Tyrannosaurus rex ruled North America just before the mass extinction 66 million years ago that ended the "age of dinosaurs." However, according to new research, its ancestors came from Asia.

T. rex is iconic. It is sometimes called the "king of dinosaurs" because of its enormous size. It was one of the largest land predators of all time, measuring up to 13 meters long and weighing up to 9 tons.

The species is known from fossils found in North America. The most famous of these is the Hell Creek Formation in the northern United States.

Other Tyrannosaurus species also dominated their respective habitats on the northern continents.

Before Tyrannosaurus rex, the top predators were the species Dacrestosaurus and Gorgosaurus. Further north, the Albertosaurus ruled. In Asia, the top predator was the Tarbosaurus, which was almost as large as the T. rex.

The southern continents had their own massive predators that filled the same ecological niche as the tyrannosaurs in the northern hemisphere.

Paleontologists have been unable to agree on where tyrannosaurs came from.

A new article published in the journal Royal Society Open Science argues that the ancestors of Tyrannosaurus rex crossed a land bridge from Asia to North America more than 70 million years ago.

"The geographical origin of T. rex is the subject of fierce debate. Paleontologists are divided on whether its ancestor came from Asia or North America," said lead author Cassius Morrison, a PhD student at University College London (UCL) in the United Kingdom.

"Dozens of T. rex fossils have been found in North America, but our findings suggest that fossils of T. rex's direct ancestor may still be undiscovered in Asia," he explained.

Morrison's team also refuted a claim made last year that the newly discovered North American relative of T. rex, Tyrannosaurus mcraeensis, preceded it by 3 to 5 million years. This discovery seemed to suggest a North American origin.

The team claims that this T. mcraeensis fossil has not been reliably dated.

The new study also traces and models how tyrannosaurs and their cousins, the megaraptors, spread around the world.

Tyrannosaurs split into tyrannosaurids, such as T. rex, and megaraptors.

Megaraptors are a mysterious group of carnivorous dinosaurs, about which there is only scant evidence from fossils in Asia, South America, and Australia. Unlike the sturdy tyrannosaurids with small arms, megaraptors developed slender skulls and long arms.

The new study shows that megaraptors were more widespread than previously thought. They probably appeared in Asia around 120 million years ago and spread to Europe and then to the large southern supercontinent Gondwana.

The two groups probably evolved differently because of their different prey.

In the southern continents, megaraptors may have hunted young long-legged sauropods, while in the north, tyrannosaurs had to contend with large and powerful species such as Triceratops, Edmontosaurus, and Ankylosaurus.

Both tyrannosaurs and megaraptors reached gigantic sizes at approximately the same time. This period was a period of climate cooling after the peak of global temperatures about 92 million years ago.

This rapid growth was followed by the extinction of other giant carnivores, the carcharodontosaurids, which left a vacuum at the top of the food chain.

Tyrannosaurs and megaraptors were probably better adapted to lower temperatures than other groups of carnivorous dinosaurs.

"Our findings shed light on how the largest tyrannosaurs appeared in North and South America during the Cretaceous and how and why they became so large by the end of the dinosaur era," said co-author Charlie Sherrer, also from University College London.

"At the beginning of their evolutionary history, around 120 million years ago, megaraptors were part of a widespread and diverse dinosaur fauna," said co-author Mauro Aranchaga Rolando from the Bernardino Rivadavia Museum of Natural Sciences in Argentina.

"As the Cretaceous period progressed and the continents that once formed Gondwana drifted apart, these predators became increasingly specialized. This evolutionary change led them to inhabit more specific environments. While in regions such as Asia, megaraptors were eventually replaced by tyrannosaurs, in areas such as Australia and Patagonia, they evolved to become apex predators, dominating their ecosystems," he added. | BGNES