New Mosasaur Species Was Like A Giant Sea Dragon With "Angry Eyebrows"

This giant reptile was a prehistoric prince of the sea. Just don't make fun of their eyebrows.


Tom Hale

Tom is a writer in London with a Master's degree in Journalism whose editorial work covers anything from health and the environment to technology and archaeology.

Senior Journalist

An illustration of two Jormungandr walhallaensis mosasaurs fighting it out in the ocean.

An illustration of two Jormungandr walhallaensis mosasaurs fighting it out in the ocean. 

Image credit: © Henry Sharpe

A ferocious sea beast that lived 80 million years ago has been discovered by palaeontologists. Defined by its “angry eyebrows” and gargantuan size, the new species of aquatic lizard has aptly been named after a sea serpent in Norse mythology, Jörmungandr, and the tiny city in North Dakota near where the fossil was found, called Walhalla. 

Jormungandr walhallaensis is a mosasaur, a large group of extinct reptiles that ruled the oceans from about 82 to 66 million years ago.


Judging by the shape and size of the fossilized remains, the researchers estimate it was about 7 meters (24 feet) long from snout to tail, comparable to the size of a male orca. It featured “angry eyebrows” caused by a bony ridge on the skull, plus a relatively stumpy tail.

“If you put flippers on a Komodo dragon and made it really big, that’s basically what it would have looked like,” Amelia Zietlow, lead study author and a PhD student in comparative biology at the American Museum of Natural History’s Richard Gilder Graduate School, explained in a statement

The specimen, first discovered in 2015 in North Dakota, consists of a nearly complete skull, jaws, cervical spine, and a few vertebrae.

A new study of the remains showed it’s an especially interesting example as it’s a mish-mash of features seen in two other well-known mosasaurs: Clidastes, a smaller and more primitive form of mosasaur; and Mosasaurus, a gigantic member of the family that could measure over 15 meters (50 feet) in length. 

A line drawing of the skull of Jormungandr walhallaensis
A line drawing of the skull of J. walhallaensis.
Image credit: © Henry Sharpe

J. walhallaensis swam the oceans around 80 million years ago during the late Cretaceous. At this time, sea levels were high and much of Africa was flooded. The ocean had pulled nutrient-rich bottom waters to the surface, creating a thriving marine ecosystem – and the perfect conditions to be an apex predator of the seas. Mosasaurs boomed during this time until they were killed off around 66 million years ago in the same mass extinction event that wiped out the dinosaurs.

“As these animals evolved into these giant sea monsters, they were constantly making changes,” Zietlow said. “This work gets us one step closer to understanding how all these different forms are related to one another.”

The dating of the specimen, as well as its anatomy, suggests that it was perhaps a precursor to the great Mosasaurus. However, this chapter of mosasaur history is a little hazy, so the researchers hope that J. walhallaensis might be able to fill in some gaps. 

“This fossil is coming from a geologic time in the United States that we don’t really understand,” added co-author Clint Boyd, from the North Dakota Geological Survey. “The more we can fill in the geographic and temporal timeline, the better we can understand these creatures.”


The study is published in the journal Bulletin of the American Museum of Natural History


  • tag
  • animals,

  • prehistoric animals,

  • marine reptile,

  • extinct,

  • Palaeontology,

  • mosasaurs,

  • Mosasaurus