Trilobites and Horseshoe Crabs

Diagram of a trilobite Illustration of horseshoe crabs by Heinrich Harder, circa 1916
Illustration of horseshoe crabs by Heinrich Harder, circa 1916

Known only from the fossil record, trilobites were marine arthropods that roamed the seas during the Paleozoic Era from the Cambrian to the Permian Period, 521 to 252 million years ago. Once widely dispersed and diversified, they survived a number of catastophic events over hundreds of millions of years before disappearing completely during the mass extinction event at the end of the Permian. The name “trilobite” means “three lobes”, which refers to their three longitudinal sections, an axial lobe (2) between two pleural lobes (1 and 3).

Do trilobites or their descendants still exist today?

Trilobites fall into the Arthropoda subphylum of Chelicerata, which includes horseshoe crabs, sea spiders, and arachnids like scorpions and spiders. No trilobites or descendants of trilobites have been identified. If you feel a bit disappointed that there are no more trilobites, just remember that the remarkable horseshoe crab has survived in nearly its present form since the days of the trilobites! 

Horseshoe crabs – a “living fossil”

Horseshoe crabs are found in the fossil record alongside trilobites. In 2008, horseshoe crab fossils were discovered in rocks in Manitoba, Canada, dating from the Ordovician Period, 445 million years ago. In Illinois, their fossils have been found in Pennsylvania Period rocks in the Mazon Creek-Braidwood area of Will and Grundy counties, where they lived in shallow seas by Coal Age forests. While a number of varieties evolved over time, the only surviving species of horseshoe crab is Limulus polyphemus.