Exclusive: There’s a new T. rex in town—and this giant beast ruled the seas

What do you call a massive prehistoric reptilian tyrant, with large, powerful jaws and sharp serrated teeth?

If you said Tyrannosaurus rex, that’s understandable: it’s the most famous dinosaurian predator ever. But this mystery beast swam the ocean depths, where no land-stalking Tyrannosaurus ever dove. What would you call that?

If you said “T. rex,” you’d be correct!

Paleontologists have discovered an enormous new species of Tylosaurus, a sea lizard belonging to the mosasaur family. Measuring up to 43 feet long—the size of a humpback whale—this 80-million-year-old apex predator is among the biggest known mosasaurs. 

They named it Tylosaurus rex, or “king of the Tylosaurs,” a name that comes with an iconic abbreviation. 

“If any animal deserves it, it’s this animal,” says Amelia Zietlow, a paleontologist previously at the American Museum of Natural History in New York and now at the History Museum at the Castle in Wisconsin. “Half of its characteristics are around it having a bigger jaw and bite.”

Zietlow and her colleagues announced the new underwater terror Thursday in the journal Bulletin of the American Museum of Natural History. The finding helps paleontologists better understand the evolution of these enormous sea lizards, including how and when separate lineages of them achieved success as massive marine monsters. 

(The 80-foot-long Ichthyotitan was the killer whale of its time)

“Tylosaurs are already notorious for being one of the largest-sized mosasaurs that ever lived,” says Tiago Simões, an evolutionary biologist at Princeton University. “I think this study further supports that, making them the largest group of mosasaurs that ever lived.” 

In addition to describing the new species, Zietlow and her team also assembled a comprehensive dataset on mosasaur anatomy, updating the older, rickety version from scratch with head-to-tail measurements of 300 mosasaur specimens and modern lizards

“I hope people are able to use it and give tylosaurs the attention I don’t think they’ve had,” says Zietlow. 

Measuring sea monsters

The first known Tylosaurus species, T. proriger (“prow-bearing knob lizard”), was named 150 years ago for the battering-ram-like tips of its jaws. At around 30 feet long, “it was the largest North American mosasaur known for quite some time,” says Mike Polcyn, a vertebrate paleontologist at Southern Methodist University and an author on the study. “Because the snout was so easily recognizable, people could differentiate it easily from other mosasaurs.” 

While paleontologists have described other species of Tylosaurus over the years, most of the specimens found in the 84-million-year-old rocks of Kansas are assigned to T. proriger. By custom, several giant specimens originally found in the 80-million-year-old rocks around Dallas, Texas, including a large one now mounted at the Perot Museum of Nature and Science, were also given this classification. 

In the 1960s, a Texas paleontologist named John Thurmond wondered in a personal letter whether the largest T. proriger specimens might be their own species, which he half-jokingly dubbed “Tylosaurus thallasotyrannus,” or “sea tyrant.” But he never formally published the idea.

(This sea monster sank ships off Constantinople for 50 years—was it an orca?)

In 2010, when Polcyn received a donation of an immense Tylosaurus specimen from an amateur fossil hunter who had collected it near the city, something about its features struck him as odd. As a side project, he spent years accumulating additional specimens from fossil hunters and reviewing other Texas collections in Austin and Waco. Increasingly, he noticed repeated anatomical traits “that were clearly pointing to something that was not T. proriger,” he says. 

Zietlow was also studying mosasaurs, first as an undergraduate, then as part of her doctoral research at the AMNH. Looking over the AMNH’s extensive collection of Tylosaurus fossils, a big specimen originally collected from Texas also jumped out at her. When, in 2022, Polcyn came on as an outside advisor for her dissertation, she asked him about it. Polcyn initially played it coy, Zietlow says. Then he told her he had tracked down several such colossal specimens. 

Zietlow spent the next few years visiting several institutions in Texas, New York, and Massachusetts to remeasure many of their Tylosaurus fossils. The team found that more than a dozen of the largest Tylosaurus specimens held in various museums—including the 31-foot-long “Heath Mosasaur” at the Perot Museum, “Sophie” at Yale Peabody Museum, and the enormous “Bunker Mosasaur” at the University of Kansas National History Museum—shared similar unusual features. They all had finely serrated teeth, an unusual trait in mosasaurs. All had adaptations in their skull and neck linked to more powerful jaws. 

“It was pretty clear that this thing was consistently different,” Zietlow says. 

To make sure these giants weren’t just adult T. proriger, as she had initially assumed in her undergrad work, Zietlow also measured the skeletons of 130 modern lizard species, examining skull features that change over their lifespans. The anatomical differences seen between T. proriger and the large Tylosaurus were not the same features that changed as lizards grew. That, in addition to the four-million-year time difference they had with T. proriger, suggested that the big Texas Tylosaurus were their own species, Zietlow says. 

Naming a new T. rex

While the largest T. proriger specimens max out at about 30 feet long, T. rex specimens all ranged from 25 feet to 43 feet. 

It was “a danged big lizard,” says Ron Tykoski, a curator at the Perot Museum and author of the paper. T. rex (the sea lizard) seems to have been a formidable and aggressive animal. One of several specimens held at the Perot Museum in Dallas, nicknamed “The Black Knight,” is missing the tip of its snout and has a fractured lower jaw. 

“It’s evidence of really energetic, violent interactions between members of the same species,” Tykoski says. “The only thing that could have done this was another Tylosaurus of the same size.” (Interestingly, the dinosaur T. rex also seems to have had a habit of biting each other’s faces.) 

Inspired by a stray thought she’d had looking at the Bunker mosasaur as an undergrad, Zietlow floated the name T. rex to Polcyn as a joke. 

“Mike laughed really hard,” she says, and agreed, noting that the name serves both to describe its anatomy and as an homage to Thurmond’s suggestion from the 1960s. 

(Teen T. rex or totally different dinosaur? The debate over Nanotyrannus gets a new twist. )

It’s also not the first time a team has cheekily dubbed a non-dinosaur “T. rex.” In 2001, a team named an extinct prehistoric beetle trapped in amber “Tyrannasorus rex,” says Tom Holtz, a vertebrate paleontologist at the University of Maryland who was not involved in the paper. 

Since the more famous Tyrannosaurus rex and the newly discovered mosasaur did not live and hunt in the same environment, he adds, “there shouldn't be too much confusion.” For much of the 20th century, Tylosaurus was the standard mosasaur in prehistoric popular culture.

Indeed, if there’s going to be a "T. rex of the sea", Holtz said he’s perfectly happy for it to get the name. 

“I do think it deserves it,” Zeietlow says. “But we’re also having a little bit of fun.”