This updated evolutionary timeline stems from a study published in Royal Society Open Science, which combines fossil distribution data with evolutionary trees and paleoclimatic models. Scientists now believe that the immediate precursors to T. rex arrived in North America over 70 million years ago, paving the way for its emergence on the ancient continent of Laramidia.
For decades, paleontologists have debated whether the evolutionary roots of T. rex lay in Asia or North America. Previous studies favored the theory of an Asian origin, based on anatomical similarities between T. rex and Asian tyrannosaurids like Tarbosaurus.
But this new research suggests that while its ancestors came from Asia, the species itself evolved on North American soil. Understanding this migration sheds light on dinosaur dispersal during the Late Cretaceous period and offers new perspectives on how environmental changes shaped evolutionary pathways.
Evidence Points to Asian Ancestors, but a North American Evolution
The research team, led by doctoral student Cassius Morrison at University College London, developed a model tracing tyrannosaurid fossil locations, timelines, and evolutionary relationships. Their findings indicate that T. rex‘s direct ancestor likely migrated from Asia to North America between the Late Campanian and Early Maastrichtian ages, approximately 72 million years ago.
According to Live Science, this supports the anatomical data linking T. rex more closely with Asian species like Tarbosaurus than with native North American tyrannosaurids such as Daspletosaurus. The fossils of T. rex are widely found in areas that once formed the island continent of Laramidia, including modern-day Montana, South Dakota, and Alberta.
These findings challenge a long-held assumption that the species itself arrived from Asia and instead propose a scenario in which its ancestors made the journey, with T. rex evolving locally.
Environmental Shifts May Have Favored Giant Predators
The study also investigates how T. rex and related groups like megaraptors achieved their massive body sizes. Researchers propose that both lineages began growing significantly after a major global warming event around 92 million years ago, known as the Cretaceous Thermal Maximum. This period saw a rise in atmospheric greenhouse gases, with ocean temperatures in tropical regions reaching up to 95°F (35°C).
After this climatic peak, global temperatures began to drop. According to the study, tyrannosaurids and megaraptors may have survived better than other large predators, whose extinction opened up ecological niches. “They likely grew to such gigantic sizes to replace the equally giant carcharodontosaurid theropods that went extinct about 90 million years ago,” said study co-author Charlie Scherer, as cited by the media. This adaptation allowed them to dominate the predatory landscape of the Late Cretaceous.
Fossil Record Remains Incomplete, Especially in Asia
Despite the abundance of T. rex fossils in North America, the study notes a gap in the fossil record of its direct ancestors in Asia. While dozens of T. rex specimens have been unearthed in the U.S. and Canada, fossils of its immediate precursors have yet to be found on the Asian continent. This suggests that key discoveries may still lie buried in remote parts of Asia.
Paleontologist Steve Brusatte of the University of Edinburgh, who co-authored a 2016 study supporting Asian origins for T. rex, described the new work as “a fine scholarly piece.” As reported by Live Science, Brusatte remarked that while T. rex is often considered the quintessential American dinosaur, “it was actually an immigrant.”
