- Scientists have named a new species of giant marine reptile Tylosaurus rex, which ruled the Western Interior Seaway roughly 81 to 79 million years ago.
- Body length estimates for confirmed specimens range from 25 to 43 feet, making it among the largest mosasaurs known from North America.
- The species was misidentified for decades as Tylosaurus proriger, with oversized traits dismissed as signs of age or growth.
- Lead author Amelia R. Zietlow and colleagues analyzed multiple specimens and identified distinct features, including a three-lobed jaw joint, that separate T. rex from other species.
- The discovery suggests other new species may be hiding in museum collections, mislabeled due to historical lack of careful comparison.
Long before Texas was cattle country, it was ocean floor and something enormous was patrolling those ancient waters. Scientists have formally named a new species of mosasaur, a giant marine reptile that ruled the seas roughly 81 to 79 million years ago, giving it a name that should sound familiar: Tylosaurus rex.
Body length estimates for confirmed specimens range from about 25 to 43 feet, placing it among the largest mosasaurs known from North America. Even a very large modern shark would have been shorter than the biggest individuals described in this study, published in the Bulletin of the American Museum of Natural History.
The paper is based on fossils pulled from marine rock formations in northeast Texas. Mosasaurs, the broader group of ocean-going reptiles to which T. rex belongs, were not dinosaurs, but they lived at the same time and were every bit as formidable. They were massive, flippered, sea-going lizards with long skulls, powerful jaws and serrated teeth.
Tylosaurus rex sat at the very top of the food chain in what scientists call the Western Interior Seaway, a shallow inland sea that once split North America in two from Canada down through Texas.
A king that had been hiding in plain sight for decades
The species name rex, Latin for king, was not chosen lightly. The paper's authors trace the name back to a letter written in 1967, in which a researcher named J. Thurmond informally suggested calling a large Texas mosasaur Tylosaurus thalassotyrannus, essentially sea tyrant, because of its enormous size.
The animal earns it. According to the paper, the species name is further earned by its massive skull, large, serrated teeth and accommodation space for powerful jaw and neck musculature, underscoring its position as a top predator of its domain.
The specimen that officially defines the species is a nearly complete skull, jaws and partial skeleton now housed at the Perot Museum of Nature and Science in Dallas. It was discovered in the fall of 1979 by the Newman family while boating along the eastern shoreline of Lake Ray Hubbard, east of Dallas. For years it was known informally as the Heath Mosasaur, named for the city of Heath, Texas. It sat in collections, scientifically important but not fully understood, for decades.
Mislabeled in museum collections across the country
One of the more revealing aspects of this paper is that Tylosaurus rex wasn't entirely unknown. It was misidentified. Several specimens now officially assigned to this new species had been labeled in museum databases as Tylosaurus proriger, the most commonly found mosasaur species from the Niobrara Formation, a well-studied rock unit spanning Kansas and neighboring states.
Because the oversized traits of T. rex had previously been chalked up to the animal simply being older or more developed, the way a puppy grows into a large dog, researchers hadn't looked closely enough at whether something more fundamental was different.
Lead author Amelia R. Zietlow of the American Museum of Natural History and her colleagues took a harder look. They analyzed specimens from multiple institutions, including Yale's Peabody Museum, the University of Kansas Natural History Museum and several Texas collections. After detailed comparisons of skull bones, jaw structure, tooth counts and other physical features, the team concluded that T. rex and T. proriger are genuinely different animals, not just different life stages of the same one.
Critically, some specimens of both species overlap in body size, which means size alone doesn't explain the differences. Even when a T. proriger and a T. rex specimen are roughly the same length, they can still be told apart by specific features of the jaw joint, skull bones and teeth that remain consistent within each species. One key identifier is a jaw joint with three lobes rather than the standard two, along with a specific groove on the outer rim of that same joint bone. A statistical test found a measurable difference in the height of a particular jaw bone between T. rex and three other Tylosaurus species.
What this discovery means for museum collections everywhere
Altogether, the researchers examined over a dozen specimens. The largest known individual was found not in Texas but in Wallace County, Kansas, a reminder that the species ranged beyond its home formation.
Perhaps the most sobering takeaway isn't about T. rex specifically. It's about what the discovery points to for everything else sitting in museum storage. Many North American mosasaur specimens were collected during the 1800s and early 1900s, often without precise information about where they came from or which rock layer they were pulled from. When researchers later sorted those specimens into species, they frequently did so without carefully comparing them to the original defining specimens of each species.
Other new species may be hiding in plain sight, quietly mislabeled in collection drawers across the country. Traits dismissed as signs of age or growth may actually signal separate species entirely.
As noted by BrightU.AI's Enoch, Tylosaurus rex spent millions of years as an unrecognized king. It likely won't be the last prehistoric creature to finally claim its name. The authors acknowledge several limitations in this study, including that precise locality and stratigraphic data are lacking for many historically collected North American mosasaur specimens, which makes it difficult to confirm where exactly some referred specimens originated.
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