Tylosaurus rex: The 13-Metre “T. rex of the Seas” That Ruled the Late Cretaceous Oceans 80 Million Years Ago

In a May 21, 2026 paper in the Bulletin of the American Museum of Natural History, paleontologists have formally named Tylosaurus rex—a colossal new mosasaur species that prowled the Western Interior Seaway of North America 80 million years ago. Reaching lengths of up to 13.1 metres (43 feet), this apex predator is one of the largest mosasaurs ever documented and has earned the nickname “T. rex of the seas.”

Led by Amelia Zietlow (AMNH), the team re-examined more than a dozen fossils long housed in museum collections, many previously ᴀssigned to T. proriger. Detailed morphological analysis revealed consistent differences in skull architecture, jaw musculature, and vertebral proportions that cannot be explained by growth stage. The new species possesses reinforced jaw and neck muscles, serrated teeth, and a suite of traits suggesting powerful bite forces and rapid prey capture.

“This is the king of the tylosaurs,” Zietlow stated. “At over 13 metres, it dwarfed most contemporaries and dominated the marine food web alongside sharks, plesiosaurs, and giant fish.”

The holotype, now on display at the Perot Museum, comes from northern Texas. Body-size estimates range from 7.7 to 13.2 metres, consistently larger than the biggest T. proriger specimens. The discovery underscores that tylosaurines were the first mosasaur clade to achieve true gigantism and forces a re-evaluation of Late Cretaceous marine predator guilds.

After decades misfiled as a single species, Tylosaurus rex has emerged as a distinct, fearsome ruler of the ancient seas—proving that the oceans had their own tyrant king long before the dinosaur T. rex roamed the land.