The pore-systems of 17 extant species of Loxoconcha around Japan were studied in order to understand their phylogeny and evolution. The phylogeny was estimated by two steps. First, the 17 species were divided into two groups, Group A (12 species) and Group B (five species) by Pore pattern Below Eye tubercle (PBE) analysis. Then, intragroup relationships were estimated by Differentiation of Distributional pattern of Pore-system (DDP) analysis. PBE analysis reveals that species of Groups A and B have on average different ecological preferences. Species of Group A, which appeared in the late Pliocene, are more diverse, have both phytal and bottom-dwelling modes of life, possess fewer pore-systems in the ventral area, and inhabit normal marine environments. Species of Group B, whose oldest fossil record is the lower Miocene, are less diverse, have only bottom-dwelling species, possess more pore-systems in the ventral area, and tend to inhabit brackish water environments. The results of this study suggest that the differences in ecology may have had an impact on the late Cenozoic diversification around Japan. The primary invasion of Group B occurred before the lower Miocene,with no subsequent diversification. Group A invaded after the late Pliocene and immediately diversified, which created the present abundance of Loxoconcha species around Japan in both species diversity and variety of modes of life.

A new ostracod, Loxoconcha kamiyai sp. nov. in the Family Loxoconchidae, is described from the Pleistocene Omma Formation of Japan. Its geological and geographical distributions suggest that this species was once endemic to the Japan Sea, where it would have evolved until the Pliocene. Since the early Pleistocene, this species would have become extinct within this marginal sea during glacial maxima, probably due to its narrower salinity tolerances and geographical distributions than those of extant species inhabiting the euryhaline environments in other seas. The distributional patterns of pore systems in this species strongly suggest its closest phylogenetic affinities to a living species, Loxocorniculum mutsuense. These two species show a unique adult sexual dimorphism in the anterior element of the hingement. Taking the female hingement morphology as a standard, the male hingement can be explained in terms of heterochrony, i.e. paedomorphosis. Sexual hingement dimorphism with paedomorphosis occurs in only one phylogenetic group of the genus Loxoconcha, which is distinguished by the ontogenetic distributional patterns of pore systems. This morphology may represent relict primitive characters of ancient ostracods and could be an important character for evaluating the history of sexual dimorphism in ostracods since the Palaeozoic.

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