Pity the hadrosaur. The duck-billed dinosaur had no horns, armor or tusks for defense when Tyrannosaurus rex was on the hunt. And it was too big to escape by climbing a tree or burrowing into the ground. To top it all off, the herbivore was slow of foot. Luckily, the layout of a hadrosaur's leg and tail muscles may have helped it escape the massive jaws of tyrannosaur predators.

T. rex would win in a sprint, but a hadrosaur would outrun it in a longer race, paleontologist W. Scott Persons argues in a study about the dinosaurs' caudofemoralis muscles, published in November 2014 by Indiana University Press. The left and right caudofemoralis of theropod dinosaurs were large tail muscles anchored to the upper leg bones. Contractions would swing a hind limb backward, propelling the dinosaur forward. T. rex fossil impressions show that its caudofemoralis muscles were attached to the femur near the hip socket. Extrapolating from 3-D computer modeling of modern-day reptiles “means the physical distance the muscle has to contract to swing the leg through a single arc is very, very short,” Persons says. Short contractions allowed it to take fast, long strides.

In contrast, the hadrosaur's similarly sized caudofemoralis muscle was attached much farther down on the femur, making the muscle contraction considerably longer, which in turn made its strides shorter and slower. Advantage: T. rex.

An extended race, however, would yield a different result. The proximity of T. rex's caudofemoralis muscle to its femur also meant the carnivore had to expend enormous amounts of energy to swing its leg, so it tired quickly. (Imagine the effort required to open and close a door if the doorknob were positioned three inches from the hinge.) For the hadrosaur, superior leverage and slower muscle contractions would mean that it tired less over great distances.

Thus, the only chance T. rex had to nab a hadrosaur was to take it by surprise, Persons notes. But unlike a big, agile cat, which can slink through high grass to sneak up on its prey, T. rex's size would have easily given it away. With a head start and the push of its leveraged legs, the duck-billed dino would live to run another day.