On the appendicular anatomy of the xiphosurid Tachypleus syriacus and the evolution of fossil horseshoe crab appendages

Xiphosurida—crown group horseshoe crabs—are a group of morphologically conservative marine chelicerates (at least since the Jurassic). They represent an idealised example of evolutionary stasis. Unfortunately, body fossils of horseshoe crabs seldom preserve appendages and their associated features; thus, an important aspect of their morphology is absent in explorations of their conservative Bauplan. As such, fossil horseshoe crab appendages are...