Strong-arming the T. rex forelimb.

Strong-arming the T. rex forelimb 

It had long, powerful hindlimbs to chase after enemies, teeth that cut like steak knives, and muscle-bound jaws that could hold struggling animals in a death grip. But Tyrannosaurus rex, the largest of the carnivorous dinosaurs, also had 3-foot-long forelimbs hat seemed downright diminutive compared with the rest of its 40-foot-long body.

Paleontologists long assumed that the small size signified floppy, feeble forelimbs that served no critical function. But two researchers, armed with a comprehensive study of the most complete T. rex skeleton ever unearthed, have gone out on a limb with a new theory. These front appendages packed enormous power, capable of holding some 426 pounds of prey, they assert. Moreover, they say, the two claws attached to each forelimb faced in opposite directions and could dig into an animal like grappling hooks, immobilizing the creature until T. rex's razor-sharp teeth finished the kill.

"People had been looking at [forelimb] function based on proportionate size. I don't think that's appropriate," says Matt B. Smith, a research assistant at Montana State University's Museum of the Rockies in Bozeman. Rather than follow traditional thinking, Smith and paleontologist Kenneth Carpenter of the Denver Museum of Natural History took a new tack, examining the capacity of a T. rex forelimb to bear weight without comparing the limb to other body parts of the giant dinosaur.

From studies of a nearly intact Albertasaurus skeleton, Smith calculated that the forelimbs of this large dinosaur -- with a body build similar to T. rex -- could hold more weight than previously thought. Two years ago, the arrival of a rare T. rex forelimb from a paleontological site near Bozeman prompted him to extend his studies. He and Carpenter measured the width of the fossil's "scar" -- the spot where a tendon would attach the biceps muscle in the living dinosaur. They inferred that the biceps muscle was huge -- about the diameter of an entire human thigh -- and capable of bearing a stationary force of about 426 pounds, more than any other dinosaur biceps and roughly 10 times more than a human biceps. The finding prompted Carpenter to dub T. rex "the Schwarzenegger of dinosaurs."

As excavators at the site uncovered and identified additional pieces of the T. rex skeleton, Smith and Carpenter found themselves examining finger bones as well as the carpal and metacarpal bones that make up the forelimb. Using wax to hold the bone joints together, they discovered the forelimb's two claws have an unusual feature. Unlike the human thumb and forefinger, which meet like pincers to grasp objects, the two dinosaur claws face away from each other like the barbs of a fishing hook, each embedding separately in the doomed animal's flesh.

Smith says some researchers remain doubtful that T. rex used its forelimbs to capture prey. Skeptics reason that the forelimbs -- however mighty -- were still woefully short and would have required the dinosaur to either lower its chest on top of an animal or hug the prey to its chest in preparation for killing it. Instead, some suggest, T. rex may have used its forelimbs to pick up dead animals from the ground.