A living fossil could inspire tomorrow's armor. Engineers at the Massachusetts Institute of Technology, funded by the U.S. Army, investigated the primitive fish Polypterus senegalus, nicknamed the “dinosaur eel” for the suit of armor it sports. In experiments mimicking bites from a predator, the researchers found that each scale is made of three layers on a bone support that all complement one another to defy penetration. The outer coat is the hardest and most resistant to sharp teeth. The middle is softer and dissipates energy by deforming. The last layer has a plywoodlike structure, which prevents cracks from spreading. The precise sequence of these layers critically preserves armor strength—for instance, replacing the outer and middle layers in simulations increased risk of the scale coming apart. These findings, posted online July 27 by Nature Materials, could illuminate how fish evolved and lead to more effective ways of designing armor.