Here's how Tyrannosaurus ended up with a big head and small arms

By Will Dunham

May 29 (Reuters) - Tyrannosaurus rex possessed a preposterously massive skull - 5 feet long (1.5 meters) and built to enable bone-crunching bite force - but presented preposterously puny arms. And many other meat-eating dinosaurs shared this mismatched combination of traits. So how did this come to be?

Researchers studying this phenomenon have now documented how ‌skull robustness in meat-eating dinosaurs started to evolve first, as plant-eating dinosaurs that they hunted became bigger soon after dinosaurs became Earth's dominant land animals, leading to a subsequent ‌reduction in the arms.

They identified five lineages of theropods - the two-legged group that spans all the meat-eating dinosaurs - in which this phenomenon independently appeared, underscoring the evolutionary advantages that drove these traits.

T. rex's tiny arms long have been the ​object of fascination, even spawning online memes mocking the fearsome predator's inability to do things like clap its hands, scratch its nose or perform a push-up.

Dinosaurs first appeared roughly 230 million years ago during the Triassic Period, then dominated the landscape during the subsequent Jurassic and Cretaceous Periods before being doomed by an asteroid strike 66 million years ago. The earliest theropods had well-developed arms useful for subduing prey, but that changed following the appearance of ever-larger plant-eaters including the long-necked sauropods.

"Body size in dinosaurs increased massively from the Triassic to the end-Cretaceous, so it's likely that the increase in body size drove ‌some theropods to shift towards using their heads more than their ⁠limbs in hunting. Effectively, the forelimbs became redundant in hunting," said Charlie Scherer, a University College London doctoral student in paleontology and lead author of the study published in the journal Proceedings of the Royal Society B.

"Natural selection will act on the traits which allow an animal to survive and ⁠thrive in its ecosystem. If that means sacrificing the size of the arms for a stronger head, which is the primary weapon for the animal, then that's likely what will happen," Scherer said.

The researchers created a new methodology for quantifying skull robustness based on traits including skull dimensions, bite force, tooth shape and cranial bone-fusion patterns. Tyrannosaurus, which lived in North America during the Cretaceous, scored the highest, followed by Tyrannotitan ​of ​Cretaceous South America.

A CLOSE ASSOCIATION

The researchers found that skull robustness was closely associated with forelimb reduction.

The theropod lineages in ​which this phenomenon occurred were: tyrannosaurs including Tyrannosaurus; carcharodontosaurs including Carcharodontosaurus of ‌Cretaceous Africa; megalosaurs including Megalosaurus of Jurassic England; ceratosaurs including Ceratosaurus of Jurassic North America and Europe; and abelisaurs including Abelisaurus of Cretaceous South America.

These lineages included apex predators that relied on large body size and strong jaws to tackle various types of large plant-eating dinosaurs including sauropods, horned dinosaurs, armored dinosaurs and duck-billed dinosaurs.

One of the earliest theropods to exhibit this phenomenon was Eoabelisaurus, which lived in Jurassic South America about 170 million years ago.

Some lineages of large theropods retained long and strong arms. These included such dinosaurs as Spinosaurus of Cretaceous Africa and Megaraptor of Cretaceous South America.

These theropods "have incredibly large and mobile arms for their body size, which suggest a more prominent role for them in hunting compared to something like T. rex," Scherer said.

Small theropods also ‌kept useful arms, including the lineage that led to birds.

For theropods like Tyrannosaurus, scientists are not sure what ​function small arms served. Not only were the length and strength of the arms of Tyrannosaurus diminished, but it ​retained only two fingers on its hands.

"Potentially, they did nothing with them - they were just ​useless. This raises the question: why did they have tiny arms, rather than no arms? If the tiny arms are still there, then it is ‌possible that they still retain some kind of function that we are ​not aware of," University College London paleontologist and study ​co-author Paul Upchurch said.

"For me, however, this is unlikely, and I think something else is going on," Upchurch said.

When an anatomical structure is no longer helpful, genetic changes can result in its reduction so the animal does not waste energy and resources building something it does not need, Upchurch said.

"But we know that genetics is complicated, and very ​often genes have more than one role. For example, a gene might ‌be involved in building something that the animal no longer needs, but the same gene might also be doing something in another part of the body that ​the animal does still need. This means that the gene is maintained because it is still doing something useful, so the useless structure persists in a reduced ​form rather than disappearing completely," Upchurch said.

(Reporting by Will Dunham in Washington; Editing by Daniel Wallis)