Jan Bartek – AncientPages.com – A recent study published in Royal Society Open Science reveals that the Tyrannosaurus rex, which evolved in North America, has its direct ancestor originating from Asia. This ancestor crossed a land bridge connecting the continents over 70 million years ago.
During the end of the Cretaceous period, the fauna in the Northern Hemisphere was primarily dominated by Tyrannosaurids, including species like Tyrannosaurus rex. Additionally, hadrosaurs and ceratopsian ornithischian dinosaurs were prevalent during this time. These groups played significant roles in their ecosystems and are key to understanding the biodiversity of that era. Credit: Pedro Salas and Sergey Krasovskiy.
The research also indicates that tyrannosaurids, including T. rex, and a related group known as megaraptors experienced rapid growth in size during a period of global climate cooling following peak temperatures 92 million years ago. This finding suggests that T. rex and its relatives may have been better adapted to cooler climates than other dinosaur groups at the time, potentially due to features like feathers or warm-blooded physiology.
The study was conducted by an international team of researchers from universities including Oxford, Pittsburgh, Aberdeen, Arizona, Anglia Ruskin, Oklahoma, and Wyoming.
“The geographic origin of T. rex is the subject of fierce debate. Palaeontologists have been divided over whether its ancestor came from Asia or North America.
Our modelling suggests the ‘grandparents’ of T. rex likely came to North America from Asia, crossing the Bering Strait between what is now Siberia and Alaska.
This is in line with past research finding that the T. rex was more closely related to Asian cousins such as the Tarbosaurus than to North American relatives such as Daspletosaurus.
Dozens of T. rex fossils have been unearthed in North America but our findings indicate that the fossils of T. rex’s direct ancestor may lie undiscovered still in Asia,” Lead author Cassius Morrison, a PhD student at UCL Earth Sciences, said in a press release.
The research team has concluded that the Tyrannosaurus rex evolved in North America, specifically in the region known as Laramidia, which was the western half of the continent where it was widely distributed. This conclusion challenges previous findings from last year that suggested a T. rex relative, Tyrannosaurus mcraeensis found in New Mexico, predated T. rex by three to five million years and indicated a North American ancestry for T. rex. The researchers argue that the dating of this T. mcraeensis fossil is not reliable.
In their new study, researchers examined how tyrannosaurids and their relatives, megaraptors, dispersed globally using mathematical models based on fossils, evolutionary trees of dinosaurs, and historical geography and climate data. These models importantly incorporate uncertainties due to gaps in the fossil record.
Due to the scarcity of their fossils, megaraptors are considered enigmatic among large carnivorous dinosaurs. Unlike T. rex, with its robust build and powerful bite, megaraptors had slender heads and long arms equipped with claws up to 35 cm (14 inches) long.
The study suggests that mega raptors were more globally distributed than previously believed. They likely originated in Asia around 120 million years ago before spreading to Europe and across Gondwana, a supercontinent including present-day Africa, South America, and Antarctica.
This implies that mega raptors may have inhabited regions like Europe and Africa where no fossils have yet been discovered. Their evolution might have diverged from that of tyrannosaurids due to different hunting strategies. While mega raptors possibly used their claws to hunt juvenile sauropods in southern Gondwana regions like South America or Antarctica, Tyrannosaurus rex hunted species such as Triceratops or Edmontosaurus within Laramidia’s ecosystem.
During the period following the Cretaceous Thermal Maximum, approximately 92 million years ago, both tyrannosaurids and megaraptors experienced significant growth in size.
This development coincided with a global cooling trend after a peak in temperatures. The extinction of carcharodontosaurids, previously dominant giant carnivores, created an ecological niche at the top of the food chain that these dinosaurs filled.
Researchers propose that tyrannosaurs—encompassing both tyrannosaurids and megaraptors—were potentially more adept at thriving in cooler climates compared to other dinosaur groups. By the end of the dinosaur era, T. rex could weigh up to nine tonnes, comparable to a very large African elephant or a light tank, while megaraptors could grow up to 10 meters in length.
Co-author Charlie Scherer, an MSci Earth Sciences graduate and soon to be PhD student at UCL, and founder of UCL’s Palaeontology Society, said: “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.
Credit: Pedro Salas and Sergey Krasovskiy.
“They 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.”
Co-author Dr Mauro Aranciaga Rolando, from the Bernardino Rivadavia Natural Sciences Argentine Museum, Buenos Aires, Argentina, said: “At the beginning of their evolutionary history, around 120 million years ago, megaraptors were part of a widespread and diverse dinosaur fauna.
“As the Cretaceous period progressed and the continents that once formed Gondwana began to drift apart, these predators became increasingly specialised. This evolutionary shift led them to inhabit more specific environments.
“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.”
The study was published in Royal Society Open Science
Written by Jan Bartek – AncientPages.com Staff Writer
