Paleontologists Discover an Ancient Marine Reptile They’ve Dubbed the T. Rex of the Sea, Crowning Another King of the Cretaceous

Scientists figured out that the predators were lumped in with a previously named mosasaur species. The new one, called Tylosaurus rex, could grow to up to 43 feet long, about the length of a school bus

Margherita Bassi | Daily Correspondent

May 22, 2026 5:00 p.m.

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Earth was a terrifying place during the Cretaceous period, some 66 million to 145 million years ago. During its tail end, Tyrannosaurus rex prowled the land. And a little before that, another kind of T. rex stalked the ancient ocean, according to a new fossil analysis.

In a study published May 21 in the Bulletin of the American Museum of Natural History, researchers reveal a newly discovered species of mosasaur—an ancient aquatic lizard—that they’ve dubbed Tylosaurus rex for its giant size. The name means “king of the knob lizards,” much like the land T. rex’s translation, “king of the tyrant lizards,” and, of course, it comes with the same recognizable abbreviation.

“If any animal deserves it, it’s this animal,” study co-author Amelia Zietlow, a paleontologist at the History Museum at the Castle in Wisconsin, tells National Geographic’s Asher Elbein. “Half of its characteristics are around it having a bigger jaw and bite.”

The extinct animal was identified from 80-million-year-old fossils, mostly found in northern Texas, gathered over several decades and now housed in museums. They were previously identified as members of a different mosasaur species, T. proriger.

Study co-author Michael Polcyn, a paleontologist at Southern Methodist University, first noticed something strange in the species about 15 years ago, when he saw odd anatomical features in an abnormally large T. proriger, he tells the Dallas Morning News’ Niamh Ordner.

More recently, Zietlow noticed that one of the T. proriger specimens at the American Museum of Natural History seemed surprisingly huge, per a statement. Comparing this oddball to T. proriger’s holotype fossil, the specimen that represents the species, convinced the researchers that it had been misidentified.

So, the team scoured collections at several museums across North America and found more than a dozen similarly large mosasaur fossils. Analyses revealed that the animals would have been taller than T. proriger and that they had mouths full of finely serrated teeth, an unusual feature in mosasaurs. What’s more, most T. proriger specimens are from modern-day Kansas and are about 84 million years old. But the newly identified group of fossils is around four million years younger and mostly comes from Texas.

The evidence hinted at a newfound, enormous species that could grow to up to 43 feet long—about the length of a school bus. For comparison, T. proriger reached up to 31 feet long.

“Tylosaurs are already notorious for being one of the largest-sized mosasaurs that ever lived,” Tiago Simões, an evolutionary biologist at Princeton University who was not involved in the study, tells National Geographic. “I think this study further supports that, making them the largest group of mosasaurs that ever lived.”

Polcyn and Zietlow weren’t the first to notice something strange among T. proriger specimens. In the late 1960s, paleontologist John Thurmond, then at Southern Methodist University, realized that tylosaurs from northeast Texas were unusually large and might represent a previously unrecognized species. He unofficially called them “Tylosaurus thalassotyrannus,” meaning “sea tyrant,” in reference to the iconic land dinosaur.

Polcyn happened to find a note that Thurmond had written about it, and the team’s new species title pays homage to the nickname. “It was serendipity,” he tells the Dallas Morning News. “I came across that letter when I was looking for something else completely.”

The T. rex of the sea had bone features indicative of incredibly strong jaw and neck muscles, pointing to its role as a terrifying predator—and probably a way “meaner” one than other mosasaurs, study co-author Ron Tykoski, curator of vertebrate paleontology at the Perot Museum of Nature and Science, says in the statement. The team found evidence of violence within the newly identified species that surpasses that seen in other Tylosaurus specimens, he explains.

For example, a T. rex fossil at the Perot Museum dubbed “The Black Knight” is missing part of its snout and features a broken lower jaw. Those wounds were probably caused by other members of the ancient predators’ species, according to researchers.

Ultimately, the study reveals yet another king of the Cretaceous. “Everything is bigger in Texas and that includes the mosasaurs, apparently,” Zietlow says in the statement.

Margherita Bassi | Read More

Margherita Bassi is a freelance journalist and trilingual storyteller. Her work has appeared in publications including BBC Travel, Discover magazine, Live Science, Atlas Obscura and Hidden Compass.