The tibia suggests that the dinosaur’s ancestors came from North America
When it comes to identifying fossil species, a solitary leg bone doesn’t amount to much. Yet a new analysis of a large fossil tibia suggests it offers a clue to the origins of Tyrannosaurus rexthe towering, sharp-toothed apex predator that dominated the twilight of the dinosaur age.
THE the size of the bone suggests that it was a tyrannosaurusa group that includes the most massive members of the tyrannosaur family treesay the researchers in a study published on March 12 in Scientific reports. Tyrannosaurids lived in the Late Cretaceous, between 83 and 66 million years ago, and were only found in Asia and North America. This tibia was discovered in rocks approximately 74 million years old.
T. rex evolved in the latter part of the Cretaceous period in what is now northern North America about 68 to 66 million years ago, the youngest, largest and most specialized predator of the group. Yet the origins of the iconic dinosaur are obscure. The most widely accepted hypothesis is that its large ancestors migrated via a land bridge from Asia; which is supported by T. rex striking similarity with Tarbosaurusa tyrannosaurid that lived in what is now Mongolia and China.
But a large tyrannosaurid living a few million years earlier in southern North America supports a different hypothesis, explains Nick Longrich, a paleontologist at the University of Bath in England. Instead of coming from Asia, tyrannosaurids lived in what is now southern North America. may have migrated northhe said.
The tibia, which is about 96 centimeters long, is part of a set of bones found in the Kirtland Formation of New Mexico and housed for decades at the New Mexico Museum of Natural History and Science in Albuquerque. The bone was surprisingly massive, much more so than those of ancient tyrannosaurs such as Albertosaurus found elsewhere in North America, Longrich says.
“He was a big bruiser on the shin,” Longrich said. The team estimated that the creature it belonged to must have had a body mass of around 4.5 tonnes. For comparison, Albertosaurus weighed up to 3 metric tons, while T. rex weighed up to 9 metric tons.
The owner of the tibia was perhaps “small by Tyrannosaurus standards, but maybe 50 percent larger than anything we knew at that time,” Longrich says. “Just really big.”
But it’s still just a leg bone, other researchers say, and that’s simply not enough to draw definitive conclusions about what type of animal it belonged to, let alone questions of T. rex origins. “These are pretty incredible claims about a single bone that is not well preserved,” says Thomas Carr, a paleontologist at Carthage College in Kenosha, Wisconsin, who was not involved in the study.
Carr says he’s not convinced there’s enough evidence to suggest the bone must have belonged to a tyrannosaurus, as opposed to, say, Bistahieververtera smaller tyrannosaur nicknamed “Bisti Beast” and which was already known to live at the same time and in the same place. “In my opinion, the null hypothesis is that the tibia came from a large, heavy body. Bistahieververtersince no other tyrannosaurid is known in this geological unit.
The new study suggests that the leg bone is both too big and poorly shaped to belong there. Bistahieververterbut the leg bones of tyrannosaurs are delicate, Carr says. The leg bones of juvenile tyrannosaurids such as T. rex are known to be markedly different from adult leg bones, in that they are thinner and more curved. As the creature grows, its leg bones swell to support the animal’s weight, otherwise they break. “Functionally, [these creatures are] all the same: they run around killing things, then get old and big and go around killing things.
“The bottom line,” Carr says, “is that they haven’t convincingly demonstrated that the similarities between this tibia and those of tyrannosaurids are not simply a consequence of large size.”