The evolution of pedicellariae in echinoids: an arms race against pests and parasites

Sea urchins ( Echinoidea) have evolved a diverse array of jawed appendages termed pedicellariae to deter pests and predators. Pedicellarial structure and function are reviewed and their distribution mapped in 75 extant genera. Using a phylogeny of echinoids at family level constructed from 353 skeletal characters scored across 162 extant and fossil taxa, the evolution of pedicellarial form and function is reconstructed. For much of the Palaeozoic echinoids possessed a very restricted pedicellarial armament. By the early Mesozoic a diverse array of pedicellarial types had become established, implying that the threat from predators and pests markedly increased at this time. Since the Triassic, echinoids have continued to improve their defensive capability by evolving more effective venom delivery in globiferous pedicellariae, developing spatulate- tips and curved blades for a more efficient grab in tridentate pedicellariae, and stouter, more robust valves with a stronger bite in ophicephalous pedicellariae to disable and remove ectoparasites. However, pedicellarial types are shown to be particularly prone to subsequent secondary loss, especially among infaunal echinoids, and thus have higher homoplasy levels than other phylogenetically useful skeletal structures.