Newly identified species could shed light on the evolution of prehistoric ‘sea monsters’

CNN
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A huge sea serpent from Norse myth, born of the trickster god Loki, and growing large enough to circle the world, now has the namesake of a different kind of “monster” – a newly discovered species of the famously large carnivorous marine reptile. Like the mosasaur, which lived about 80 million years ago.

Paleontologists recently described a previously unknown mosasaur from fossils found near the town of Walhalla in North Dakota. The city’s name comes from Valhalla, the banquet hall in Norse mythology where dead heroes gather, so scholars have called the mosasaur Jörmungandr and the Halensis. Its name refers to the Norse mythology of Jormungandr, the serpent of Midgard, as well as the location of the fossil’s discovery, the researchers reported Monday in the journal. Bulletin of the American Museum of Natural History.

The fossil itself has a somewhat less poetic name: NDGS 10838. It includes a nearly complete skull with a bony ridge above the eyes as well as jaws and some skeletal fragments, including 11 ribs and 12 vertebrae. In life, the animal was about 24 feet (7.3 meters) long and had a long, thinner face than its mosasaurid cousins, said Amelia Zietlow, the study’s lead author and American Museum of Natural History paleontologist and doctoral candidate Richard Gilder. Graduate school in New York City.

Overall, Jormungandr walhallaensis looked like most dinosaurs — “kind of like if you took a Komodo dragon, made it 30 feet long and gave it the fins and tail of a shark,” Zietlow told CNN.

However, on the other hand, the animal was unique. The study’s authors reported that a combination of features in its skull bones made it unexpectedly difficult for scientists to classify the newcomer and hinted that the mosasaurid group included more diverse forms than expected.

The fossil was collected in 2015 by the North Dakota Geological Survey, a state agency dedicated to geology and public education about minerals and fossils. In fact, Zitlow said, NDGS 10838 was discovered in a hillside by someone who participated in one of the agency’s programs and thus was able to identify the object as a fossil and was aware of alerting agency officials.

When scientists examined the skull, they quickly realized they had something unusual on their hands. Its ear bones, which were somewhat rectangular, resembled those of Mosasaurus, a genus of giant mosasaurs. But the shape and large number of its teeth were closer to a genus of smaller mosasaurs: Clidastes. Meanwhile, the angle and number of teeth on the bony palate at the roof of its mouth were unlike anything seen in either of these groups of mosasaurs.

“It has features that look in some ways like Mosasaurus, and in some ways like Clidastes. And in other ways, they are completely unique to this individual,” Zitlow said. This combination of features convinced the researchers that what they were looking for was a new genus and species.

However, fossilization often distorts bones, and it is possible that abnormalities in the fossil were formed through natural processes after the animal died, said paleontologist Takuya Konishi, an associate professor in the Department of Biological Sciences at the University of Cincinnati. (The authors acknowledge this possibility; their study included idealized illustrations of an intact skull showing what it looked like before it fossilized.)

When the researchers analyzed the data, their evolutionary tree showed a result called “polyploidy” — “when a group of different species cluster together in one place” — with Jörmungandr, Halensis and Clydastes, Zietlow said. “They are closer to each other than anything else. But within this group of things, it is not entirely certain how related they are.”

Additional fossils of the newly discovered species could help refine Jormungandr walhallaensis’s position in the mosasaur family tree, said Konishi, who studies mosasaur evolution and was not involved in the study.

“How different J. walhallaensis is from Clidastes has not yet been done further,” Konishi told CNN in an email. “Future discoveries may favor an alternative hypothesis that it is a new species of Clidastes.”

Other unusual details in the fossil are the holes and scratches that scar the vertebrae. Researchers identified these as bite marks. The marks do not appear to have healed, suggesting that they occurred toward the end of the animal’s life or that they were the work of a scavenger that tore up the mosasaur after it died.

“This may be why we didn’t get the rest of the skeleton,” Zitlow said.

More questions about why the marks appeared — and whether Jörmungandr and Halaensis survived an attack — will be addressed in future research by study co-author Clint Boyd, a senior paleontologist with the North Dakota Geological Survey and curator of the North Dakota State Paleontological Collection. . Zitlow said.

Mosasaurs and evolutionary mysteries

Mosasaurs were a diverse group of predators that swam the world’s oceans during the latter part of the Cretaceous Period, about 98 to 66 million years ago. They lived alongside dinosaurs but are more closely related to modern lizards and snakes.

Some mosasaurs were only a few feet long, while the largest — in the genus Mosasaurus — were about 60 feet (18.2 meters) long. While mosasaurus fossils are relatively abundant, scientists have “only scratched the surface of the ‘true’ mosasaurs,” Konishi said. “Diversity.” New mosasaur specimens, such as NDGS 10838, are helping experts unravel “the rich evolutionary history of these rather charismatic predators of the Cretaceous seas,” he said.

To that end, he added, the new study makes a significant contribution by providing “rich anatomical detail documented by a very capable mosasaur worker, Ms. Zietlow.”

“It is clear that the authors have provided a very comprehensive and accurate skeletal description of the new specimen,” Konishi said, creating a trove of exceptional data.

Although mosasaurs were aquatic, their ancestors lived on land and then evolved to return to the sea. They were not the only animal group to do this; Many species of reptiles and mammals—including plesiosaurs, whales, sea turtles, and seals—adapted to life in the oceans from their terrestrial ancestors, long after their tetrapod ancestors left the seas for land. Zitlow said mosasaurs are an important animal group to study this transition because their fossils are so abundant.

“There are a lot of them, thousands of samples in the United States alone,” she said. “This makes them good for studying big picture, statistical-type evolutionary questions.”

Despite the abundance of specimens, many fossil mosasaurs have not been documented as comprehensively as Jörmungandr valhallensis (and in some cases, were barely annotated at all when they were first described, Zietlow said).

Addressing this discrepancy in newly discovered fossils – and revisiting known specimens – will play a big role in helping scientists solve these evolutionary mysteries.

“I spent a lot of time putting these shapes together, showing the bones in each view, showing all the little lumps and bumps and things, so that people in the future can look at these shapes and learn about the anatomy and then apply that in making new shapes,” Zitlow said. “The characters and discovering new differences between this animal and other animals.” “This generally helps everyone understand the anatomy of these things a little better.”

Mindy Weisberger is a science writer and media producer whose work has appeared in Live Science, Scientific American, and How It Works.