Giant Mosasaur Species from Texas Named T. rex

When someone mentions T. rex, you’re likely to think about the giant apex dinosaur Tyrannosaurs rex.

But have you heard about T. rex the ancient marsupial lion (Thylacoleo rex)? Or T. rex the blind, cave-dwelling salamander (Typhlomolge rex)? Or T. rex the parasitic leech (Tyrannobdella rex)?

Now, researchers are adding a new T. rex to the taxonomy books, and it might be every bit as fearsome as the original “tyrant king.” 

Growing up to 43 feet long with serrated teeth, the newly described Tylosaurus rex, or “king of the tylosaurs,” is one of the largest mosasaurs known to date. This massive marine reptile lived during the age of dinosaurs and its description is based on 80-million-year-old fossils found primarily in northern Texas decades ago. The study was published today by the Bulletin of the American Museum of Natural History.

A reconstruction of Tylosaurus rex in the Cretaceous-era Western Interior Seaway of North America that was created by Alderon Games with scientific consultation from the study’s lead author Amelia Zietlow. 
Courtesy of Alderon Games - Path of Titans

“Everything is bigger in Texas, and that includes the mosasaurs, apparently,” said Museum Research Associate Amelia Zietlow, lead author of the new study, who is now at the History Museum at the Castle in Wisconsin. 

Zietlow began this work as a comparative biology Ph.D. student in the Museum’s Richard Gilder Graduate School, when she came across a mosasaur fossil in the Museum’s research collection that appeared to be misidentified as the mosasaur Tylosaurus proriger.

Zietlow and colleagues from the Perot Museum of Nature and Science in Dallas and Southern Methodist University compared this specimen with T. proriger’s holotype fossil, the name-bearing specimen that was described more than 150 years ago and is in the collections at the Harvard Museum of Comparative Zoology. This led them to suspect that the Museum specimen, as well more than a dozen similar fossils held at other institutions, were a different animal.

Among the evidence, the researchers found that the outlier fossils are larger than T. proriger, ranging from 25 feet to 43 feet—about the length of a school bus. They also have finely serrated teeth, an uncommon feature for mosasaurs, along with a suite of adaptations for exceptionally strong jaw and neck muscles, suggesting that it was a powerful predator.

And while the majority of T. proriger specimens are found in what is now Kansas and are estimated to be about 84 million years old, these other mosasaur fossils are predominantly from Texas and are 4 million years younger. The researchers gave the name T. rex to this distinct apex predator.

The holotype for the newly described T. rex is a giant specimen displayed at the Perot Museum that was first discovered in 1979 along an artificial reservoir near Dallas.

“Besides being huge, roughly twice the length of the largest great white sharks, T. rex appeared to be a much meaner animal than other mosasaurs,” said study co-author Ron Tykoski, vice-president of science and curator of vertebrate paleontology at the Perot Museum. “Through our study and examination of well-preserved fossils collected throughout the north Texas region, we have evidence of violence within this species to a degree not previously seen in other Tylosaurus specimens.”

Some of this aggressive behavior can be seen in a T. rex specimen housed in the Perot Museum’s collection, nicknamed “The Black Knight.” It is missing the tip of its snout and has a fractured lower jaw, damage that researchers say could only be inflicted by its own species.

Other well-known mosasaur specimens that were previously known as T. proriger and will now take the name T. rex include “Bunker,” a massive specimen discovered in 1911 that is on display at the University of Kansas, and YPM 64607, sometimes called “Sophie,” which is on display in the Yale Peabody Museum.

Mosasaur specimens that were previously known as T. proriger and will now take the name T. rex include YPM VP 64607, sometimes called “Sophie,” which is on display in the Yale Peabody Museum.
Courtesy of the Yale Peabody Museum

The research also addresses a long-standing problem in mosasaur evolutionary studies. The dataset traditionally used to analyze relationships among mosasaurs has remained largely unchanged for nearly three decades. As part of the new T. rex study, the researchers assembled a comprehensively revised dataset and a new arrangement of evolutionary relationships among tylosaurs.

“This discovery is not just about naming a new species,” Zietlow said. “It highlights the need to revisit long-standing assumptions about mosasaur evolution and to modernize the tools we use to study these iconic marine reptiles.”