• Tyrannosaurus rex's ancestors likely migrated from Asia to North America over 70 million years ago via a land bridge.
• The rapid growth of tyrannosaurids and megaraptors coincided with a global cooling period after the Cretaceous Thermal Maximum.
• The extinction of carcharodontosaurids created an ecological niche that allowed tyrannosaurs to grow to enormous sizes.

A recent study published in Royal Society Open Science, led by University College London paleontologists, suggests that Tyrannosaurus rex's ancestors originated in Asia and migrated to North America via the Bering Strait over 70 million years ago. This finding challenges previous assumptions about the dinosaur's origins and sheds light on the evolutionary factors contributing to its immense size. The study also examines the parallel evolution of megaraptors, a related group of predatory dinosaurs.

• What: A study suggests Tyrannosaurus rex's ancestors migrated from Asia to North America via a land bridge. This migration, along with climate shifts and the extinction of other large predators, influenced the size and evolution of both tyrannosaurids and megaraptors.
• Who: Cassius Morrison (University College London Ph.D. student), Charlie Scherer (UCL Earth Sciences graduate), Dr. Mauro Aranciaga Rolando (Bernardino Rivadavia Natural Sciences Argentine Museum), Steve Brusatte (University of Edinburgh paleontologist)
• When: Migration occurred over 70 million years ago. Climate shifts and the extinction of carcharodontosaurids occurred around 90-92 million years ago. The study was published in May 2025.
• Where: Origin in Asia, migration to North America via the Bering Strait (between modern-day Siberia and Alaska). Megaraptors thrived in Gondwana (South America, Africa, Antarctica, Australia, and India).

The study provides compelling evidence for the Asian origin of Tyrannosaurus rex ancestors. The use of mathematical models helps to overcome gaps in the fossil record and supports the hypothesis that climate change and ecological factors played a significant role in the evolution of these giant predators. The divergent evolution of megaraptors in Gondwana further illustrates how environmental conditions can shape the development of different dinosaur species. The disagreement regarding the dating of Tyrannosaurus mcraeensis highlights the ongoing debates and uncertainties in paleontological research. Further fossil discoveries, particularly in Asia and the southern continents, are crucial for refining our understanding of tyrannosaurid and megaraptor evolution.

Our modeling suggests the ‘grandparents’ of T. rex likely came to North America from Asia, crossing the Bering Strait between what is now Siberia and Alaska.

Tyrannosaurs likely grew to such gigantic sizes to replace the equally giant carcharodontosaurid theropods that went extinct about 90 million years ago. This extinction likely removed the ecological barrier that prevented tyrannosaurs from growing to such sizes.

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.

It seems like tyrannosaurs were able to get big multiple times independently, when cooler climates promoted increases in size. 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.

The study offers significant insights into the origin and evolution of Tyrannosaurus rex and its relatives. While the Asian origin theory gains further support, ongoing research and fossil discoveries are essential to fully understand the complex interplay of factors that shaped these iconic dinosaurs. The study emphasizes the importance of considering climate change, ecological interactions, and geographical distribution when reconstructing the evolutionary history of extinct species.