Taxonomy and sexual dimorphism of a new species of Loxoconcha (Podocopida: Ostracoda) from the Pleistocene of the Japan Sea

Abstract

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.

The History of Sexual Dimorphism in Ostracoda (Arthropoda, Crustacea) Since the Palaeozoic

“…However, except for the The History of Sexual Dimorphism in Ostracoda (Arthropoda, Crustacea) Since the Palaeozoic 65 horn-like protuberance, the carapace appearance of species of this genus, including Loxocorniculum mutsuense from Japan, is very similar to that of the genus Loxoconcha as noted by Ishii et al [63]. The phylogenetic independence of Loxocorniculum in Japan as a genus distinct from Loxoconcha has been debated [16]. Therefore this chapter tentatively includes Loxocorniculum mutsuense, first proposed as a new species from Japan by [66], in the genus Loxoconcha following the opinion of Ishii et al [63].…”

“…Sexually dimorphic and paedomorphic morphological characteristics of the inner carapace were recently reported in two unrelated podocopid taxonomic groups from Japan [16] [17]. These, together with their ecological and evolutionary significance for ostracods since the Palaeozoic, are reviewed below.…”

“…Recently, our research group found a rare case of sexual dimorphism in ostracod hingements with a paedomorphic character within only one phylogenetic group of one family. This morphology could be an important characteristic for evaluation of the history of sexual dimorphism in ostracods since the early Palaeozoic [16]. The author discusses this together with the history of ostracod sexual dimorphism from the The History of Sexual Dimorphism in Ostracoda (Arthropoda, Crustacea) Since the Palaeozoic 57 Palaeozoic to Recent, reviewing another case of ostracod sexual dimorphism with paedomorphosis reported by [17].…”

“…A number of Japanese Cenozoic and living Podocopida genera, such as Finmarchinella, Loxoconcha, Semicytherura and Vestalenula (e.g., Figures 5 and 6), can be recognised by their distinct carapace shape and size sexual dimorphism [2,9,16,17,33]. In many cases, the male forms of these podocopids have relatively longer and narrower carapaces than the females ( Figures 5 and 6) [34][35][36].…”

Three new species of the genus Loxoconcha (Crustacea, Ostracoda, Podocopida) from the Okinawa Islands, southern Japan

“…Palmoconcha, Sagmatocythere and Loxocorniculum, the effective species number of Loxoconcha must be smaller. More than twenty living and four fossil species have been described so far from Japan (Ishii et al 2005;Ozawa & Ishii 2008).…”