Researchers studying the evolutionary family tree of tyrannosauroids reveal that Tyrannosaurus rex may have migrated from Asia to North America, where the species took over immediately. The work, published in Scientific Reports last month, also suggests that the short-armed, deep-skulled body plan of the colossal bone cruncher evolved in a piecemeal fashion. 

The 13-meter-long (43 feet) T. rex reigned at the top of the food chain in North America at the end of the Cretaceous some 66 million years ago. The species was one of the last survivors of a major carnivorous group called the tyrannosauroids, which originated 100 million years earlier. These dinosaurs used to be widely distributed, small, and ecologically marginal. In fact, it wasn’t until shortly before the end-Cretaceous asteroid struck that T. rex and its closest kin (called the tyrannosaurids) became enormous. 

In the last few years, many new species have been discovered, raising new questions about the origin and evolutionary relationships (or phylogenetics) of the group. To construct an updated phylogeny, University of Edinburgh’s Stephen Brusatte and Thomas Carr of Carthage College studied 366 anatomical characters of 28 tyrannosauroids. 

They discovered that the famous T. rex body plan didn’t develop rapidly. Features that enabled stronger and stronger bite forces evolved incrementally, and cranial ornamentation (such as skull crests) gradually became more elaborate.

At about 9 meters (30 feet) long, Yutyrannus and Sinotyrannus are early examples of large bodies. They had shallower skulls and smaller jaw muscles than T. rex, but Yutyrannus had a large, three-fingered arm, not a withered two-fingered forearm. So while early Early Cretaceous tyrannosauroids were capable of evolving moderately large sizes during the middle portion of their evolutionary history, colossal size – 1.4 tonnes (1.5 tons) or more – came much later, about 80 million years ago. Oddly enough, the late Early Cretaceous Xiongguanlong – the earliest to exhibit a broad U-shaped snout and other familiar features of the giant end-Cretaceous tyrannosaurids – showed no sign of developing gigantism. Its body mass was a modest 170 kilograms (375 pounds). 

Furthermore, the researchers found no clear division between northern and southern species in western North America, called Laramidia. (The east and west coasts would have been separated by a seaway.) This suggests that tyrannosaurids weren’t provincial; rather, they were dynamic and capable of spreading around.

And finally, the team reveal that T. rex may have been an Asian migrant to North America. During the last 20 million years of the Cretaceous, the large-bodied meat eaters lived on both continents. The team nested Tyrannosaurus within a sub-lineage that includes two Asian tyrannosaurids: Tarbosaurus and Zhuchengtyrannus. The three were very similar. It’s possible that the two Asian lineages dispersed from North America; it’s equally possible that Tyrannosaurus originated in Asia, then immigrated to North America. 

“In other words, it may be that T. rex was an invasive migrant species that spread across Laramidia,” the authors write. This may explain why there was just one large tyrannosaurid in North America during the late Late Cretaceous, when there were multiple over in Asia. T. rex likely pushed others out as soon as it arrived.