Evrim Yazgin
Cosmos science journalist
Tyrannosaurus rex dominated North America just before the mass extinction 66 million years ago which spelled the end of the “Age of Dinosaurs”. But its ancestors came from Asia according to new research.
T. rex is iconic. It is sometimes referred to as the “king of the dinosaurs” due to its massive size. It was one of the largest land predators of all time, measuring up to 13m long and 9 tonnes.
The species is known from fossils found in North America, most famously the Hell Creek Formation in the northern US.
Other tyrannosaurid species also dominated their respective habitats in northern continents.
Before T. rex, species Daspletosaurus and Gorgosaurus were the apex predators. Further north, Albertosaurus ruled. In Asia, the Tarbosaurus – almost as big as T. rex – was the ultimate predator.
The southern continents had their own massive carnivores which filled the same ecological niche as the tyrannosaurids in the northern hemisphere.
Palaeontologists have been unable to agree on where tyrannosaurs originated.
A new paper published in the journal Royal Society Open Science suggests T. rex ancestors crossed a land bridge from Asia to North America more than 70 million years ago.
“The geographic origin of T. rex is the subject of fierce debate,” says lead author Cassius Morrison, a PhD student at University College London (UCL) in the UK. “Palaeontologists have been divided over whether its ancestor came from Asia or North America.”
“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,” Morrison says. “This is in line with past research finding that the T. rex was more closely related to Asian cousins such as the Tarbosaurus, than to North American relatives such as Daspletosaurus.”
“Dozens of T. rex fossils have been unearthed in North America but our findings indicate that the fossils of T. rex’s direct ancestor may lie undiscovered still in Asia.”
Morrison’s team also refuted the claim made last year that newly discovered North American T. rex relative, Tyrannosaurus mcraeensis, predated T. rex by 3 to 5 million years – a finding which seemed to suggest a North American ancestry.
The team argued that this T. mcraeensis fossil was not reliably dated.
The new study tracked and modelled how tyrannosaurids and their cousins the megaraptors spread around the world.
Tyrannosaurs split into tyrannosaurids, like T. rex, and megaraptors.
Megaraptors are a mysterious group of carnivorous dinosaurs which are known from scant fossil evidence in Asia, South America and Australia. In contrast to the robust, small-armed tyrannosaurids, megaraptors evolved slender skulls and long arms.
The new study shows that megaraptors were more widely distributed than previously thought, likely originating in Asia about 120 million years ago and spreading to Europe and then throughout the large southern landmass of Gondwana.
The 2 groups likely evolved differently because of different prey.
In the southern continents, megaraptors may have hunted juvenile long-necked sauropods, while in the north the tyrannosaurids would have had to contend with large and heavily armoured species like Triceratops, Edmontosaurus and Ankylosaurus.
Both tyrannosaurids and megaraptors grew to gigantic sizes at around the same time. This period was a time of climate cooling following a peak in global temperatures about 92 million years ago.
This rapid growth followed the extinction of other giant meat-eaters, carcharodontosaurids, which left a vacuum at the top of the food chain.
Tyrannosaurids and megaraptors were probably better suited to the cooler temperatures than other groups of carnivorous dinosaurs.
“Our findings have shined a light on how the largest tyrannosaurs appeared in North and South America during the Cretaceous and how and why they grew so large by the end of the age of dinosaurs,” says co-author Charlie Scherer, also from UCL.
“At the beginning of their evolutionary history, around 120 million years ago, megaraptors were part of a widespread and diverse dinosaur fauna,” adds co-author Mauro Aranciaga Rolando, from the Bernardino Rivadavia Natural Sciences Argentine Museum in Argentina.
“As the Cretaceous period progressed and the continents that once formed Gondwana began to drift apart, these predators became increasingly specialised. This evolutionary shift led them to inhabit more specific environments.
“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.”
Read this Cosmos article to learn more about Australia’s strange assemblage of carnivorous dinosaurs.
