In 1983, near Kettle Top Butte in southeastern New Mexico, researchers discovered a partial skull identified as belonging to a distinct Tyrannosaurus species named Tyrannosaurus mcraeensis. This dinosaur predates the T. rex, sharing a similar size but featuring subtle differences, such as a shallower and more curved lower jaw and lower-positioned hornlets above the eyes. While 25% of the skull has been recovered, crucial parts like the braincase and upper jaws are still undiscovered. The observed disparities in T. mcraeensis challenge established notions of Tyrannosaurus evolution, influencing discussions on feeding habits and mate selection. Despite ongoing debate, this finding suggests giant Tyrannosaurus species evolved millions of years earlier than previously believed, originating in southern North America. Consequently, it prompts a reevaluation of the timeline and geographic distribution of these formidable creatures during the prehistoric era.

Paleontologist Nick Longrich from the University of Bath emphasizes the consistent differences in the various bones of T. mcraeensis compared to known T. rex specimens. Some skeptics within the scientific community attribute these distinctions to relative maturity and individual variation, adding complexity to the ongoing debate. Furthermore, disputes arise over the estimated age of the fossil, with some questioning the proposed timeline of 71–73 million years. This discovery challenges established notions, contributing to a deeper understanding of dinosaur evolution during the twilight of the prehistoric era.