T. rex ancestors crossed from Asia to North America via land bridge 70 million years ago, says study

CNN

06-05-2025

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The direct ancestors of Tyrannosaurus rex would have arrived in North America after crossing a land bridge from Asia, according to a new study. The report is the latest to weigh in on the fierce debate among paleontologists over the origins of the king of the dinosaurs.
A team led by Cassius Morrison, a doctoral student of paleontology at University College London, or UCL, used mathematical modeling to conclude that T. rex precursors likely arrived in North America after crossing the Bering Strait between modern-day Siberia and Alaska around 70 million years ago.
The finding tracks with past research that suggests T. rex was more closely related to the large carnivore Tarbosaurus in Asia compared with top predators in North America such as Daspletosaurus, Morrison said in a statement.
At the time, the area would have been home to temperate rainforests, with a climate somewhat similar to British Columbia today, Morrison told CNN on Tuesday.
T. rex ancestors — tyrannosaurids — would have been fewer in number within their environment compared with the herbivorous dinosaurs they preyed on, much like apex predators such as lions are today, Morrison said.
'And because they are fewer, there are also fewer chances for them to then be preserved in the fossil record,' he said.
Faced by this lack of evidence, Morrison and his coauthors instead used mathematical models that incorporate data from the existing fossil record and the T. rex family tree, as well as climatic and environmental conditions, Morrison said.
The modeling also accounts for gaps in the fossil record, meaning that it can be updated if new discoveries are made in future research, he added. For example, Morrison said the new study's findings suggest fossils of these T. rex ancestors may still remain undiscovered in Asia.
The team also found that tyrannosaurids such as T. rex experienced a rapid increase in size during a period in which global temperatures were falling, suggesting that these dinosaurs were better able to thrive in cooler climates, perhaps thanks to their feathers or the fact that they were more warm-blooded.
The rapid growth in size also came after another group of giant meat-eating dinosaurs known as carcharodontosaurids went extinct, leaving 'a vacuum at the top of the food chain,' according to a news release from UCL on Tuesday.
This growth meant that, by the time dinosaurs went extinct, T. rex could have weighed as much as 9 metric tons, 'about the same as a very large African elephant or a light tank,' according to the release.
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Study coauthor Charlie Scherer, a master's graduate in Earth sciences and soon-to-be doctoral student at UCL, said in a statement that the 'findings have shined a light on how the largest tyrannosaurs appeared in North and South America during the Cretaceous (Period) and how and why they grew so large by the end of the age of dinosaurs.'
'They likely grew to such gigantic sizes to replace the equally giant carcharodontosaurid theropods that went extinct about 90 million years ago,' Scherer said.
'This extinction likely removed the ecological barrier that prevented tyrannosaurs from growing to such sizes.'
Steve Brusatte, a paleontologist at the University of Edinburgh in Scotland who was not involved in the study, told CNN that the paper 'is fine scholarly work that forensically tracks tyrannosaurs and other meat-eating dinosaurs over time, and compares their evolution with changes in climate.
'Even the very largest and most dominant dinosaurs were affected by the weather. It seems like tyrannosaurs were able to get big multiple times independently, when cooler climates promoted increases in size,' he said.
'It was easier to be big when temperatures were cool. The kings of the dinosaurs were not predestined to rule, but were helped along by the climate,' Brusatte added.
The study was published in the journal Royal Society Open Science.