A large tyrannosaurid dinosaur may have stalked the floodplains of what is now New Mexico nearly 74 million years ago, according to a team of paleontologists from the University of Bath, Montana State University and the New Mexico Museum of Natural History and Science.
Bistahieversor sealeyi hunting Pentaceratops sternbergii. Image credit: A. Belov / CC BY 3.0.
“The Tyrannosauridae were among the last and the largest of the predatory dinosaurs,” lead author Dr. Nicholas Longrich from the University of Bath and his colleagues wrote in their paper.
“Following the extinction of the carcharodontosaurs in the mid-Cretaceous, tyrannosaurs diversified and evolved large size, becoming the dominant predators of the latest Cretaceous in both North America and Asia.”
“By the Late Campanian, multiple groups of tyrannosaurids, including Albertosaurinae, Daspletosaurini, and Teratophonei, had achieved large sizes of 2-3 tons.”
“Their evolution culminated in the appearance of the giant Tyrannosaurus, both one of the last tyrannosaurids and the largest tyrannosaurid, and perhaps the largest known predatory dinosaur ever to evolve.”
In their study, the paleontologists examined an unusually large tyrannosaur tibia (shinbone) from the Hunter Wash Member of the Kirtland Formation, New Mexico.
The specimen measures 96 cm in length and 12.8 cm in diameter, about 84% and 78% the dimensions of the largest known Tyrannosaurus.
Based on comparisons with known tyrannosaur species, the researchers estimated the animal weighed roughly 4 to 5 tons.
“This represents the oldest known giant tyrannosaur from North America and may represent the oldest known member of the Tyrannosaurini,” they wrote in the paper.
The tibia shares several features with later tyrannosaurs, especially Tyrannosaurus rex.
The scientists considered three possibilities: that the fossil belonged to an unusually large individual of the previously-known New Mexican tyrannosaur Bistahieversor sealeyi; that it represented a previously unknown lineage of giant tyrannosaurs; or that it was an early member of the group Tyrannosaurini, which includes Tyrannosaurus rex and its Asian relatives.
After comparing the fossil with other tyrannosaurs and conducting a phylogenetic analysis, the authors concluded that the third explanation was the most likely.
“Regardless of which hypothesis is adopted, the unusual size of the Hunter Wash tyrannosaur is significant, as it represents a previously unrecognized appearance of large tyrannosaurids in the Late Campanian, and shows that they evolved earlier than previously believed,” they wrote.
For decades, paleontologists have debated where giant tyrannosaurs first evolved.
Some have argued that the lineage originated in Asia before dispersing into North America.
Others have proposed that giant tyrannosaurs emerged in the southern portion of the western North American landmass known as Laramidia.
The newly-described fossil strengthens the southern-Laramidian-origin hypothesis.
“The Hunter Wash tyrannosaur emphasizes the marked endemicity of Laramidian dinosaurs; while smaller Albertosaurinae and Daspletosaurini inhabited the north, giant tyrannosaurins occurred in the south,” the paleontologists concluded.
The team’s paper was published in March 2026 in the journal Scientific Reports.
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N.R. Longrich et al. 2026. A large tyrannosaurid from the Late Cretaceous (Campanian) of North America. Sci Rep 16, 8371; doi: 10.1038/s41598-026-38600-w
