Phylogeny and zoogeography of three new species of the genus Hexabathynella (Crustacea, Malacostraca, Bathynellacea) from North America

Three new species of the genus Hexabathynella , H. otayana sp. n., H. hessleri sp. n. and H. muliebris sp. n., from North America are described. A comparison with previous studies results in the reevaluation of characters already known and the discovery of new constitutive characters. The distal spine of the mandibular spine row is regarded as a synapomorphy of the three new species and two previously known species, H. knoepffleri , H. minuta , which constitute a monophyletic group within the genus, the knoepffleri-group. The phylogenetic relationships between the five species are depicted with a cladogram. The setation on the uropodal exopod is shown to be not only a new unique character for the genus, but also a significant one in classifying its species. The presence of the ancestor species of the genus before the Triassic and dispersal during Mesozoic continental drift are assumed to explain the transatlantic occurrence of the knoepffleri-group and the distribution pattern of the other species.