A new species of mosasaur, large marine reptiles that thrived during the late Cretaceous period, has been unearthed by scientists.
This newly discovered species, named Jǫrmungandr walhallaensis after a sea serpent in Norse mythology and the North Dakota city Walhalla, bridges the characteristics of two well-known mosasaurs. Details outlining this remarkable creature have been released today in the Bulletin of the American Museum of Natural History.
Describing the findings, Amelia Zietlow, the lead author of the study and a Ph.D. student in comparative biology at the Richard Gilder Graduate School of the American Museum of Natural History, likened the appearance of Jǫrmungandr walhallaensis to envisioning a magnified Komodo dragon equipped with flippers.
Mosasaurs have intrigued researchers for over two centuries, predating even the term “dinosaur.” Despite this long history, numerous enigmas about these creatures persist, including the frequency of their flipper evolution toward complete aquatic life—estimated at least three times, if not more—and their closer affiliation with monitor lizards or snakes. The ongoing quest to discern the interrelationships between distinct mosasaur groups gains a new perspective through this recent study.
The pivotal fossil forming the crux of the study was unearthed in 2015 during an excavation in the northeastern region of North Dakota. It comprised an almost intact skull, jaws, cervical spine, and several vertebrae.
Thorough scrutiny and surface scanning of the fossil remnants led Zietlow and her colleagues to conclude that this specimen represents a distinct species, displaying a combination of features found in two emblematic mosasaurs: Clidastes, a smaller and more primitive variant, and Mosasaurus, a colossal form spanning nearly 50 feet and coexisting with Tyrannosaurus rex. Estimated at around 24 feet in length, Jǫrmungandr walhallaensis sported flippers, a shark-like tail, distinctive brow ridges creating an “angry eyebrows” appearance, and a slightly truncated tail shorter than its body.
“Throughout their evolution into these colossal sea creatures, they underwent continual modifications,” explained Zietlow. “This research propels us closer to unraveling the connections between these varied forms.”
The study proposes that Jormungandr served as an antecedent to Mosasaurus and existed roughly 80 million years ago.
“This fossil heralds from a geological epoch in the United States that remains relatively enigmatic,” remarked co-author Clint Boyd from the North Dakota Geological Survey. “Expanding our knowledge of the geographic and temporal context enhances our comprehension of these creatures.”
Co-author Nathan Van Vranken from Eastern West Virginia Community and Technical College emphasized, “The account of Jormungandr paints a vivid picture and contributes significantly to our insights into the northernmost realms of the interior seaway, particularly regarding mosasaurs. Discoveries like these spark scientific curiosity.”
Header Image Credit : Henry Sharpe