The Daily Beast has an article about T-Rex's tiny arms. What were they used for, grabbing, slashing, mating? Steven Stanley, a geologist at the University of Hawaii at Manoa, presented a paper at the Geological Society of America's annual meeting.
The conventional wisdom, as far as T. rex arms go, is that they atrophied over evolutionary time as natural selection in the animals came to favor the bitey, bitey over the grabby, grabby. Tyrannosaurus arms may have ultimately had some use, but it wasn’t much of one. They simply had too little strength and not enough reach to do a whole lot.
Sure, some theories of function have been floated. Perhaps they used them to help themselves up from a squat, or to grasp prey in a fight, or to hold on to a mate during sex. Stanley finds these proposals largely unsupported, and frankly laughable. He, on the other hand, presents five bits of anatomical evidence that suggest that T. rex arms were custom-made for raking prey to shreds.
Notably, Tyrannosaurus has two fingers on each hand, when its close relatives tended to have three. This, Stanley suggests, would give each claw 50 percent more slashing power per unit of force. Having two instead of one provides for increased stability at the wrist. Also, the T. rex claw has a similar shape to a bear claw, and bears are the large-bodied slashers of the modern world.
In Stanley’s telling, a T. rex kill looked something like this: The predator ambushes the prey, perhaps picking a sick or young animal as an easy target, and jumps on its back. Now at close range, it lets lose those claws, inflicting several wounds in a matter of seconds. “Things would not have recovered from this,” he said. “They would have been in great pain, they would have suffered blood loss. Nerves would have been slashed, blood vessels, muscles… it would have been hideous.” Now, the prey is too injured to get away, and the T. rex goes in with its primary weapon—its impressive jaws.
