The horseshoe crab may be a descendant of trilobites. According to research by paleontologists, the oldest ancestor of the horseshoe crab may be the trilobite that appeared during the Cambrian period, because its embryonic development goes through the same stages as the trilobite. The earliest horseshoe crab fossil discovered by humans belongs to the Ordovician period, which is 400 million years old. Moreover, horseshoe crabs have not undergone any obvious evolution in these years. From the appearance point of view, they have not changed much compared to 400 million years ago.

The horseshoe crab, also known as the king crab, belongs to the phylum Arthropoda, the class Limulus, and the order Xiphophora. It is a lower invertebrate marine animal that evolved from trilobites in the Paleozoic Era, so it is called a "living fossil".