Horseshoe crabs are marine and brackish water arthropods. Their fossil record goes back over 440 million years to the Ordovician period, with the oldest representatives of the modern family horseshoe crabs dating to approximately 250 million years ago during the Early Triassic. As such, the extant forms have been described as "living fossils". Despite their name, horseshoe crabs are most closely related to arachnids such as spiders, ticks, and scorpions.