Dunkleosteus terrelli boasted a 20-foot-long body covered with bony plates of armor. Fossil records indicate that this huge fish, one of the first large jawed vertebrates in the ocean, was an aggressive predator.
The razor-sharp edges of bones in this animal's jaws served as cutters. As they rubbed against each other, the opposing jaw blades acted like self-sharpening shears. These bones continued to grow as they were worn down by use.
Hall of Vertebrate Origins