genus of extinct jawless fishlike vertebrates found as fossils in Early Devonian rocks (those 398 million to 416 million years old) in North America and Europe. Pteraspis was approximately 16 cm (6.5 inches) in length and had a heavy, rounded, bony shield that covered the anterior parts of the body. The remainder of the body was encased in small scales; it is probable that Pteraspis was an active, though inefficient, swimmer.