasaur was a sea creature, how did its remains end up in an inland river?

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The answer lay in the chemistry of the tooth enamel. Using advanced isotope analysis at the Vrije Universiteit in Amsterdam, the team compared the chemical composition of the mosasaur tooth with its neighbors.

The key was the ratio of oxygen isotopes. 

The mosasaur teeth contained a higher proportion of the lighter oxygen isotope than is typical for mosasaurs living in saltwater. This specific isotopic signature, along with the strontium isotope ratio, strongly suggests that the mosasaur lived in a freshwater habitat.

Analysis also revealed that the mosasaur did not dive as deep as many of its marine relatives and may have fed on unusual prey, such as drowned dinosaurs. 

The isotope signatures indicated that this mosasaur had inhabited this freshwater riverine environment. When we looked at two additional mosasaur teeth found 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,” explained Melanie During, the study author.

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Transformation of the Seaway

The adaptation occurred during the final million years of the Cretaceous period.

It is hypothesized that the mosasaurs were adapting to an enormous environmental shift in the Western Interior Seaway, the vast inland sea that once divided North America.

Increased freshwater influx gradually transformed the ancient sea from saltwater to brackish water, and finally to mostly freshwater, similar to the modern Gulf of Bothnia. 

The researchers hypothesize that this change led to the formation of a halocline: a structure where a lighter layer of freshwater rested atop heavier saltwater. The findings of the isotope analyses directly support this theory.

The analyzed mosasaur teeth belong to individuals who successfully adapted to the shifting environments. 

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This transition from marine to freshwater habitats (reverse adaptation) is considered less complex than the opposite shift and is not unique among large predators. 

Modern parallels include river dolphins, which evolved from marine ancestors but now thrive in freshwater, and the estuarine crocodile, which moves freely between freshwater rivers and the open sea for hunting.

Findings were published in the journal BMC Zoology on December 11.



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North Dakota High School Boys Basketball State Champions, Runners-Up, & Third Place

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North Dakota High School Boys Basketball State Champions, Runners-Up, & Third Place


The 2026 North Dakota high school boys basketball season has come to an end, and champions have been crowned across each classification.

After months of competition, the teams listed below rose above the field to capture state titles in their respective divisions.

High School On SI has the completed brackets along with the champions, runner-ups, and third place for every classification.

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Take a look below to see the teams that finished on top and the full path each squad took to reach the state finals.

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Champions: Century Patriots

Runner-Ups: Bismarck Demons

Third Place: Dickinson Midgets

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Champions: Beulah Miners

Runner-Ups: Central Cass Squirrels

Third Place: Devils Lake Firebirds


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Champions: Central McLean Cougars

Runner-Ups: Our Redeemer’s Christian Knights

Third Place: Ray Jays


More Coverage From High School On SI



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New York Giants met with WR RaJa Nelson at North Dakota State Pro Day

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New York Giants met with WR RaJa Nelson at North Dakota State Pro Day


The New York Giants have their eyes on North Dakota State wide receiver RaJa Nelson this offseason.

KPRC 2 Houston’s Aaron Wilson reports that the Giants spoke with Nelson at NDSU’s Pro Day on March 19.

Nelson recorded 404 receiving yards in 2025, along with four touchdowns. His 14.4 yards per reception marked a career high. The Minnesota native also contributed in the return game, totaling 168 kick return yards on eight attempts last season.

The Giants lost slot receiver Wan’Dale Robinson to the Tennessee Titans in free agency. Robinson was New York’s leading receiver in 2025, finishing with 1,014 yards. The Kentucky native’s departure leaves the Giants without one of their two 1,000-yard wide receivers heading into next season.

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Nelson is a prospect with whom the Giants have done their due diligence this offseason. New York has just under five weeks remaining before the draft begins on April 23.



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Special Olympics of North Dakota hosts ‘Polar Plunge’

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Special Olympics of North Dakota hosts ‘Polar Plunge’


GRAND FORKS – Taya Lulay, 6, of Michigan, North Dakota, looked a bit apprehensive but eager as she waited with her team to participate in the annual “Polar Plunge” on Saturday, March 21, at Choice Health and Fitness.

Taya was among about 40 courageous souls who braved the 39-degree temps, chilly winds and a tank of frigid water to raise funds for the Special Olympics of North Dakota organization.

“She’s the reason we’re doing this,” said Erick Houde, of Grand Forks.

The daughter of Brandon and Amy Lulay attended the event last year and apparently convinced five others to join her in taking the plunge, all for the first time.

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Members of “The ReNewed Penguins,” ranging in age from 6 to 53, have raised about $700 for the Special Olympics of North Dakota.

Erick Houde, Brandon Lulay and Luke Syrup are employees of PCL Community Contractors.

“We do a lot of volunteering” as employees, Houde said, “and we like to keep on doing it. We decided to do it as our own team this year.”

“And it’s fun,” said Joan Houde, Erick Houde’s wife, who confessed to being “a little bit” nervous before the plunge. “It’s just the cold.”

“We felt the water,” she said, “and we shouldn’t have.”

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Joan Houde, of Grand Forks, along with five others on “The ReNewed Penguins” team, braved chilly waters as they participated in the annual Polar Plunge on Saturday, March 21, near Choice Health and Fitness in south Grand Forks

Pamela Knudson / Grand Forks Herald

Dressed in black-and-white penguin costumes, Erick Houde added that “the layers are pretty cozy.”

At the far end of the tank, three volunteers from Grand Forks Water Rescue were positioned in the water to help the plungers steady themselves after the jump and exit the tank.

Others on hand to help, if needed, included Altru ambulance, fire and police personnel.

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This is the 17th year the Polar Plunge event has been held here, said Nancy Hanson, president and CEO. The funds raised support year-round competition and sports training for Special Olympics of North Dakota athletes.

Anyone who raised $100 or more before the event was welcome to walk-up and participate, Hanson said.

The organization set a goal of raising $25,000 through this event, about the same as last year, Hanson said. People could participate as individuals or as members of a team.

Travis Nelson, a trooper with the North Dakota Highway Patrol, was taking the plunge for the 12th time. He has served on the event’s organizing committee since 2011.

He does it because Special Olympics “is a great organization to raise money for,” he said, emerging from the water.

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Awards were given for best costume, best belly flop, oldest and youngest plungers, the team and individual who raised the most funds for the Special Olympics of North Dakota organization.

Judges for the event were Rod Clark, UND chief of police; Mike Hedlund, East Grand Forks police chief; Jeremy Moe, Grand Forks Police Department; Captain Joel Lloyd, Grand Forks County Sheriff’s Office; and Tyler Lee, SOND athlete.

This spring, SOND will host another major fundraising event, “Dancing for Special Stars,” on April 16. Local celebrities and other well-known residents team up with dance instructors to learn routines which they perform for the audience. The dance couple with the most votes wins the competition at the Mirror Ball Trophy; a People’s Choice Award is also presented.

Pamela Knudson
Pamela Knudson is a features and arts/entertainment writer for the Grand Forks Herald.

She has worked for the Herald since 2011 and has covered a wide variety of topics, including the latest performances in the region and health topics.

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Pamela can be reached at pknudson@gfherald.com or (701) 780-1107.





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