An immense new species of marine reptile, a mosasaur reaching an astonishing 13.2 meters (43 feet) in length and possessing formidable serrated dentition, has been identified and christened with an unexpected moniker: T. rex, a shorthand for Tylosaurus rex.
A life-size depiction of Tylosaurus rex navigating the Cretaceous period’s Western Interior Seaway within North America. Image courtesy of Alderon Games / Path of Titans.
Mosasaurs constituted a lineage of colossal marine vertebrates that thrived during the latter part of the Cretaceous period, approximately from 100 million to 66 million years ago.
While frequently likened to dinosaurs, these creatures were, in reality, more evolutionarily proximate to extant monitor lizards and ophidians.
The mosasaur clade is typically categorized into four principal subfamilies: Mosasaurinae, Plioplatecarpinae, Tylosaurinae, and Halisaurinae. Each of these groups independently developed natatorial appendages, sinuous aquatic anatomies, and, in certain instances, prodigious dimensions.
Among these, the tylosaurine mosasaurs were distinguished by their dentition-less rostrums, attenuated caudal regions, and comparatively less ossified limbs — adaptations indicative of a pelagic lifestyle.
Furthermore, they represented the vanguard of mosasaurs to attain truly titanic proportions, surpassing a length of 8 meters (26 feet).
Fossilized remains of tylosaurines have been unearthed across Europe, Asia, New Zealand, Africa, and Antarctica, though they exhibited a pronounced abundance within North America’s Western Interior Seaway.
“The sheer scale of organisms in Texas appears to extend even to its mosasaurs, as evidenced by this discovery,” remarked the study’s principal investigator, Dr. Amelia Zietlow, a paleontological expert affiliated with the American Museum of Natural History.
Dr. Zietlow’s research initiative commenced during her doctoral studies when she encountered a mosasaur fossil within the American Museum of Natural History’s repository that had been misclassified as Tylosaurus proriger. An examination of this specimen against the acknowledged holotype of Tylosaurus proriger led her and her colleagues to the compelling conclusion that their fossil, along with over a dozen similar osteological findings housed in other academic institutions, belonged to a distinct mosasaur species.
The divergences were significant: the newly identified fossils indicated a creature of greater magnitude than Tylosaurus proriger, and they also presented finely serrated dentition — a characteristic not commonly observed among mosasaurs.
Moreover, whereas most Tylosaurus proriger specimens originate from what is presently Kansas and date back approximately 84 million years, the newly delineated fossils are primarily from Texas and are estimated to be around 4 million years more recent.
“The foundational specimen, or holotype, for the recently elucidated Tylosaurus rex is a colossal fossil that was originally discovered in 1979 adjacent to an artificial reservoir near Dallas and is currently showcased at the Perot Museum,” the paleontological team stated.
In addition to its imposing size, the novel Tylosaurus rex species exhibited a suite of anatomical features indicative of exceptionally robust jaw and cervical musculature, suggesting it was a formidable apex predator.
“Beyond its immense dimensions, roughly twice the length of the largest extant great white sharks, Tylosaurus rex appears to have been a significantly more aggressive entity compared to its mosasaur kin,” commented Dr. Ron Tykoski, a co-author of the study and the curator of vertebrate paleontology at the Perot Museum.
“Our investigations, involving the meticulous examination of well-preserved fossils recovered from across the northern Texas region, provide evidence of intraspecific aggression within this species to a degree previously undocumented in other Tylosaurus specimens.”
“A portion of this pronounced aggressive behavior is observable in a Tylosaurus rex specimen residing within the Perot Museum’s collection, affectionately known as ‘The Black Knight.’ This individual bears damage, including a missing mandibular tip and a fractured lower jaw, injuries that researchers posit could only have been inflicted by conspecifics.”
“Other well-recognized mosasaur specimens, previously identified as Tylosaurus proriger, will henceforth be classified under the designation Tylosaurus rex. These include ‘Bunker,’ a substantial fossil unearthed in 1911 and exhibited at the University of Kansas, and ‘Sophie,’ currently on display at the Yale Peabody Museum.”
The collective research findings of the team are published in the current edition of the Bulletin of the American Museum of Natural History. A direct link to the publication is available here: paper.
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Amelia R. Zietlow et al. 2026. A gigantic new species of Tylosaurus (Squamata: Mosasauridae) from Texas, and a revised character list for phylogenetic analyses of Mosasauridae. Bulletin of the American Museum of Natural History 482; doi: 10.1206/0003-0090.482.1.1
