There’s a New T. Rex in Town & It Ruled the Ocean

A misidentified 43-foot marine reptile from Texas has been crowned Tylosaurus rex, a newly recognized mosasaur that’s forcing paleontologists to redraw the family tree of dinosaur-age sea monsters.

By Serrae Bell

Friday, July 3, 2026

The holotype of Tylosaurus rex, PMNS 8029. (Figure via. Bulletin of the American Museum of Natural History.)

EARTH, Laniakea Supercluster—Move over T. rex, there’s a new dinosaur-age species in town, and it’s a 43-foot marine reptile also called T. rex.

Research recently published in the Bulletin of the American Museum of Natural History describes a new species of mosasaur called Tylosaurus rex—or, translated, knob lizard king. This sea-dwelling, school-bus-sized tyrant was previously misidentified as the species Tylosaurus proriger before Amelia Zietlow, lead author of the study, noticed a big difference (emphasis on the ‘big’) in the fossils that led to an exciting discovery.

“We've known about [tylosaurs] for 150-something years, right?” Zietlow told Milky Way News. “So what are the chances that one of them would be new? It turns out, much higher chances than we thought.”

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Tylosaurs were the first kind of mosasaur to reach official gigantic status, which is classified as more than 8 meters, or 26 feet, long. Tylosaurus proriger typically ranged from 12 to 31 feet, but researchers estimated that the new T. rex could have been anywhere from 25 to 43 feet long. That is one massive knob lizard.

At first, the 80-million-year-old T. rex fossil, which was housed in the American Museum of Natural History when Zietlow observed it, was thought to be an atypically large T. proriger, but there were other factors that set it apart. Most T. proriger specimens were found in Kansas, but apparently that town ain’t big enough for the both of them, because the fossil identified as T. rex was found in northern Texas.

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This mosasaur, which was more than twice the size of the biggest great white shark, boasted finely serrated teeth and an unusual neck and jaw structure. These features were first chalked up to ontogenetic differences, or variations in an individual organism as it grows and develops over the span of its life, but then Zietlow compared the fossil to those of known T. proriger specimens and confirmed that it was, in fact, an entirely different species.

“A lot of species variation was, I don't want to say ignored, because it was recognized, but it was considered to be within species variation,” said Zietlow. “And so what that does is kind of mask these true patterns of diversity more broadly.”

Additionally, many specimens identified as T. proriger were not compared to the holotype, which is the specimen that sets the standard for a species—the OG, if you will. It defines the characteristics of a species and allows scientists to identify others of the same kind. Without comparing fossils to the holotype, over a dozen specimens that have since been identified as the new T. rex were mistaken for other types of tylosaurs.

One of these re-identified specimens is known as “The Black Knight” due to severe injuries to its skull. “The Black Knight” is housed at the Perot Museum in Dallas, and is evidence that, while the Tyrannosaurus rex stomped about on land, the Tylosaurus rex wreaked utter havoc in the sea.

The researchers even speculate that the damage to “The Black Knight” was done by one of its own kind, and that the T. rex’s strong neck and jaw muscles allowed for aggression and violence.

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According to Zietlow, tylosaurs, and mosasaurs in general have more intra-species variation than ever thought before, and the discovery of this Cretaceous-era sea monster calls for changes in the way these creatures are classified.

“Scientifically, mosasaurs are a lot more diverse than is currently recognized, and so people should take this as a sign to give them another look,” said Zietlow.

Researchers have created a new phylogenetic character list—essentially documentation of traits used to track the evolution and history of a species—and have updated the standard for what makes a tylosaur a tylosaur.

This re-identification may even open doors for naming more new species as fossils and specimens that vary from their holotypes are reclassified. This tylosaur could be the first of many.

Zietlow said the discovery is about more than just identifying a new species. It promotes engagement in local museums and paleontology, as the specimens used in the study were not newly uncovered fossils, but ones already housed in museum collections, many of which were found by hobbyist paleontologists.

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“Museum collections are an incredibly important resource and new discoveries are always hiding, if you will, in museum collections, and so they deserve attention,” Zietlow said. “And the second point is that everyone and anyone can get involved in paleontology.”

If average people found the fossils that led to this identification of this new tylosaur, who knows what could be uncovered (literally) next? If the next discovery is the scale of either T. rex, the impact is sure to be huge (pun intended).