The origin and evolution of the famous Tyrannosaurus rex have been a topic of fierce debate among the palaeontologists for a long time. A new study led by UCL researchers suggests that T-Rex might have evolved in North America, but its direct ancestors arose and came from Asia when the sea levels fell, providing a land bridge connecting the continents more than 70 million years ago. The study also found that the rapid growth in size of tyrannosaurids as well as a closely related group called megaraptors coincided with a cooling of the global climate following a peak in temperatures 92 million years ago.

Evolutionary Origins

According to Cassius Morrison, the lead author of the new study, the direct ancestors of the T-Rex reached North America via Bering Strait around 72 million years ago. The new research aligns with a 2016 study that found T-Rex shares more anatomical similarities with Asian tyrannosaurids like Tarbosaurus than with North American ones like Daspletosaurus.

The researchers used a model based on where and when various tyrannosaurid species had been discovered, their evolutionary trees, and local climate. They found that T-Rex fossils are widely dispersed in Laramidia, and the ancestor of T-Rex was present in both Asia and Laramidia, indicating that the T-Rex’s ancestor likely migrated from Asia to North America between the Late Campanian and the Early Maastrichtian ages, around 72 million years ago.

Reason behind the size

The study also explored why T-Rex and its cousins reached such massive sizes. Tyrannosaurids and their relatives, the megaraptors (which reached up to 33 feet), experienced a growth surge after a climatic event known as the Cretaceous Thermal Maximum (92 million years ago), when global temperatures peaked due to elevated greenhouse gases from volcanic activity.

As temperatures declined following the CTM, many other large dinosaur species went extinct, including the carcharodontosaurids. This opened ecological niches that tyrannosaurids and megaraptors filled, allowing them to grow significantly larger.