Past and Present

Living Fossils

The horseshoe crab has remained virtually the same for millions and millions of years. In virtue of that fact, the horseshoe crab, which is estimated to have inhabited the Earth now for over 300 million years, is often called a "living fossil."

Trilobite Courtesy UD Department of Geology

Most scientists believe that horseshoe crabs are the closest living relative of the trilobite, a marine animal that has long been extinct. Scientists believe that horseshoe crabs were among the dominant creatures some 300 million years ago. There may have been many different species of horseshoe crabs.

Today, four species of horseshoe crabs inhabit the earth's oceans. Limulus polyphemus is found along the western Atlantic and Gulf coasts from southern Maine to the Yucatan Peninsula, with the Delaware Bay as the center of the population.

Tachypleus gigas and Carcinoscorpius rotundicauda are found in the Indo-Pacific region from the Bay of Bengal to Indonesia and Borneo. Tachypleus tridentatus ranges from the Philippines to the southwestern seas of Japan.

Continents on the Move

Our world looked very different 350 million years ago. The continents were merging into a large land mass called Pangea (click here to see what it looked like). A tropical climate enveloped North America, Europe, and Southeast Asia. Warm shallow seas and coastal lagoons surrounded these areas, perfect habitat for horseshoe crabs. Where in the world did horseshoe crabs live at that time? Although the fossil record of horseshoe crabs is limited because horseshoe crab shells are made of chitin (a cellulose-like compound that breaks down quickly over time), some fossils of horseshoe crabs and their tracks have been found. Remember, you need to envision the world as it looked when the fossils were being created — over time the fossil record has moved along with the Earth's land masses.

As the European land mass began to form, shallow seas disappeared. The ensuing loss of habitat may have forced the horseshoe crabs to migrate east and west to the places we find them today. Changes in habitat initiated the movement, but regional differences in hydrology and geology have helped to define the current distribution of horseshoe crab species. Water temperature influences the dispersal of horseshoe crabs along coastal regions. Neither T. tridentatus nor L. polyphemus extends into the northern reaches (above 45°N latitude) of the continental shelf, where cold-water currents prevail. Seafloor geology may also play a role in dispersal patterns. For example, the deep water basin between the Caribbean islands and Cuba prevents L. polyphemus from migrating farther south.

Click here to see the oldest known horseshoe crab fossil at the University of Oslo's Paleontology museum.