The uropods are the specialised sixth pair of pleopods of eumalacostracan Crustacea. Their quite variable morphology is suggestive of a good potential as a phylogenetic signal. Because uropods have hitherto been neglected in analyses of malacostracan phylogeny, we examined them in 11 representative species of Eumalacostraca and the sixth pleopods of a phyllocarid malacostracan for outgroup comparison. Uropods have apparently evolved in the stem species of Eumalacostraca, possibly being leaf-shaped in the ground-pattern state but already with stabilising carinae along their surface and marginal setae enlarging the effective surface of the rami. Functionally, uropods aid in locomotion and, within the taxon Caridoida, also add to the special tail-flip mechanism. From an original leaf-shaped design in adaptation to different functions and lifestyles uropods may have become rod-shaped, as in Bathynellacea, Cumacea and Amphipoda, but independently so in the first taxon, while this shape might have developed in a common stem species of the latter two, inter alia. Among the taxa that have retained the leaf shape, mysidacean uropods possess a basipod that is drawn out medio-proximally into an outgrowth; Lophogastrida have a triangular median keel there. The uropods of both Euphausiacea and Decapoda also have a laterodistal prolongation, while those of Decapoda are special in bearing a longitudinal median keel on their basipod. All these differences appear to be exclusive to the respective higher taxa, demonstrating the value of uropods, and pleonal structures in general, for phylogenetic considerations.

Pleopods, phylogenetic systematics, morphology, basipod, endopod, exopod, pleon.