Tyrannosaurus rex, the iconic apex predator of the Late Cretaceous period, evolved in North America after its direct ancestors crossed a land bridge from Asia more than 70 million years ago, according to a new study led by researchers at University College London (UCL). The findings shed light on the long-standing debate among paleontologists regarding the geographic origins of T. rex.
"The geographic origin of T. rex is the subject of fierce debate. Paleontologists have been divided over whether its ancestor came from Asia or North America," said Cassius Morrison, a PhD student at UCL Earth Sciences and the lead author of the study, as reported in Live Science.
The research team utilized mathematical models that incorporated fossils, dinosaurs' evolutionary trees, and the geography and climate of the time to trace the movements and evolution of tyrannosaurs.
"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," Morrison added.
Once in North America, T. rex evolved and became widely distributed across Laramidia, the western half of the continent during the Late Cretaceous period. Most T. rex fossils have been discovered in the U.S. states of Montana and South Dakota, as well as in Alberta, Canada. The species lived between 67 to 66 million years ago, reaching immense sizes of up to 12.5 feet tall at the hips and up to 41 feet long, weighing as much as nine tonnes.
"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," said co-author Charlie Scherer, an MSci Earth Sciences graduate and soon-to-be PhD student at UCL.
The study also modeled how tyrannosaurids—the group that included T. rex—and their cousins the megaraptors grew to gargantuan sizes. Both groups experienced rapid increases in size coinciding with a cooling of the global climate following a peak in temperatures 92 million years ago, known as the Cretaceous Thermal Maximum (CTM). During the CTM, sea surface temperatures in some tropical regions may have reached 95 degrees Fahrenheit (35 degrees Celsius) due to a spike in atmospheric CO₂ and methane from volcanic and tectonic activity.
After the CTM, atmospheric greenhouse gas levels and global temperatures fell, creating a cooler climate. The researchers suggested that tyrannosaurs, both tyrannosaurids and megaraptors, may have been able to better exploit these cooler temperatures than rival dinosaur groups. "This extinction likely removed the ecological barrier that prevented tyrannosaurs from growing to such sizes," Scherer added.
Megaraptors, a closely related group of large, meat-eating dinosaurs, experienced similar rapid growth in size, evolving to lengths of about 33 feet around the same time as tyrannosaurids. Megaraptors are considered the most mysterious of the large theropods, as few fossils have been found. They evolved slender heads and arms as long as a person is tall, with claws up to 35 centimeters (14 inches) long.
"At the beginning of their evolutionary history, around 120 million years ago, megaraptors were part of a widespread and diverse dinosaur fauna," said co-author Dr. Mauro Aranciaga Rolando from the Bernardino Rivadavia Natural Sciences Argentine Museum in Buenos Aires, Argentina. "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," added Aranciaga Rolando.
The study concluded that megaraptors were more widely distributed across the globe than previously thought, spreading throughout the large southern landmass of Gondwana, including present-day Africa, South America, and Antarctica. This suggests that megaraptors lived in parts of the world where no megaraptor fossils have been found so far.
The researchers proposed that both tyrannosaurids and megaraptors evolved differently to adapt to their environments. Megaraptors may have developed killing claws rather than a powerful bite, unlike their tyrannosaurid cousins. The better adaptation to cooler climates might be due to having feathers or a more warm-blooded physiology.
"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," said Steve Brusatte, a paleontologist at the University of Edinburgh who was not involved in the study, according to Live Science.
The study, published in Royal Society Open Science, provides new insights into how tyrannosaurs moved around the globe and evolved into some of the largest predators of their time. The international research team involved researchers from multiple universities, including Oxford, Pittsburgh, Aberdeen, Arizona, Anglia Ruskin, Oklahoma, and Wyoming.
The article was written with the assistance of a news analysis system.