New species of massive mosasaur with serrated teeth is named T. rex

There’s a new T. rex in the fossil record, only this one terrorized the ancient seas. New research led by scientists at the American Museum of Natural History (New York City, NY), the Perot Museum of Nature and Science, and SMU uncovers a new, massive species of mosasaur, a marine reptile that lived during the age of the dinosaurs. One of the largest mosasaurs known to date—stretching up to 43 feet long—this top predator was described from 80-million-year-old fossils that were found primarily in northern Texas over the span of several decades. It was named Tylosaurus rex, or T. rex for short, meaning “king of the Tylosaurus.” The holotype fossil—the name-bearing specimen—for this new species is part of the Perot Museum’s collection and is exclusively on view in the Life Then and Now exhibit hall.

“Active paleontological research is central to the work we do at the Perot Museum, and as a leading science and nature institution, we are committed to sharing these discoveries as they emerge,” said Dr. Linda Silver, Eugene McDermott chief executive officer of the Perot Museum. “We are particularly excited to have not only contributed to the important research on the T. rex, but to be the home of the holotype sharing this new specimen with our community. This commitment to access and research is especially important for prehistoric animals native to North Texas, many of which can be seen in our Museum halls.”

Amelia Zietlow, a research associate at the American Museum of Natural History who is now at the History Museum at the Castle in Wisconsin, 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 research collection that appeared to be misidentified as Tylosaurus proriger.

After comparison with T. proriger’s holotype fossil which was described more than 150 years ago and is in the collections at the Harvard Museum of Comparative Zoology, Zietlow and colleagues suspected that the American Museum of Natural History specimen, as well more than a dozen similar fossils held at other institutions, were a different animal. Larger in stature than T. proriger, these fossils also have finely serrated teeth, a trait that’s uncommon among mosasaurs. 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 fossils are predominantly from Texas and are 4 million years younger.

“Everything is bigger in Texas and that includes the mosasaurs, apparently,” said Amelia Zietlow, lead author of the new study, which was published by the Bulletin of the American Museum of Natural History.

The researchers gave the name T. rex to this group, an homage to paleontologist John Thurmond, who, in the late 1960s, first recognized that tylosaurs from northeast Texas stood out for their size and might belong to a new species. Thurmond, at that time an SMU paleontology graduate student, informally referred to them as “Tylosaurus thalassotyrannus,” or “sea tyrant,” along with a note acknowledging the cliché.

The holotype for the newly described T. rex is a giant specimen displayed at the Perot Museum that was first discovered in 1979 at Lake Ray Hubbard near Dallas. Beyond T. rex’s impressive size, ranging from 25 feet to 43 feet—about the length of a school bus—the new species had a suite of adaptations for exceptionally strong jaw and neck muscles, suggesting that it was a powerful predator.

“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 Dr. Ron Tykoski, vice-president of science & 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,” which 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 on display at the University of Kansas that was discovered in 1911, and “Sophie,” which is on display in 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. These results suggest that mosasaur relationships need to be re-examined, as most prior studies relied on the same, minimally modified dataset for decades.

“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.”

Co-author Dr. Michael Polcyn, an SMU paleontologist, added: “These findings reshape both the physical and evolutionary picture of mosasaurs, underscoring Texas as a key region for understanding ancient marine ecosystems and signaling a new era of research into the evolutionary history of these formidable predators.”

Many of the fossils in this study were discovered by amateur fossil hunters in the north Texas area, underscoring the important role of the citizen scientist. The Perot Museum and Shuler Museum of Paleontology at SMU paleontologists have worked closely with the amateur community for decades to ensure these important fossils are preserved in perpetuity for research and education.  

Support for the T. rex research was provided in part by the National Science Foundation, Grant # 1938103, the Dallas Paleontological Society, the Society of Systematic Biologists, the Richard Gilder Graduate School, the Gingrich Fund, and the Carter Fund.