The horseshoe crabs (Xiphosura) are a group of large aquatic arthropods known from the East coast of the USA, and the Southern and Eastern coasts of Asia. Despite their name, they are not actually crabs at all, but are chelicerates (the group containing spiders and scorpions). As a group, the horseshoe crabs possess an extremely long fossil record, reaching as far back as the Ordovician Period, some 480 million years ago. Since that time, they would appear to have undergone very little change, leading the horseshoe crabs to become the archetypal 'living fossils'. Joining us for this two-part episode is Dr Russell Bicknell, University of New England, Australia. We discuss what makes a horseshoe crab, before taking questions from our listeners as to all aspects of horseshoe crab ecology and what we can infer from them about other extinct arthropods.