Scientists have found horseshoe crab fossils that date back 445 million years ago, making the species much older than the dinosaurs, who first arrived on Earth nearly 200 million years later. Today, there are four living horseshoe crab species (one type can be found in the Atlantic coastal waters of North America and the Gulf of Mexico; the other three can be found in the coastal waters of Asia). Since they haven’t evolved much, horseshoe crabs are often referred to as “living fossils.”