The Horseshoe Crab - Limulus polyphemus
Photo by David Richman. Used with permission.)


The subclass Xiphosura (Xiphos meaning sword and uros meaning tail) are a small however highly successful group within the Arthropoda that first appeared during the Palaeozoic era. Extant Xiphosura, also known as horseshoe crabs or "living fossils", consist of 4 species occurring in 3 genera, having been far more diverse during the Palaeozoic yet evolving very little since the Triassic. Members within the class have a large hinged carapace that covers the cephalothorax giving them a crab-like resemblance although they are more closely related to the Chelicerata (arachnids) and the extinct Eurypterids than to the Crustaceans. Modern horseshoe crabs are predatory, living in shallow marine environments at depths of about 100 feet, feeding on small marine invertebrates. They migrate ashore in large numbers, to reproduce and lay their eggs in the warm sands around Asia and North America. This strategy has allowed xiphosurans to survive unchanged for 250 million.

Author: Andrew Przewieslik
Last updated: 21/11/05
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