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The Xiphosurida, or horseshoe crabs, are often cited as a classic example of arrested evolution. They have been so consistently associated with this concept, in both professional and popular literature, that their reputation for extreme conservatism in form and behavior is probably more widely known than any other single aspect of their biology or history. Ironically, however, there have been no published measurements of evolutionary rates (either morphologic or taxonomic) for horseshoe crabs, let alone any rigorous comparisons of evolutionary rates between horseshoe crabs and groups that supposedly evolve more rapidly. Rather, their status as an archetypal bradytelic group has been founded primarily on the judgment and authority of specialists who have perceived only minor morphologic differences between the living forms and certain fossil relatives (Fig. 1A-D).