Stegosaur tails packed a punch. (Paleobiology).

A mathematical analysis of a fossil stegosaur's bones leaves little doubt thatthe creature's spike-studded tail was an effective defense against predators.

Stegosaurus stenops was a 9-meter-long, 2-ton herbivore that had two rows of finlike plates running along its back and two pairs of meter-long spikes adorning its tail. Those pointy skewers, wrist thick at their base, projected backward and almost horizontally from the sides of the tail at angles of about 35[degrees] and 60[degrees], says Frank Sanders of the Denver Museum of Natural History. The slightly flattened spikes were covered with keratin, the same protein found in horns, fingernails, and claws.

Sanders and his colleagues estimate that Stegosaurus could flex its tail about 13[degrees] to the left or fight but only a few degrees up and down. Despite this limited range of motion, the animal's tail muscles probably could accelerate the tail to strike with a force of about 35 kilograms. Although that force sounds small, when concentrated at the sharp tip of a Stegosaurus' spike, it generates a pressure more than 1,000 times that of the atmosphere at sea level. That's more than sufficient to puncture tough dinosaur hide and enough to pierce bone, says Sanders. He's found a tail bone from a meat-eating Allosaurus that shows a silver-dollar-size hole and a wide gash that a Stegosaurus' spike could have inflicted.

But self-defense sometimes has a cost. The Denver museum contains fossils of a stegosaur for which a tail spike had snapped off near the base. That injury, which left fossil evidence of a massive and possibly fatal infection, may have resulted when the spike broke off inside an attacker's body, says Sanders.--S.P.