A new genus and species of mosasaur being named Jormungandr walhallaensis has been discovered by American Museum of Natural History Ph.D. student Amelia Zietlow and colleagues.
Reconstruction of Jormungandr walhallaensis shown engaging in intraspecific combat. Image credit: Henry Sharpe.
Mosasaurs were a group of large, carnivorous aquatic reptiles that inhabited all of the world’s oceans during the Late Cretaceous epoch, between 90 and 66 million years ago.
These creatures went extinct during the end-Cretaceous extinction event which killed non-avian dinosaurs and 75% of life on the planet.
The first mosasaur was discovered more than 200 years ago, and the word ‘mosasaur’ predates the word ‘dinosaur.’
But many questions about these animals remain, including how many times they evolved flippers and became fully aquatic — paleontologists think it was at least three times, and maybe four or more — and whether they are more closely related to monitor lizards or snakes.
“After 200 years of scientific study, new mosasaur species are still being discovered as new localities are explored and specimens collected long ago are reevaluated using modern standards of species delimitation,” Zietlow and co-authors said.
“Even so, the phylogenetic positions of many key taxa are unresolved and therefore our understanding of mosasaur macroevolution is muddled.”
Jormungandr walhallaensis lived in the Cretaceous oceans approximately 80 million years ago.
Its fossilized skull and skeleton (cervical spine and a number of vertebrae) were discovered in 2015 in the Pierre Shale Formation in Cavalier County, North Dakota, the United States.
The species shows a mosaic of features seen in two iconic mosasaurs: Clidastes, a smaller and more primitive form of mosasaur; and Mosasaurus, a larger form that grew to be nearly 15 m (50 feet) long and lived alongside Tyrannosaurus rex.
Jormungandr walhallaensis is estimated to be about 7.3 m (24 feet long), and in addition to flippers and a shark-like tail, it would have had ‘angry eyebrows’ caused by a bony ridge on the skull, and a slightly stumpy tail that would have been shorter than its body.
“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 researchers suggest that Jormungandr walhallaensis was a precursor to Mosasaurus.
“This fossil is coming from a geologic time in the United States that we don’t really understand,” said Dr. Clint Boyd, a paleontologist at the North Dakota Geological Survey.
“The more we can fill in the geographic and temporal timeline, the better we can understand these creatures.”
“The tale of Jormungandr walhallaensis paints a wonderful picture and helps contribute to our understanding of the northernmost regions of the interior seaway, especially with the mosasaurs, and discoveries such as these can pique scientific curiosity,” said Dr. Nathan Van Vranken, a paleontologist at Eastern West Virginia Community and Technical College.
The discovery of Jormungandr walhallaensis is described in a paper published in the Bulletin of the American Museum of Natural History.
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Amelia R. Zietlow et al. 2023. Jormungandr walhallaensis: a new mosasaurine (Squamata: Mosasauroidea) from the Pierre Shale Formation (Pembina Member: Middle Campanian) of North Dakota. Bulletin of the American Museum of Natural History 464; https://hdl.handle.net/2246/7332
