The discovery of Tyrannosaurus mcraeensis, a sister species of the famous Tyrannosaurus rex, suggests that tyrannosaurid dinosaurs originated on Laramidia, an island continent that existed between 100 and 66 million years ago and stretched from modern-day Alaska to Mexico.

“Tyrannosaurid dinosaurs were the dominant predators in North America and Asia during the Late Cretaceous epoch,” said Dr. Sebastian Dalman from the New Mexico Museum of Natural History and Science and his colleagues.

“Evolving from small-bodied ancestors in the mid-Cretaceous, they became apex predators in the latest Cretaceous, and finally saw the appearance of T. rex.”

“T. rex, characterized by a robust skeleton and powerful, bone-crushing jaws, was the dominant carnivore in the Late Maastrichtian of western North America.”

“Growing to 12 m long and 10 tons in weight, T. rex was the largest terrestrial predator of its time, and perhaps of all time.”

“Where and when Tyrannosaurini (T. rex and kin) originated remains unclear. Competing hypotheses place their origins in Asia, or western North America (Laramidia).”

The newly-identified species, Tyrannosaurus mcraeen, lived in what is now the United States between 71 and 73 million years ago, or between 5 and 7 million years before T. rex.

The dinosaur’s fossilized partial skull was found in the Hall Lake Formation of New Mexico.

Although the specimen was initially assigned to T. rex and was comparable in size to those of T. rex, the paleontologists propose that it belongs to a new species due to the presence of multiple subtle differences in the shape of, and joins between, the skull bones of the specimen and T. rex.

An analysis of the relationships between Tyrannosaurus mcraeen and other theropod dinosaur species indicates that Tyrannosaurus mcraeen may have been a sister species to T. rex, making it the closest known relative of T. rex.

The authors suggest that Tyrannosaurini may have originated in southern Laramidia, an island continent that existed between 100 and 66 million years ago and stretched from modern-day Alaska to Mexico.

Additionally, they propose that Tyrannosaurini may have evolved a giant body size by approximately 72 million years ago, alongside other giant dinosaurs from southern Laramidia such as ceratopsians, hadrosaurs, and titanosaurs.

They speculate that the evolution of giant tyrannosaurins may have been driven by the giant body sizes of herbivores that they preyed on in southern Laramidia.

“Evolution of giant tyrannosaurs in southern North America, alongside giant ceratopsians, hadrosaurs, and titanosaurs suggests large-bodied dinosaurs evolved at low latitudes in North America,” they said.

The study was published in the journal Scientific Reports.

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S.G. Dalman et al. 2023. A giant tyrannosaur from the Campanian–Maastrichtian of southern North America and the evolution of tyrannosaurid gigantism. Sci Rep 13, 22124; doi: 10.1038/s41598-023-47011-0