Tyrannosaurus rex was one of the dominant predators at the end of the Late Cretaceous. Adapted from © Pedro Salas and Sergey Krasovskiy.
The ‘king’ of the dinosaurs might not be the all-American predator it appears to be.
While fossils of Tyrannosaurus rex are exclusively found in the USA and Canada, new research suggests its ancestors spread to the continent via a now-lost connection to Siberia.
A “fierce debate” over the origins of Tyrannosaurus rex may have finally been laid to rest.
While films like Jurassic Park and Jurassic World have made the dinosaur one of the most famous in the world, the origins of T. rex are surprisingly murky. Though its fossils have only ever been found in North America, it’s more similar to Asian dinosaurs like Tarbosaurus than its North American relatives.
By combining information on the fossils and evolutionary trees of tyrannosaurs and their close relatives the megaraptors, an international team of researchers has come down in favour of an Asian origin for T. rex’s ancestors.
Cassius Morrison, a PhD student who led the research, says that the dinosaurs would have reached North America more than 70 million years ago by crossing a land bridge.
“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,” Cassius explains.
“This is in line with past research finding that the T. rex was more closely related to its Asian cousins than to North American relatives such as Daspletosaurus. Our findings indicate that, while dozens of T. rex fossils have been unearthed in North America, the fossils of its direct ancestor may lie undiscovered still in Asia.”
The findings of the study were published in the journal Royal Society Open Science.
Since being named more than a century ago, Tyrannosaurus rex has become one of the most famous of all dinosaurs. © The Trustees of the Natural History Museum, London
While the common image of the tyrannosaurs is of large, meat-eating predators, they didn’t start off that way. Middle Jurassic relatives like Proceratosaurus were about three metres long and only weighed around 30 kilogrammes, which is about the same as a Dalmatian dog.
Later relatives could be even smaller. For example, the 125-million year old Dilong was just 10 kilogrammes and so would have been light enough to carry.
While larger tyrannosaurs are known from the Early Cretaceous, they wouldn't become truly gigantic until the aftermath of the Cretaceous Thermal Maximum around 92 million years ago. Volcanic activity and rising levels of carbon dioxide caused global temperatures to spike, with ocean temperatures around 5 to 7ºC higher than they are today.
The planet then cooled sharply, with the dramatic climate shifts believed to have contributed to the extinction of another group of already extremely large predatory dinosaurs called the carcharodontosaurids.
Charlie Scherer, a co-author of the paper, says that tyrannosaurs were able to exploit this vacuum at the top of the food chain.
“Tyrannosaurs likely grew to such gigantic sizes to replace the equally giant carcharodontosaurid theropods that went extinct about 90 million years ago,” Charlie explains. “This extinction likely removed the ecological barrier that prevented tyrannosaurs from growing to such sizes.”
Being better able to tolerate cooler temperatures than their rivals, tyrannosaurs grew larger to become the apex predators of the Late Cretaceous. By 68 million years ago, T. rex was 12 metres long and weighed as much as nine tonnes – heavier than an African elephant.
Megaraptors, relatives of the tyrannosaurs, are currently known from only a handful of fossils. Adapted from © Pedro Salas and Sergey Krasovskiy.
The study also sheds light on the evolution of the megaraptors, a different group of predatory dinosaurs that were closely related to the tyrannosaurs.
While their relatives developed powerful jaws to take down prey, making their arms largely redundant, megaraptors took the opposite approach. They had powerful forelimbs tipped with claws that could be as much as 35 centimetres long, allowing them to hunt dinosaurs up to the size of young sauropods.
Though it’s likely megaraptors would have been important predators, very little is known about these dinosaurs. Few fossils have ever been found and the skeletons that are known are far from complete.
By accounting for gaps in the fossil record, the researchers believe that the megaraptors probably split from the tyrannosaurs around 170 million years ago in Asia. The megaraptors then spread south to what is now Antarctica and Australia, where they became widespread between 122-110 million years ago.
Dr Mauro Aranciaga Rolando, another co-author of the study, says that the movement of these continents would have spurred further megaraptor evolution.
“As the Cretaceous period progressed and the continents that once formed Gondwana began to drift apart, these predators became increasingly specialised,” Mauro says. “This evolutionary shift led them to inhabit more specific environments.”
“While megaraptors were eventually replaced by tyrannosaurs in regions like Asia, they evolved to become apex predators in areas such as Australia and Patagonia, dominating their ecosystems.”
The team hope that their modelling will help palaeontologists to uncover more early tyrannosaurs and megaraptors, offering new insights into these iconic dinosaurs.
Find out what Museum scientists are revealing about how dinosaurs looked, lived and behaved.
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