Paleontologists have unveiled a colossal new species of mosasaur: Tylosaurus rex, dubbed the “king of the tylosaurs.” This massive marine reptile reached lengths of up to 13.1 meters (43 feet), making it one of the largest mosasaurs ever known.

Fossils of this apex predator, long mislabeled as the related species Tylosaurus proriger, sat in U.S. museum collections for decades. A new study led by Amelia Zietlow of the American Museum of Natural History (AMNH), along with colleagues from the Perot Museum and Southern Methodist University, has now formally described the species. It was published in the Bulletin of the American Museum of Natural History.

A True Cretaceous Ocean Tyrant

Tylosaurus rex dominated the Western Interior Seaway approximately 80 million years ago during the Campanian stage of the Late Cretaceous. At that time, a vast inland sea covered much of what is now Texas and the central United States.

Key features that distinguish T. rex include:

• Significantly larger size than T. proriger (body length estimates 7.7–13.2 m vs. 3.9–9.5 m for the largest T. proriger).
• Powerful jaws with finely serrated teeth — a relatively rare trait among mosasaurs that likely enhanced its bone-crushing capabilities.
• Adaptations suggesting increased jaw and neck musculature, making it an even more formidable predator.

This oceanic giant was bigger and likely deadlier than its close relative T. proriger, earning it the nickname “Sea T-Rex.”

Reclassification of Museum Specimens

Many specimens previously assigned to T. proriger — including well-known fossils like “Bunker” at the University of Kansas and “Sophie” at the Yale Peabody Museum — have now been reclassified as Tylosaurus rex. The re-examination highlights how historical assumptions about mosasaur growth and variation may have hidden other undescribed species.

Does It Dethrone the Land T. rex?

While Tyrannosaurus rex ruled the land, Tylosaurus rex was the undisputed tyrant of the Cretaceous seas. With its immense size, serrated teeth, and powerful build, it terrorized marine ecosystems long before the final days of the dinosaurs.

This discovery not only adds a new giant to the roster of prehistoric marine reptiles but also underscores the importance of revisiting old museum collections with modern analytical tools.

The full scientific paper is available as open access from the AMNH.