Babbage | Tyrannosaurus's forelimbs

Useful, though probably not for dancing

T. rex’s arms were, indeed, functional. Paradoxically, though, it is still unclear what they were for

By M.K. | RALEIGH

A FULLY grown Tyrannosaurus rex would have been a magnificent sight 66m years ago on the landscape of what was then not yet North America. Huge, powerful and armed with teeth like daggers, everything about it was awesome. Except its arms, which resembled nothing so much as the twigs that sometimes fulfil that role on a child’s snowman.

What those arms were for is a puzzle. In fact, many palaeontologists argue that they were for nothing at all. In their view, Tyrannosaurus arms were vestigial organs, like the blind eyes of cave-dwelling fish.

Sara Burch, a researcher at Stony Brook University, in New York, has been investigating the question, and she has just presented her conclusions to the annual meeting of the Society of Vertebrate Palaeontology, which is taking place in Raleigh, North Carolina. Those conclusions, in a nutshell, are that T. rex’s arms were, indeed, functional. But she still does not know what they were for.

She came to this paradoxical answer by looking at muscle-attachment points in the arms and pectoral girdles of T. rex and its ancestors. She reasoned that if the arms were useless, and were thus atrophying under the influence of natural selection, this would show up in the fossil record.

Ms Burch studied T. rex and ten of its relatives. The size of the attachment points indicated how large the missing muscles were. Their arrangement let her work out what the muscles were doing. She then plotted this information onto the Tyrannosaurus family tree, to see if any atrophying trends showed up.

They didn’t. In fact, the sizes of the muscles concerned waxed and waned, apparently at random. There was certainly no steady reduction. What she did see, though, at a point on the tree about 70m years ago, just before T. rex emerged, was a sudden burst of alterations in the muscles’ actions.

The animals’ forearms, for example, increased their ability to flex in the way that a human flexes his biceps. Their ability to pull their arms close in towards their torsos was reduced. Their ability to draw their arms out away from their bodies, however, went up.

Clearly, some change in function was going on. And for function to change, there must have been function in the first place.

That does not solve the problem of what the function actually was. The arms may have been there to hold onto prey. Or they may, as a more recent suggestion has it, have had some role in signalling between individual dinosaurs. They might even have been used as levers, to help an animal that had fallen over get up again. They clearly, however, had a job to do—however stupid they might have looked.

(Photo credit: EPA)

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