According to the tales of Norse mythology, a sea serpent surrounds the world, circling its waters in a continuous cycle, by biting onto its own tail. When the creature eventually releases its bite, the stories say, a world-ending battle will ensue, destroying the old and ushering in the new.

In a similar narrative of rebirth and revival, the name of this sea serpent, Jörmungandr, has recently taken on a new meaning. This year, a team of researchers gave the name to a specimen of mosasaur — a massive marine reptile — that may have met its end, around 80 million years ago, at the mouth of another mosasaur.

“From the vertebrae, we know that this thing was bitten at one time by another mosasaur,” says Amelia Zietlow, a paleontologist at the Richard Gilder Graduate School at the American Museum of Natural History and a member of the research team. “There was nothing else we know of at that time that could have made a bite that big or that deep.”

According to researchers, the newly described Jormungandr walhallaensis species was much more than a meal for other mosasaurs, however. Today, the species reveals important information about the way that mosasaurs evolved, becoming some of the biggest predators that ever swam through the world’s oceans.

What Did J. walhallaensis Look Like?

The specimen was discovered in 2015 during an annual dig held by the North Dakota Geological Survey. The fossils took a few years to clean and analyze. It was only after Zietlow joined the research team in 2022 that she and her collaborators were able to describe the specimen as a member of a new species.

Jormungandr Fossils

The fossils consist of a number of face bones, including a large forehead bone that prompted Clinton Boyd, a colleague of Zietlow’s, to nickname the specimen “Eustace” after a characteristically cranky character from the cartoon Courage the Cowardly Dog thanks to the bone’s grumpy appearance. “They have a built-in angry eyebrow,” Zietlow says.

The Structure of Jormungandr Bones

The specimen also features some of the snout, upper jaw, and upper vertebrae. A part of Eustace’s shoulder was also preserved, though that portion was un-described in the recent paper that Zietlow and her colleagues published in the Bulletin of the American Natural History Museum.

Many of these same bones have been found in other mosasaur species. But the bones of the North Dakota specimen are distinct enough that Zietlow and her colleagues argue the fossils are not only a new species of mosasaur, but also a new genus.

How Big Was Jormungandr?

The skull was about 2.2 feet long, while the whole specimen may have been about 24 feet long, putting it in the medium-sized category of mosasaurs. Eustace also had a shorter tail than many other mosasaurs, and flippers with less cartilage and more bone.

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When Did Jormungandr Live?

Dating of the clay deposits around the specimen has revealed with a degree of precision that this individual lived about 80 million years ago, Zietlow says.

Where Did Jormungandr Live?

At that time in the Cretaceous, the sea level was much higher. North America was split down the middle by an inland sea which stretched from the Gulf of Mexico to Canada, meaning parts of North Dakota and the Canadian province of Alberta were underwater, among other regions.

It was in these waters — the warm, shallow seas of North America — that the Jormungandr specimen once swam, spending its time hunting and doing its best to avoiding being hunted, in turn.

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What Does J. walhallaensis Mean?

The whole name is Norse in theme, with Zietlow and her colleagues selecting the first part, Jormungandr, to reference the massive monster that the ancient Norse imagined to usher in the end of the world, or Ragnarök.

The second part, walhallaensis, was inspired by the town of Walhalla in North Dakota, near the dig site. The name is a variation on the mythical hall of Valhalla presided over by the god Odin — a place where warriors went after death.

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Why Is Jormungandr Important?

More important than its mythology, is that the specimen answers some of the lingering questions that paleontologists have had regarding the evolution of mosasaurs as a group. In fact, when Zietlow first looked at the bones, it almost appeared as if they were a chimera, a collection of fossils from both earlier and later species, positioned together in the same place.

Once the team realized the fossils were all from one individual, however, they could see that the creature likely sat somewhere in evolutionary time between the small, primitive mosasaur Clidastes and Mosasaurus — the sophisticated, sometimes 50-foot-long sea monster that was featured in the 2015 film Jurassic World.

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How Did J. walhallaensis Die?

Many mosasaur fossils are so fragmentary that it’s impossible to tell how individual mosasaurs died. This specimen’s end isn’t completely ambiguous, though.

Some of the fossils show signs of being bitten by something even bigger than Jormungandr walhallaensis. It’s impossible to tell from the remains whether this bite was the killing blow, or whether the larger predator merely scavenged Eustace’s remains after it had died of some other cause. But based on the fact that the bones show no sign of healing, Zietlow says it was unlikely that the bite occurred earlier in Eustace’s life.

Did Jormungandr Die of a Shark Bite?

Sharks were around at the time, but the bite marks aren’t really consistent with that type of jaw. “Mosasaur bites will be like punctures and crushes, and that’s what we see on our guy,” Zietlow says.

Did Jormungandr Die of a Mosasaur Bite?

It’s unclear which mosasaur would have bitten it, but a few potential candidates were around at the time. It might have been a Plioplatecarpines or a Prognathodon — researchers know the latter hunted other mosasaurs based on other findings in the fossil record. Or it could have been a Tylosaurus, a marine reptile that got nearly as big as Mosasaurus and had “big thick railroad-spiked teeth,” Zietlow says.

Future work may reveal more about the type of creature that bit Eustace. For now, Zietlow is betting on the latter of the three potential suspects. “My hunch is that this thing was killed by a Tylosaurus,” she says.

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