ABSTRACT
Decapods of the infraorder Polychelida are unusual in having chelate pereopods 1-4 and reduced eyes in extant species. Polychelidans traditionally have been included with the achelate lobsters in the infraorder Palinura. Polychelida, however, is depicted as basal in the Reptantia by most recent studies. The polychelidan fossil record extends back to the Upper Triassic, with four families recognized to date, of which only Polychelidae is extant. Interrelationships of the fossil and living polychelidan lobsters were studied by cladistic analysis of morphology, with emphasis on Polychelidae. Coleiidae was found to be sister to Polychelidae, to the exclusion of Palaeopentacheles, previously placed in the latter. A new family, Palaeopentachelidae, is recognized for Palaeopentacheles. All other recognized polychelidan families are also diagnosed. An incomplete fossil taxon from the Upper Triassic attributed to Polychelidae, Antarcticheles antarcticus, is confirmed as a polychelid and is most closely related to the extant genus Willemoesia. The strong similarities between Willemoesia and Antarcticheles indicate that differentiation of the ‘polychelid form’ was well established by the late Jurassic. Among extant Polychelidae, Willemoesia is least derived, though the shallow dorsal orbits, regarded by some as plesiomorphic, are a derived condition. Stereomastis is removed from the synonymy of Polycheles. Six extant polychelid genera are recognized: Cardus, Homeryon, Pentacheles, Polycheles, Stereomastis, and Willemoesia. All extant polychelid genera are diagnosed, and keys to genera and species are provided. Phylogenetic trends within Polychelida include a general narrowing of the carapace and abdomen; shortening of the carapace front with respect to the anterolateral margins, leading to a shift in eye orientation from anterior to transverse; dorsal exposure of the base of the antennules and development of a stylocerite; and a shift in the form of the major chelipeds from relatively robust with short, triangular carpi to elongated and slender, with slender carpi. These trends within Polychelida appear to correspond to a shift from a shallow-water, epibenthic habit to the deep-water, fossorial lifestyle currently evident in Polychelidae. Phylogenetic trends within Polychelidae include a consistent reduction in length of the maxilliped 3 and pereopodal epipods. Epipod length is not known for any of the fossil taxa, but character polarization among extant taxa predicts that extinct taxa bore well-developed epipods.
