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An artist’s reconstruction of Tylosaurus rex swimming in the Cretaceous seas of North America.(Image credit: Courtesy of Alderon Games/Path of Titans)Share this article 0Join the conversationFollow usAdd us as a preferred source on GoogleSubscribe to our newsletter
There’s a new T. rex in town, but this one didn’t hunt on land. It ruled the ancient seas.
Scientists have described a new species of mosasaur, a marine reptile lineage that coexisted with dinosaurs during the Cretaceous period (145 million to 66 million years ago). The recently named species belongs to an established genus: Tylosaurus. However, its new species designation, Tylosaurus rex — T. rex, in brief — distinguishes it from other mosasaur species within the group.
The mosasaur T. rex reached lengths of up to 43 feet (13 meters), comparable to the size of a tour bus. It possessed finely serrated teeth, remarkably powerful jaws, and fossil evidence indicating aggressive confrontations with others of its own kind.
“Everything is bigger in Texas and that includes the mosasaurs, apparently,” study first author Amelia Zietlow, a research associate of paleontology at the American Museum of Natural History in New York City, said in a statement.
A misidentified species
While examining a fossil housed at the American Museum of Natural History, Zietlow observed that a specimen cataloged as Tylosaurus proriger — a well-known mosasaur species first detailed in 1869 — did not entirely align with others of its classification. This unusual fossil was discovered in 1979 near an artificial reservoir outside Dallas.
Following a comparison of the specimen with the original type fossil of T. proriger housed at Harvard’s Museum of Comparative Zoology, Zietlow and her colleagues concluded it represented a previously undiscovered species. This specimen was not the sole misclassified fossil of this species; over a dozen other fossils at various institutions shared similar characteristics.
In contrast to T. proriger, the newly identified T. rex was 13 feet (4 m) longer, featured finely serrated teeth (a trait absent in T. proriger), and existed several million years later. The majority of T. proriger fossils have been found in what is now Kansas and date back approximately 84 million years, whereas the fossils now identified as T. rex are primarily from Texas and date to around 80 million years ago. At that time, the Western Interior Seaway extended from the Gulf of Mexico to the Arctic, hosting a diverse array of marine life, including mosasaurs.
A Tylosaurus specimen, originally found in 1979, is now on display at the Perot Museum of Nature and Science in Dallas.
(Image credit: Courtesy of the Perot Museum of Nature and Science)A violent tyrannical king
The new T. rex’s anatomy suggests that, like its mosasaur relatives, it was a formidable marine predator. In addition to its massive size, T. rex had strong jaw and neck muscles.
Some fossils bear marks of brutal injuries. One specimen in the collection of the Perot Museum of Nature and Science in Dallas, nicknamed “the Black Knight,” is missing the tip of its snout and has a fractured lower jaw. The researchers suspect the damage was inflicted by another individual of the same species.
The “Sophie” specimen, displayed at the Yale Peabody Museum, was once classified as Tylosaurus proriger and will now be reclassified as Tylosaurus rex.
(Image credit: Courtesy of the Yale Peabody 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 observed in other Tylosaurus specimens,” study co-author Ron Tykoski, vice president of science and curator of vertebrate paleontology at the Perot Museum of Nature and Science, stated in the release.
A name to remember
Other renowned mosasaur specimens previously identified as T. proriger will now be reassigned to T. rex. These include “Bunker,” a substantial specimen exhibited at the University of Kansas discovered in 1911, and “Sophie,” a specimen showcased at the Yale Peabody Museum.
The name also references an earlier concept. In the late 1960s, paleontologist John Thurmond proposed that large tylosaurs from northeast Texas might represent an unknown species. He informally dubbed them Tylosaurus thalassotyrannus, meaning “sea tyrant.”
This is not the first instance of “T. rex” having a naming complication. The terrestrial Tyrannosaurus rex almost received the name Manospondylus gigas, following paleontologist Edward Cope’s description of two partial backbones from South Dakota in 1892. Those bones were later associated with T. rex, but they were too incomplete to definitively define the species when initially described. By the time the bones were linked to T. rex, its name was already firmly established in scientific literature and popular culture.
Tyrannosaurus rex almost had a different designation.
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While the aquatic T. rex seems to have resolved its misidentification issues, the new study also addressed another persistent challenge in mosasaur research: the dataset used for studying mosasaur evolutionary relationships has remained largely unchanged for almost 30 years. By re-examining the evolutionary data on the species, the team also proposed a new evolutionary family tree among tylosaurs and argued for a broader reassessment of mosasaur evolution.
“This discovery is not just about naming a new species,” Zietlow commented. “It underscores the necessity of re-evaluating long-held assumptions about mosasaur evolution and modernizing the methodologies we employ to study these iconic marine reptiles.”
T. rex quiz: How much do you really know about the king of the dinosaurs?