T. rex’s ancestors may have walked from Asia to North America

A new study led by UCL researchers has traced the roots of Tyrannosaurus rex back to Asia. Before ruling North America as a colossal predator, T. rex likely crossed a land bridge from Asia more than 70 million years ago, according to the team.

Published in the journal Royal Society Open Science, the study not only examines T. rex’s migration but also explores the evolutionary rise of megaraptors, suggesting that climate played a significant role in their development.

Cooler Earth, bigger dinosaurs

Global temperatures peaked around 92 million years ago, but a cooling trend soon followed. This climatic shift coincided with the rapid growth in size of tyrannosaurids and megaraptors.

Despite the cooling climate, Morrison’s team found no direct link between temperature and body mass. Instead, they suggest that climatic shifts may have indirectly fueled gigantism by altering predator-prey dynamics.

“Our modelling suggests the ‘grandparents’ of T. rex likely came to North America from Asia, crossing the Bering Strait between what is now Siberia and Alaska,” said lead author Cassius Morrison, a PhD student in Earth Sciences at UCL.

T. rex came from Asia

The debate surrounding T. rex’s origins is as fierce as the predator itself. Some researchers argue that T. rex originated in Asia, while others claim North America as its birthplace.

Morrison’s findings suggest that T. rex’s direct ancestor likely migrated from Asia to North America, aligning with earlier studies linking T. rex to Asian cousins like Tarbosaurus.

But despite the wealth of T. rex fossils in North America, Morrison speculates that the direct ancestor’s fossils remain undiscovered in Asia.

New Mexico fossil in doubt

Last year, a study proposed that Tyrannosaurus mcraeensis from New Mexico predated T. rex by three to five million years. Morrison’s team disputes this, arguing that the fossil’s dating is unreliable.

Their biogeographical model instead positions T. rex as a North American genus, evolving in Laramidia, a western region that once stretched from Alaska to Mexico.

Megaraptors: Mysterious hunters

While T. rex dominated the northern continents, megaraptors thrived in Gondwana, an ancient southern supercontinent that included South America, Africa, Antarctica, Australia, and India.

With slender heads, long arms, and 35-centimeter claws, these predators took a different evolutionary path.

Morrison’s team traced megaraptors to Asia around 120 million years ago. From there, they spread to Europe, Africa, and the southern continents of Gondwana. Despite this wide distribution, fossils remain scarce, leaving gaps in the evolutionary timeline.

Megaraptor’s hunting strategy

Unlike the bone-crushing jaws of T. rex, megaraptors relied on their massive claws to dispatch prey. In southern Gondwana, they likely targeted juvenile sauropods, while T. rex hunted giants like Triceratops and Edmontosaurus in Laramidia.

Both predator groups reached colossal sizes as temperatures dropped and carcharodontosaurids – another group of large predators – vanished.

Morrison’s team suggests that the extinction of these giants opened the apex predator niche, allowing tyrannosaurids and megaraptors to expand their size.

Cold climates, new predators

Despite the apparent link between cooler climates and bigger dinosaurs, the study found no direct correlation between temperature and body size.

Instead, Morrison’s team proposes that climatic shifts may have indirectly driven gigantism by reshaping ecosystems and altering prey availability.

Giants rose after rivals fell

By the end of the Cretaceous, T. rex weighed up to nine tonnes, comparable to a large African elephant. Meanwhile, megaraptors stretched over 10 meters in length.

Charlie Scherer, co-author and a UCL Earth Sciences graduate, connects this size surge to the extinction of carcharodontosaurids.

“They likely grew to such gigantic sizes to replace the equally giant carcharodontosaurid theropods that went extinct about 90 million years ago,” he said.

Ancient continents shaped predators

Dr. Mauro Aranciaga Rolando from the Natural Sciences Argentine Museum underscores the role of Gondwana in shaping megaraptor evolution.

As continents drifted apart, megaraptors adapted to diverse environments, evolving into apex predators in Australia and Patagonia.

“While in regions like Asia, megaraptors were eventually replaced by tyrannosaurs, in areas such as Australia and Patagonia, they evolved to become apex predators, dominating their ecosystems,” Rolando said.

Rise of T. rex and megaraptors

Morrison’s study emphasizes that more fossil evidence is essential to refining the evolutionary timeline of megaraptors.

Regions like Africa and Antarctica, largely unexplored, may hold crucial clues to understanding megaraptor dispersal and gigantism.

For now, the study concludes that T. rex likely evolved as a North American genus, with Asian roots still shrouded in mystery. Meanwhile, megaraptors remain enigmatic, their evolutionary story far from complete.

The study is published in the journal Royal Society Open Science.

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