Although fossils confirm that chelicerates developed in the sea, there is some debate over their evolutionary history. Some research suggests that animals in this subphylum are descendants of trilobites, the earliest known arthropods which lived 570 million years ago during the Cambrian period; other research suggests that segmented worms are their true ancestors. Whatever the case, the history of the horseshoe crab can definitely be traced back to the Ordovician period, about 500-440 million years ago.
Ancestors and relatives of the horseshoe crab include very diverse animals. For instance, horseshoe crabs are related to mites that never surpass.04 in (1 mm) long and to the biggest segmented animal that ever lived—the giant sea scorpion (Pterygotus rhenanus )—which grew to over 6 ft (180 cm) long. Members of this subphylum have adapted to nearly every habitat on land and sea, and most have retained their primitive behaviors.