A morphological comparison of the phyllopodous thoracic limbs of a leptostracan (Nebalia sp.) and a spinicaudate conchostracan (Leptestheria sp.), with comments on the use of Phyllopoda as a taxonomic category

The fourth thoracic appendage of an adult female Nebalia sp. (class Malacostraca, order Leptostraca) is compared with that of an adult female Leptestheria sp. (class Branchiopoda, order Spinicaudata). Although these limbs are "phyllopodous" (flattened, leaflike) in both orders, they differ markedly in the size and arrangement of endites, type and number of setae, and function. Setal types found on the fourth thoracopod of Nebalia sp. are variations of a basic plumose or plumodentate seta, and the diversity of setal types is low. None of the setae is annulate, a distinction in setal formation. In contrast, on the fourth thoracopod of Leptestheria sp. there is a wide variety of setal types and stout spines, some of which have been reported from other "conchostracans" (orders Spinicaudata and Laevicaudata) but not from the thoracopods of any leptostracan. Nearly all of the setae are annulate. Thus, the single character (phyllopodous thoracic appendages) supposedly linking leptostracans and branchiopods in some classificatory schemes (e.g., the class Phyllopoda sensu Schram) has clearly arisen independently in these two taxa and in several other crustacean groups, and cannot be used as an indicator of phylogenetic affinity. The rejection of the class Phyllopoda as defined by Schram is supported, as are arguments for retention of leptostracans within the Malacostraca. The use of Phyllopoda as a taxonomic name, regardless of which crustacean groups are considered to compose the taxon, is discouraged in light of the rather convoluted history and inconsistent application of this term.