A colossal theropod dinosaur, potentially a large tyrannosaurid, may have roamed the riparian zones of what is presently New Mexico approximately 74 million years ago, as indicated by a collaborative research effort involving 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 family represented some of the ultimate and most substantial of the predatory dinosaurs,” stated lead author Dr. Nicholas Longrich of the University of Bath, alongside his colleagues in their published findings.
“Following the demise of the carcharodontosaurs during the mid-Cretaceous epoch, tyrannosaurs underwent diversification and achieved greater sizes, thereby establishing themselves as the apex predators of the Late Cretaceous period across both North America and Asia.”
“By the Late Campanian stage, numerous lineages within the Tyrannosauridae, encompassing Albertosaurinae, Daspletosaurini, and Teratophonei, had attained formidable body masses ranging from 2 to 3 tons.”
“Their evolutionary trajectory culminated in the emergence of the magnificent Tyrannosaurus, which stands as both one of the final tyrannosaurids and the most gargantuan within its group, possibly ranking as the largest known carnivorous dinosaur to have ever evolved.”
In the course of their comprehensive investigation, the paleontological team meticulously analyzed an exceptionally large tyrannosaur tibia (shin bone) originating from the Hunter Wash Member of the Kirtland Formation in New Mexico.
This remarkable fossil specimen measures 96 centimeters in length and possesses a diameter of 12.8 centimeters, exhibiting dimensions that correspond to approximately 84% and 78%, respectively, of those recorded for the largest known specimen of Tyrannosaurus.
Through comparative assessments with established tyrannosaur species, the researchers have projected that this ancient creature likely weighed between 4 and 5 tons.
“This discovery signifies the earliest documented occurrence of a giant tyrannosaur in North America and potentially represents the most ancient known member of the Tyrannosaurini clade,” the authors noted in their report.
The identified tibia exhibits several anatomical characteristics that are shared with later tyrannosaurs, most notably with Tyrannosaurus rex.
The scientific contingent considered three principal hypotheses for the origin of this fossil: firstly, that it belonged to an individual of extraordinary size from the previously identified New Mexican tyrannosaur species, Bistahieversor sealeyi; secondly, that it represented an entirely new lineage of colossal tyrannosaurs; or thirdly, that it constituted an early member of the Tyrannosaurini tribal group, which prominently features Tyrannosaurus rex and its Asian relatives.
Following an extensive comparison of the fossil with other tyrannosaur specimens and the execution of a rigorous phylogenetic analysis, the researchers concluded that the third hypothesis presented the most plausible explanation.
“Irrespective of which proposed explanation is ultimately accepted, the remarkable size of the Hunter Wash tyrannosaur holds considerable significance, as it points to an previously unacknowledged emergence of large tyrannosaurids during the Late Campanian epoch, underscoring their evolution occurring earlier than previously surmised,” they articulated.
For a considerable duration, paleontologists have engaged in academic discourse concerning the geographical origins of the giant tyrannosaur lineage.
Certain researchers have posited that this evolutionary branch originated in Asia before migrating to North America.
Conversely, other scholars have put forth the theory that the giant tyrannosaurs developed within the southern regions of the western North American landmass, historically referred to as Laramidia.
The recently described fossil evidence lends substantial support to the Laramidian-origin hypothesis, particularly from its southern extent.
“The Hunter Wash tyrannosaur underscores the pronounced endemism observed among Laramidian dinosaurs; while smaller taxa such as Albertosaurinae and Daspletosaurini were indigenous to the northern territories, the giant tyrannosaurins appear to have been prevalent in the southern areas,” the paleontologists concluded.
The research team’s scholarly article was formally published in March 2026 within the esteemed 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
