The ostracod genus Trachyleberis (Crustacea; Ostracoda) and its type species

The Trachyleberididae is one of the most diverse families of the Ostracoda in Mesozoic to present-day marine environments. Its type genus is Trachyleberis Brady, 1898, first described on the basis of specimens collected alive. Unfortunately the identity and morphological characteristics of the type species of Trachyleberis have long been the subject of confusion resulting from misidentifications and misunderstandings in the literature. We review the taxonomic history of this problem, providing extensive synonymies, new diagnoses, descriptions and illustrations of relevant species. We establish the identity of the type species of Trachyleberis, Cythere scabrocuneata Brady, 1880, by reference to a previously-designated lectotype from the Seto Inland Sea, Japan, noting that a second distinct species, Trachyleberis niitsumai Ishizaki, 1971, is represented in the original syntypic series. We consider that in the interests of nomenclatural and taxonomic stability C. scabrocuneata should be retained as the type species of Trachyleberis, despite the fact that the original description of the genus was based on a species misidentified as C. scabrocuneata and which was subsequently described as a new species, Trachyleberis lytteltonensis Harding and Sylvester-Bradley, 1953. The geographical and stratigraphical distributions of Trachyleberis are reconsidered in the light of our revision. Trachyleberis now comprises 18 known species inhabiting shallow marine environments of the Northwestern Pacific from Japan to the northern coast of the South China Sea off Hong Kong. The revised stratigraphic occurrence of Trachyleberis is from the Eocene to Recent. As many as 400 living and fossil species have been previously assigned to Trachyleberis and they range from the Cretaceous to the Recent; Cretaceous species formerly attributed to Trachyleberis are now re-assigned to other genera. We suggest that Trachyleberis most likely evolved from a Cythereis-like ancestor during the Eocene-Oligocene of Japan, the same area that is the centre of its modern distribution.