Scientists have discovered a fossilized shinbone in New Mexico that belonged to a massive tyrannosaurid that lived around 74 million years ago during the Late Cretaceous period. The tibia, measuring 96 centimeters long, suggests the animal weighed between four and five tons, making it one of the largest known tyrannosaurs of its time. This find, described in a study published in Scientific Reports, may represent the oldest known giant tyrannosaur in North America.

The fossil was unearthed in the Hunter Wash Member of the Kirtland Formation and analyzed by researchers from the University of Bath, Montana State University, and the New Mexico Museum of Natural History and Science. Its size and anatomical features closely resemble those of later tyrannosaurins, including Tyrannosaurus rex, indicating an early emergence of giant body plans in this lineage.

The discovery supports the 'southern-origin' theory, which posits that giant tyrannosaurs first evolved in southern Laramidia before spreading northward. It challenges previous assumptions about the timing and geography of tyrannosaur diversification, showing that large-bodied forms existed earlier than the fossil record previously indicated.