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The phylogenetic relationships of Middle and Late Devonian species of the subgenus Leptodesma (Leiopteria) Hall 1883 are examined cladistically and a taxonomic revision of the subgenus is proposed. Six taxa previously afforded species rank are synonymized with Leptodesma (Leiopteria) laevis (Hall 1843). One new species, Leptodesma (Leiopteria) accranus, is described from the Lower to Middle Devonian of Ghana. Biogeographic analysis of the subgenus reveals less vicariant speciation than speciation associated with range expansion and dispersal. This pattern has been observed in trilobites and phyllocarid crustaceans and may represent a general pattern during the Middle and Late Devonian. Extensive range expansion may have played a role in governing biodiversity dynamics before and during the Late Devonian biodiversity crisis.