A New Tyrannosaurus Species Reveals the Evolutionary Path of the Tyrant King

Reconstruction of Tyrannosaurus mcraeensis (Image source：Sergei Krasinski， CC BY 4.0 )

Tyrannosaurus is the largest terrestrial predator ever discovered. Its ferocious appearance and immense power have left a lasting impression on the public imagination. As the dominant predator of North America during the final stage of the Cretaceous, Tyrannosaurus has attracted enormous scientific attention. Compared with many other non-avian dinosaurs, its fossil record is relatively abundant and well preserved, making it one of the most thoroughly studied dinosaurs.

Despite this wealth of fossils, the evolutionary history of Tyrannosaurus has long remained puzzling. Several tyrannosaurid species lived in North America millions of years before the appearance of the famous tyrant king. However, research has consistently indicated that these earlier forms represent more distant branches of the tyrannosaur family tree. Curiously, the Asian species Tarbosaurus and Zhuchengtyrannus appear to be more closely related to Tyrannosaurus itself. Because of this pattern, many researchers proposed that the ancestors of Tyrannosaurus originated in Asia and later migrated into North America. According to this hypothesis, these immigrant tyrannosaurs displaced their North American relatives and eventually evolved into the ultimate apex predator of the Late Cretaceous, the final ruler of the dinosaur world.

A recent study, however, challenges this long-standing idea. Based on tyrannosaur fossils discovered in the southern region of North America, researchers have identified a new species belonging to the genus Tyrannosaurus. For decades, the genus contained only a single species, Tyrannosaurus rex. The discovery of this new species provides fresh insight into the evolutionary history of the tyrant dinosaurs.

Tyrannosaurus mcraeensis

Discovery locality of Tyrannosaurus mcraeensis

The new species, Tyrannosaurus mcraeensis, was discovered in New Mexico in the United States. The fossils come from geological layers dating to the boundary between the Campanian and Maastrichtian stages of the Late Cretaceous, approximately 73 million years ago. The known fossil material consists mainly of parts of the skull. Detailed anatomical comparison revealed consistent differences between these fossils and those of Tyrannosaurus rex, leading researchers to recognize the specimen as a distinct species.

Although only a single specimen is currently known, Tyrannosaurus mcraeensis was clearly an enormous animal. Its preserved bones approach the size of the largest known Tyrannosaurus rex specimens, suggesting that it reached a body length of at least twelve meters. In other words, the new species was comparable in size to the famous tyrant king.

Morphological analysis indicates that Tyrannosaurus mcraeensis and Tyrannosaurus rex each possess their own distinctive derived traits. This suggests that the two species did not evolve directly from one another but instead descended from separate branches of a shared ancestral lineage. In other words, Late Cretaceous North America may have hosted at least two species within the genus Tyrannosaurus: Tyrannosaurus mcraeensis and the lineage that eventually gave rise to Tyrannosaurus rex. Both would have originated from an even earlier tyrannosaur ancestor.

A New Evolutionary Scenario

The discovery of Tyrannosaurus mcraeensis changes how scientists interpret the origin of Tyrannosaurus. The new species lived roughly five to seven million years earlier than Tyrannosaurus rex and inhabited a more southern region of North America. These two facts support the idea that tyrannosaurs may have originated in the southern part of the continent.

During the Late Cretaceous, North America was divided by a shallow inland sea known as the Western Interior Seaway. This sea separated the continent into two landmasses: Appalachia in the east and Laramidia in the west. According to the new study, the ancestors of Tyrannosaurus likely evolved in southern Laramidia. From there, one lineage dispersed into Asia, where it produced species such as Zhuchengtyrannus and Tarbosaurus. Another lineage remained in North America, eventually evolving into Tyrannosaurus. Over time, these tyrannosaurs expanded northward and occupied ecological niches previously held by other tyrannosaurid predators.

The enormous body size of Tyrannosaurus mcraeensis also carries important evolutionary implications. Its existence demonstrates that giant tyrannosaurs had already appeared in southern North America by the end of the Campanian stage. Previously, some scientists suggested that the massive size of Tyrannosaurus rex evolved in response to falling sea levels. As sea levels dropped, more land became available, potentially supporting larger ecosystems and larger predators.

However, the presence of Tyrannosaurus mcraeensis predates this sea-level decline. This indicates that the evolution of giant tyrannosaurs was not simply a response to expanding land area. Even when the available habitat was limited to southern Laramidia, tyrannosaurs were already evolving into enormous predators.

Finally, researchers propose that Tyrannosaurus mcraeensis likely preyed upon the giant herbivorous dinosaurs that inhabited the same region. These herbivores included large ceratopsians, hadrosaurs, and titanosaurs. Exactly why such massive herbivores evolved in this region remains unclear. Environmental factors such as climate, plant productivity, or seasonal variation may have played a role.

The unusual composition of this southern dinosaur community suggests that the ecosystems of Laramidia may have been highly regionalized. As a result, scientists must be cautious when using the diversity patterns of a single region to infer global dinosaur diversity just before the end-Cretaceous mass extinction.

Author: Bai Leng

Reference:

Dalman, S. G., Loewen, M. A., Pyron, R. A., Jasinski, S. E., Malinzak, D. E., Lucas, S. G., Fiorillo, A. R., Currie, P. J., Longrich, N. R. (2024). A giant tyrannosaur from the Campanian–Maastrichtian of southern North America and the evolution of tyrannosaurid gigantism. Scientific Reports.

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