They may look ridiculous, but T. Rex’s tiny arms actually served an important purpose

The T. Rex may have evolved its tiny arms to help it get upright again after taking a tumble, a study suggests.

While it was arguably the most ferocious land predator of all time, equipped with 60 serrated, eight-inch long teeth and a crushing bite, Tyrannosaurus Rex has been mocked in the modern world for its small forelimbs, which measure just three feet long and serve no clear purpose.

But the discovery of a new dinosaur, which also had disproportionately small arms, shows that the T. Rex’s diminutive limbs may actually have been beneficial, and played a very important role.

Meraxes gigas is a species that resembles the T. Rex, despite going extinct 20 million years before it existed. The discovery of a new M. gigas fossil in Patagonia reveals both species have a huge head, sharp teeth, a long tail, weigh several tonnes and, it transpires, tiny arms.

Convergent evolution

But the two creatures are not even remotely related, occupying opposite branches of the dinosaur tree of evolution.

This, experts say, is an example of convergent evolution, where two species independently evolve the same trait and it only happens if it has a clear and significant survival advantage.

“I’m convinced that those proportionally tiny arms had some sort of function. The skeleton shows large muscle insertions and fully developed pectoral girdles, so the arm had strong muscles,” says Juan Canale, the lead researcher on the project to excavate M. gigas from the Ernesto Bachmann Paleontological Museum in Neuquén, Argentina.

One thing they almost certainly were not used for was hunting, he said, as the enormous, muscle-clad head filled with dozens of dagger-like teeth was more than sufficient for catching food.

May have used arms for reproductive behaviour

“I’m inclined to think their arms were used in other kinds of activities,” Mr Canale said.

“They may have used the arms for reproductive behaviour such as holding the female during mating or support themselves to stand back up after a break or a fall.”

The authors, writing in the journal Cell Press, also suggest that the short limbs could be part of an evolutionarily bartering agreement. Resources can be invested in either long limbs or a large head, but not both.

Thus, there are long-limbed, small-headed dinosaurs, and big-headed, short-armed creatures. However, there can not be one with both, the researchers suggest.

“The presence of multi-ton theropods with long forelimbs, but small skulls, further confirms that forelimb reduction is not a simple function of body size in theropods, but rather that it tracks some other trait, which for large predatory species is likely skull size,” they write.