Dinosaur sculture in Germany. Image provided through Wikimedia.

Bronze cast of a Tyrannosaurus rex wishbone (from the "Sue" specimen), at the Field Museum in Chicago.

Skeleton casts mounted in a mating position. Credit: Jurassic Museum of Asturias.

With a gargantuan head flaunting the largest teeth of any predatory dinosaur,embodies the ideal nightmarish horror. So, what was such a ferociously large animal doing with such tiny forelimbs that look more like a humorous afterthought than an evolutionary tool?“Some people thinkforearms are just vestigial organs which evolved away, but I claim no,” said Scott Lee, a professor of physics at University of Toledo, who argues that acould move its forearms quickly enough to prevent a struggling prey's escape. Therefore, the arms were an integral part of the predator’s hunting tactics, he said, and not useless stubs.One of the key pieces of evidence supporting Lee’s notion are the stress fractures paleontologists have found on the handful of wishbones they’ve recovered from fossilizedremains. Today, birds are the only animals with a wishbone, which helps them achieve flight. The boomerang-shaped wishbone, also called the furcula, of acomprises part of the forelimb, and its purpose is less understood.Lee suggests that the stress fractures resulted from extreme struggles betweenand its prey. Many scientists think that, much like large, land predators today,would run down its prey from behind, grabbing the throat with its mouth upon contact. Most land predators today will latch onto the throat until the animal suffocates to death, and Lee thinks thatmight have done the same while using its arms to restrain the animal from struggling free.“They would have had to grab something which was trying desperately to get away,” Lee said.'s forelimbs may have also helped the dinosaur to push off the ground in order to stand up from a lying position or to hold on to anotherwhile mating. But neither of these exercises would be rigorous enough to produce the level of stress fractures preserved in fossilized furcula, Lee argues. Moreover, Lee’s calculations indicate thathad the agility to grab prey with its arms, supporting his argument that the hunter would have used this ability for that exact purpose.Based on a three-dimensional representation of aforelimb, Lee calculated the acceleration at which the animal could attack with its claw-equipped hands. He determined thatcould shoot its hands out at an acceleration of 90 meters per second per second, or about nine times the acceleration due to Earth’s gravity. Other animals, like the Mantis shrimp, can strike with their limbs at 10,600 g’s.Althoughdid not have the speediest forelimbs, it still would have taken a good deal of muscle to achieve an acceleration of 9 g’s. Scientists estimate thatcould have lifted 439 pounds with help from its bicep muscles, which were about three and half times more powerful than the average human equivalent.“Why didhave arm muscles?” Lee asked. “Why were they so strong and able to accelerate so quickly? Mating would not necessarily involve such big accelerations.”Lee presented his results at a poster session last week at the APS March Meeting in Denver, Colorado. Stay tuned for more of Lee's work on the physics of different dinosaurs' speeds on Thursday.