A new study claims to have solved the long-standing mystery of why Tyrannosaurus rex had such disproportionately small arms.

A Royal Society research paper published May 20 suggests that T. rex, which averaged more than 40 feet long with arms stretching just 3 feet, naturally evolved smaller arms as its skull grew larger.

Researchers analyzed 85 dinosaur species and found a consistent correlation among five groups — Tyrannosaurus, ceratosaurids, megalosaurids, abelisaurids and carcharodontosaurids — each showing a pattern of larger skulls and shorter forelimbs.

While research had established that large predatory theropods tended over time toward larger bodies, larger heads, and shorter arms, the reason this pattern recurred across multiple predatory dinosaur families, which were separated by millions of years and spread across the globe, had remained unknown.

The study suggests that as prey grew bigger, tyrannosaurs and other large predatory dinosaurs evolved to use their powerful jaws as their primary weapon, leaving their arms with little evolutionary purpose.

“Reduced/vestigial forelimbs evolved in at least five theropod lineages in concert with increased cranial robusticity and gigantism,” the Royal Society said.

Among the dinosaurs studied, T. rex had the largest skull, and lead author Charlie Roger Scherer, a doctoral student in Earth sciences at University College London, said that as the head became the dinosaur’s primary weapon, its arms grew increasingly less useful.

“Everything was approached headfirst, so the head just became what came into contact with the prey,” Scherer told CNN. “That was the easiest way to bring them down, as opposed to jumping around or fighting with claws.”

Scherer added that “evolution doesn’t like to have everything all at once,” and since T. rex and other apex prehistoric predators focused on using their heads to “bring down large prey,” nature simply stopped investing in keeping their arms long and clawed.

Researchers also found that forelimbs shrank differently across species — in tyrannosaurids, each forelimb element reduced at a comparable rate — and concluded that different species likely arrived at the same outcome through different developmental pathways.

Scherer noted that while T. rex’s arms shrank over thousands of years, they weren’t completely useless.

“They obviously served some kind of function, otherwise they wouldn’t have them,” he said. “What that function is exactly, I don’t know, but hopefully we can find that out with a bit more work.”

Stephan Lautenschlager, a vertebrate paleontologist and senior lecturer in paleobiology at the University of Birmingham in England, told CNN that, unlike T. rex, large herbivores did not follow the same trend and retained their long arms to possibly grasp vegetation and defend against predators.

“In animals, investing energy in the growth of different organs and parts of the skeleton is very costly. If some organs like the forelimbs play a lesser role, it may become more beneficial to reduce the size of these and invest in other organs,” Lautenschlager said.

Tiny arms aside, T. rex is believed to have been Earth’s largest land predator, growing to over 40 feet long, weighing between 8 and 14 tons, and capable of crushing bone with over 12,000 pounds of bite force, according to the Smithsonian.

The massive carnivore dominated the forests and floodplains of what is now western North America for roughly 2 to 3 million years — until an asteroid slammed into Mexico’s Yucatán Peninsula 66 million years ago, wiping out three-quarters of all life on Earth virtually overnight.