Why did T. rex have tiny arms? A new study may finally have the answer

An analysis of 85 dinosaur species shows that tiny forearms in meat-eating dinosaurs were the result of an evolutionary trade-off.

(CNN) — Scientists may have finally solved the riddle of Tyrannosaurus rex’s small arms, which have always stood out as the oddest feature in the mightiest of dinosaurs, prompting jokes and a century-plus debate on their purpose and evolutionary history.

At about 3 feet long, the arms of T. rex were less than a third of the length of the dinosaur’s legs and looked noticeably disproportionate in a body that could span more than 40 feet in larger adults.

T. rex was one of many meat-eating dinosaurs with puny arms, and over the years scientists have come up with theories for the forelimbs’ function, including holding or pinning down prey and impressing potential mates during courtship. More recent studies have suggested that the arms became smaller to reduce the risk of being bitten during feeding frenzies, while a longstanding theory is that they are simply vestigial — they had no practical purpose and therefore shrank. But a consensus is lacking.

Now, a new study published May 20 in the journal Proceedings of the Royal Society B aims to settle the debate once and for all. Based on an analysis of 85 species of dinosaurs, the study concluded that tiny arms were an evolutionary trade-off caused by another body part becoming ever larger and taking up resources — the skull.

“If you’re a dinosaur with a very strongly put together skull, chances are you’re going to have very small forelimbs,” said Charlie Roger Scherer, a doctoral student in the department of Earth sciences at University College London and the study’s lead author. “And it doesn’t really matter how big you are — you could be 1 ton in weight, or 10 tons in weight. If you have a strong skull, you’re going to have relatively small arms.”

The reason is that “evolution doesn’t like to have everything all at once,” as Scherer put it, because it tends to prioritize one thing over another. “If you want to focus on using your head to bring down large prey, you don’t really want to be putting much effort in keeping your arms long and with claws, because you’re probably not really going to need that, so evolution kind of says, ‘We don’t need the arms anymore, so let’s shrink them down and put more energy into keeping the skull strong and using that as the primary weapon.’”

Previous research already suggested a link between shrinking forelimbs and growing skulls in carnivorous dinosaurs, but the new study is the first, according to Scherer, to identify this trend in five different groups of dinosaurs and add statistical support to the theory.

A primary weapon

To reach their conclusion, researchers measured the forelimbs and the skull bones from the pool of 85 dinosaur species, using both fossils and data from existing scientific literature.

They also devised a new way of quantifying the strength of the skull, looking at factors such as overall size, how the bones fit together and bite force. Doing so allowed them to arrange every skull on a scale. Not surprisingly, T. rex scored the highest, followed by Tyrannotitan, another massive meat eater that lived in what’s now Argentina during the Early Cretaceous, or about 30 million years before T. rex.

Other than in tyrannosaurids, the group that includes T. rex and its cousins, researchers found the correlation between large, strong skulls and small forelimbs in four other dinosaur groups — ceratosaurids, megalosaurids, abelisaurids and carcharodontosaurids — all large bipedal carnivores. The identified species lived all around the world from the start of the age of dinosaurs, the Triassic, through the end of the Cretaceous, when a large asteroid impact wiped out most non-avian dinosaurs — a time span of about 180 million years.

The new analysis suggests that shrinking limbs were not a fluke, but an evolutionary trait that occurred across different, unrelated species over an extended period. The process of shrinking was different among the groups, with some dinosaurs reducing the size of the fingers first, while others prioritized shortening the forearm.

“There’s always a common driver of it,” Scherer said, “which is that they were all preying on animals that required a bit more force to bring down, which is why they developed that very strong skull.”

As their prey became larger, these animals upped the ante by making their primary weapon bigger and more powerful, draining resources away from arms and claws. “Everything was approached headfirst, so the head just became what came into contact with the prey,” Scherer said, “and that was the easiest way to bring them down, as opposed to jumping around or fighting with claws.”

The arms, however, were not completely useless, according to Scherer. “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.”

A widespread trend

The tiny arms of T. rex are well known, but outside paleontology, not many people may be aware that small forelimbs were common in other dinosaurs too, according to Stephan Lautenschlager, a vertebrate paleontologist and senior lecturer in paleobiology at England’s University of Birmingham. He was not involved with the study.

“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 into other organs,” Lautenschlager wrote in an email. “Large theropods like T. rex pursued the most efficient strategy by investing primarily in bite force and strong jaws.” Large herbivores didn’t follow suit and retained their long arms, he added, possibly because these limbs were important to grasp vegetation and in some cases also to defend against predators.

T. rex was basically a giant land shark that did all its work with its huge head, said Steve Brusatte, a professor of paleontology and evolution at Scotland’s University of Edinburgh who also did not participate in the research.

“When we look over the course of tyrannosaur evolution, we see the heads get bigger as the arms get smaller, so there was some sort of tradeoff, with the heads taking over the functions once performed by the arms, like grabbing and killing prey,” Brusatte wrote in an email.

“And it seems a similar trend happened in other giant meat-eating dinosaurs, so it was a recurring theme in dinosaur evolution, the big predators supersizing themselves, ballooning their bodies, swelling their heads to enormous sizes, and letting their arms wither away,” he added.

Andre Rowe, a paleobiologist and a senior research associate at England’s University of Bristol, agreed that the most interesting finding of the study is how widespread the small forelimb trend was.

“Tyrannosaurs usually get all the attention, but some groups like the abelisaurs evolved even more reduced arms relative to body size,” Rowe, who was not involved in the study, said in an email.

“What makes this especially fascinating is that not all predatory dinosaurs followed the same path. Some lineages retained large functional abelisaur arms, while others evolved enormous heads and tiny forelimbs,” he added.

“This study highlights just how diverse and evolutionarily innovative dinosaurs really were,” Rowe said. “They repeatedly evolved very different solutions to the same ecological challenges, which is one reason they remain so fascinating to both scientists and the public.”

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