There is probably no animal that epitomizes the title of “survivor” than the horseshoe crab. With their shield-like carapaces and long, spined tails, these arthropods look prehistoric. When masses of one species, Limulus polyphemus, congregate on Mid-Atlantic beaches in the warmth of early summer, it is difficult not to imagine the scene as something from the deep past.
Exactly when, where and how horseshoe crabs evolved remains a matter of ongoing investigation, but the group of arthropods they belong to is thought to have diverged from their arachnid cousins around 480 million years ago. The basic horseshoe crab body plan has been around since then, although not exactly in the form we now know. The newly named, 425-million-year-old Dibasterium durgae looked roughly like a horseshoe crab from the top, though if you were to turn the arthropod over, you would have been greeted by a nest of double-branched legs used for both breathing and locomotion.
Over time, other horseshoe crab species developed other odd adaptations. Creatures like the boomerang-shaped Austrolimilus and the double-button horseshoe crab Liomesaspis represent the extremes in the group’s variation, but it is true that horseshoe crabs as we know them today have been around for a very long time—the 150 million year old Mesolimulus looks like it would fit right in on a Delaware beach. Horseshoe crabs have continued to change since then, of course. The modern Atlantic horseshoe crab is not found in the fossil record, and the specific group of horseshoe crabs to which it belongs only has a record of about 20 million years. Still, the changes within the group have been astonishingly slight when viewed against the big picture of evolution. Since the time of the horseshoe crab’s origin, the world has seen several mass extinctions, the rise and fall of the non-avian dinosaurs and shiftings of continents and climates so drastic that the world truly is a wildly different place. All the while the horseshoe crabs have been there, crawling along the seafloor. May they will continue to do so for millions of years to come.