Another T. rex debuts: A marine apex predator named Tylosaurus rex - CNN

- Paleontologists have newly described a marine reptile species called Tylosaurus rex that lived in ancient oceans.

- The massive mosasaur measured up to 43 feet long and lived 80 million years ago in waters then covering North America.

- Researchers uncovered the species after examining fossils misidentified for decades in museums.

Paleontologists have long hailed Tyrannosaurus rex as king of the dinosaurs. Now, the name “T. rex” also belongs to a newly described extinct carnivore — a massive marine reptile with the scientific name Tylosaurus rex that a trio of researchers uncovered after a hefty amount of detective work.

The freshly crowned T. rex wasn’t a dinosaur but a mosasaur, a gigantic ocean apex predator that lived about 80 million years ago — a bit earlier than the dinosaur king, which lived 68 million to 66 million years ago — and measured up to 43 feet (13 meters) long. The sleuthing scientists identified the species from fossils attributed for decades to a closely related mosasaur.

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Like the land-dwelling T. rex (rex means king in Latin), the huge creature ruled its habitat, its sawlike teeth tearing into its prey — fish, turtles and long-necked marine reptiles called plesiosaurs — “really crunching through and ripping them up,” said Amelia Zietlow, a paleontologist with the History Museum at the Castle in Appleton, Wisconsin. Zietlow is lead author of a new study describing Tylosaurus rex, published May 21 in the journal Bulletin of the American Museum of Natural History.

Fossils of the long-snouted swimmer were found in what’s now Texas and date back about 80 million years to the latter part of the Cretaceous Period, a time when an inland sea partly covered the North American continent. For the new study, Zietlow and her coauthors examined and reclassified fossils housed in more than a dozen institutions — specimens that had been misidentified as the species Tylosaurus proriger.

“Here we have two T. rexes, one the king of the dinosaurs on land, the other the king of the reptiles in the water, both about the same size, 40 feet long or so, and both dominant at the top of the food chain, as the biggest carnivores in their ecosystems,” said paleontologist Steve Brusatte, a professor of paleontology and evolution at Scotland’s University of Edinburgh, in an email. Brusatte was not involved in the research.

The discovery serves as a reminder that scientific breakthroughs can come from museum collections as well as newfound fossils, and that amateur dinosaur enthusiasts also can play an important part in identifying species new to science, Zietlow noted.

“A lot of these specimens were dug up and donated by avocational or hobbyist paleontologists in the Dallas area, so there was a lot of community involvement,” she said. “This is a really great case of what paleontology can be, if everyone works together.”

The puzzling ‘Beefcake’ fossil

For Zietlow, Tylosaurus rex’s origin story began in 2020 in New York City. She was a doctoral student in the American Museum of Natural History’s Richard Gilder Graduate School, examining AMNH’s mosasaur specimens.

“My research is focused on variation, so I wanted to get a sense of the scope of different mosasaurs that were in the collection,” Zietlow said. One large T. proriger fossil unearthed in Texas during the 19th century — she nicknamed it “Beefcake” for its size — captured her attention.

“I had never heard of Tylosaurus being found in Texas before,” she said. “Typically, they’re found in Kansas and South Dakota, so that stood out to me.” She connected with study coauthor Michael J. Polcyn, a mosasaur specialist and senior research fellow at Southern Methodist University in Dallas, around 2022, and suggested that the AMNH specimen might represent a different species of Tylosaurus.

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As it happened, Polcyn had been investigating this possibility for about a decade. Peculiarities in other T. proriger fossils caught his eye around 2012, but at the time there were not enough specimens to make a strong case for a species new to science, he told CNN in an email.

“Over the years, more specimens were discovered which started filling out the gaps in our understanding of the anatomy and natural variation of the animal,” Polcyn said.