T-Rex’s Cousin: Identifying Tyrannosaurus Mcraeensis

For decades, the Tyrannosaurus rex has reigned as the undisputed king of dinosaurs in the popular imagination. However, a significant study published in early 2024 has shaken the paleontological family tree. Researchers have identified a new, massive species of tyrannosaur—Tyrannosaurus mcraeensis—that predates the famous T. rex by millions of years. This discovery suggests that North America, specifically New Mexico, may be the true point of origin for the giant tyrant lizards.

A Case of Mistaken Identity

The fossils leading to this discovery were not dug up yesterday. In fact, a partial skull was originally excavated from the Hall Lake Formation in New Mexico back in 1983. For forty years, the specimen resided at the New Mexico Museum of Natural History and Science, categorized simply as a T. rex.

It wasn’t until a team of researchers, including Sebastian Dalman and Nick Longrich from the University of Bath, re-examined the bones that discrepancies emerged. They noticed subtle but consistent differences in the jaw and skull shape that did not align with known T. rex specimens found in Montana or South Dakota.

The Timeline Shift

The most startling aspect of Tyrannosaurus mcraeensis is its age. While the standard T. rex roamed the Earth at the very end of the Cretaceous period (approximately 66 to 68 million years ago), radio-isotopic dating of the rock layers in New Mexico places T. mcraeensis much earlier.

This new species lived between 71 and 73 million years ago. This five-to-seven-million-year gap is significant. It implies that giant tyrannosaurs evolved earlier than paleontologists previously believed. It effectively rewrites the timeline of when these predators achieved their massive sizes.

Physical Differences: How to Tell Them Apart

To the untrained eye, Tyrannosaurus mcraeensis and Tyrannosaurus rex look nearly identical. Both were apex predators roughly the same size, reaching lengths of up to 40 feet (12 meters) and weighing several tons. However, the study published in Scientific Reports highlights several distinct anatomical features that separate the two.

Key Anatomical Distinctions:

The Jawline: The most prominent difference is in the lower jaw (mandible). T. mcraeensis possesses a more slender and curved lower jaw compared to the thick, robust, and boxy jaw of the T. rex.
Lack of a Chin: T. rex is famous for a distinct chin-like projection on its lower jaw. T. mcraeensis lacks this feature entirely.
Teeth Configuration: The teeth of the new species are slightly smaller and less distinct than the bone-crushing spikes of its later cousin.
Eye Ridges: The bony ridges over the eyes (postorbital bosses) are less prominent in the New Mexico specimen than in northern T. rex fossils.

These differences suggest that while T. mcraeensis was massive, the evolutionary “arms race” that gave T. rex its unprecedented bite force (capable of crushing solid bone) was still in its early stages during the Campanian age.

Changing the Evolutionary Map

Before this identification, the prevailing theory was that the ancestors of the Tyrannosaurus lineage grew to giant sizes in Asia and then crossed a land bridge into North America roughly 67 million years ago.

The identification of Tyrannosaurus mcraeensis challenges this migration theory. Because this giant predator existed in New Mexico millions of years before the Asian migration was thought to occur, it suggests the lineage actually originated in southern Laramidia. Laramidia was an island continent that made up what is now the western coast of North America, separated from the east by a massive inland sea.

Why Did They Get So Big?

The study leads to a fascinating question: Why did they evolve to be so large in southern North America? The answer likely lies in their prey. The Hall Lake Formation also contains fossils of massive herbivores that lived alongside T. mcraeensis:

Alamosaurus: A titanosaur (long-necked dinosaur) that could grow to 100 feet long.
Sierraceratops: A large horned dinosaur similar to Triceratops.

To hunt or scavenge animals of this magnitude, a predator needed immense size. This environment likely pressured the tyrannosaurs to evolve into giants long before their descendants spread north to Montana and Canada.

Scientific Consensus and Debate

It is important to note that distinguishing dinosaur species is difficult. In paleontology, there is often a debate between “splitters” (who see new species in minor differences) and “lumpers” (who view differences as variations within the same species).

While the paper by Dalman, Longrich, and colleagues presents a compelling case for a new species based on stratigraphy (rock layers) and morphology (bone shape), some scientists argue that the differences could be due to individual variation or sexual dimorphism. However, the 5-million-year time gap remains the strongest argument that T. mcraeensis represents a distinct step in the evolutionary ladder, bridging the gap between smaller ancestors and the ultimate King of Dinosaurs.

Frequently Asked Questions

Is Tyrannosaurus mcraeensis bigger than T. rex? No, they are roughly the same size. Both species reached lengths of about 40 feet (12 meters) and weighed approximately 10 tons. T. rex generally had a heavier, more robust skull.

Where can I see the Tyrannosaurus mcraeensis fossils? The fossils are housed at the New Mexico Museum of Natural History and Science in Albuquerque.

Did T. mcraeensis fight T. rex? No. They lived millions of years apart. T. mcraeensis existed around 71–73 million years ago, while T. rex appeared around 68 million years ago. They never crossed paths.

What does the name mcraeensis mean? The name is derived from the McRae Group, the specific geological rock layers in western New Mexico where the fossil specimen was discovered.