Massive marine predator crowned T. rex of the ocean and ‘a true terror of its time’

A reconstruction depicts Tylosaurus rex in the Western Interior Seaway of North America during the Cretaceous Period. (Alderon Games/Path of Titans/Courtesy American Museum of Natural History via CNN Newsource)

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 metres) long. The sleuthing scientists identified the species from fossils attributed for decades to a closely related mosasaur.

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 co-author 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.

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.

Still, describing a species from fossils can take years. Specimens are often incomplete or poorly preserved, and other factors can muddy the waters when it comes to connecting the dots between anatomical anomalies.

“You need to rule out pathologies, how animals change while growing, and other factors that may account for the differences,” he added.

Finding a pattern in Tylosaurus museum specimens

Zietlow travelled to 22 museums across North America and Europe to do comparative data collection of presumed T. proriger fossils found in northeastern Texas, “taking pictures, taking measurements, taking surface scans in some cases, and building up a dataset to be absolutely positive that we were seeing a pattern that could be attributed to a species-level difference,” she said.

Her analysis showed that at least a dozen of those fossils displayed characteristics hinting at a novel Tylosaurus species. The Tylosaurus rex holotype — or single specimen that defines a given species — is on display at the Perot Museum of Nature and Science in Dallas. This T. rex is the most complete skeleton of the species. (Study co-author Ronald S. Tykoski is vice president of science and curator of vertebrate paleontology at the Perot Museum.)

Tylosaurus rex was larger than T. proriger, measuring on average about 30 to 35 feet (9 to 11 metres) from nose to tail. The skull of the largest specimen measured more than 5.5 feet (1.7 metres) long. The marine reptile also had serrated teeth, an extra bony pocket in its skull where neck muscles attached, and heavier jaw muscles, giving it a more forceful bite.

Tylosaurus was previously thought to be a less diverse genus than other mosasaurs. Adding T. rex to the Tylosaurus group suggests that there is greater diversity in tylosaurs than expected.

“Tylosaurus is much more interesting and much more diverse than it’s ever been given credit for,” Zietlow said.

Colossal marine reptiles ruled the seas

Illuminating the diversity of mosasaurs in general also casts the so-called age of dinosaurs in a different light, Brusatte added.

“It wasn’t only dinosaurs that thrived and dominated during this time,” he said. “Yes, they ruled the land, but there were other equally enormous and ferocious carnivorous reptiles stalking the seas. Tylosaurus rex would have been one of the biggest and most ferocious of these, a true terror of its time, and as scary as any shark that lives today.”

And there are more surprises in store from the newly crowned T. rex, Polcyn said.

“A research project always generates more questions than can be addressed in a single paper,” he said. “We currently have two other projects in process with colleagues and students, directly related to Tylosaurus rex, so stay tuned for some other exciting findings.”