Taxonomy and Sexual Dimorphism of a New Species of Loxoconcha (Podocopida: Ostracoda) from the Pleistocene of the Japan Sea

Zoological Journal of the Linnean Society, 2008, 153, 239–251. With 9 ﬁgures

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

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HIROKAZU OZAWA * and TOHRU ISHII Downloaded from https://academic.oup.com/zoolinnean/article/153/2/239/2606412 by guest on 25 March 2021

1Department of Geology, National Science Museum, Japan, 3-23-1 Hyakunin-cho, Shinjuku-ku, Tokyo 169-0073, Japan 2Department of Earth Sciences, Faculty of Science, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan

Received 20 February 2007; accepted for publication 23 July 2007

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. © 2008 The Linnean Society of London, Zoological Journal of the Linnean Society, 2008, 153, 239–251.

ADDITIONAL KEYWORDS: endemic species – hingement – paedomorphosis – phylogeny.

INTRODUCTION described (Ikeya, Tanaka & Tsukagoshi, 2003). Thus, Loxoconcha is one of the most important genera of The ostracod genus Loxoconcha (Family Loxocon- Japanese ostracods. chidae, Podocopida) is widely distributed in shallow- Species of Loxoconcha are common in late Caeno- marine environments from tropical to subarctic areas zoic strata along the coast of the Japan Sea (Ishizaki, around the world. This is one of the most diversiﬁed 1963; Ishizaki & Matoba, 1985; Kamiya, Ozawa & genera of ostracods, and about 600 species belonging Obata, 2001; Tanaka, Tsukawaki & Ooji, 2004). The to this genus have been identiﬁed (e.g. Tanaka & early Pleistocene Omma Formation in central Japan Ikeya, 2002; Horne, 2003). This genus also commonly contains many fossil ostracods, including species of occurs in and around the Japanese Islands (Ishizaki, Loxoconcha (Ozawa, 1996; Ozawa & Kamiya, 2001, 1968; Hanai et al., 1977; Zhao & Wang, 1988; Ikeya & 2005a). However, previous descriptions of several new Cronin, 1993; Zhou, 1995; Ozawa, 2003), and approxi- species of Loxoconcha from this stratum did not mately 40 fossil species of Loxoconcha have been include detailed examinations of the carapace morphology. We therefore describe a new Pleistocene *Corresponding author. E-mail: [email protected] species of Loxoconcha from the Omma Formation on

© 2008 The Linnean Society of London, Zoological Journal of the Linnean Society, 2008, 153, 239–251 239 240 H. OZAWA and T. ISHII the coast of the Japan Sea, and include the first detailed description of its carapace. 130˚E 140˚E Okhotsk Sea We discuss the palaeo-biogeographical significance, mode of life and sexual dimorphism of this species, and assess its phylogenetic relationship to other related Loxoconcha species on the basis of the distributional pattern of its pore systems. Japan Sea Ontogenetic changes in the distributional patterns Daishaka Fm. of pore systems on the carapace surface are impor- Sasaoka Fm.40˚N 40˚N Shichiba Fm. tant in assessing the phylogenetic relationships of Kaidate Fm. Kawachi Fm. ostracod species (Tsukagoshi, 1990; Irizuki, 1993; Junicho Fm. Downloaded from https://academic.oup.com/zoolinnean/article/153/2/239/2606412 by guest on 25 March 2021 Yabuta Fm. Haizume Fm. Ishii, Kamiya & Tsukagoshi, 2005). This method Nishiyama Fm. Honshu is a very useful tool for analysing ostracod Omma Fm. (Ozawa, 1996) Tokyo phylogenetic relationships, because we can apply this tool also to fossil specimens without soft Pacific Ocean parts. We therefore examined the distributional Ogikubo Fm. patterns of pore systems in the adult and A-1 juvenile stages of this new species, and compared them with those of other species of Loxoconcha from Pleistocene Japan. Pliocene 130˚E 140˚E Recent studies of Japan Sea ostracods have 30˚N summarized changes in species diversity in three Figure 1. Geographical and geological occurrences of families, the Hemicytheridae, Cytheruridae and Loxoconcha kamiyai sp. nov. based on original data, Eucytheridae, in relation to fluctuations in Pleis- except for data from the Omma Formation cited from tocene oceanographic environments induced by Ozawa (1996). Fm: Formation. glacio-eustatic cycles (Ozawa et al., 2004; Ozawa & Kamiya, 2005b; Ozawa, 2006). Important fossil MATERIAL AND METHODS occurrences of species belonging to these three families have been found along the coast of the Japan We re-examined fossil specimens of one species of the Sea, providing valuable evidence for the history of genus Loxoconcha in 33 samples from the Sakurama- the ostracod fauna in this sea since the late Cae- chi area (samples C201–C514 of Ozawa & Kamiya, nozoic (Irizuki, 1994; Ozawa, 2006). The palaeonto- 2001) of the early Pleistocene Omma Formation logical significance and geological occurrences of (c. 1.7–1.4 Ma, fide Takata, 2000), central Japan ostracods have become increasingly important in (Fig. 1). To clarify the geological distribution of this clarifying the history of the benthic fauna of the species, fossil ostracods from a series of 54 samples of Japan Sea. Pliocene and Pleistocene strata from along the Japan The Loxoconcha species described here often Sea coast (Fig. 1) previously studied by Ozawa & occurs together with extinct species in the above Kamiya (2005b) and H. Ozawa, H. Nagamori & T. three families in Pliocene and Pleistocene strata Tanabe (unpubl. data) were also examined. Additional along the coast of the Japan Sea (Ozawa, 1996), and ostracod data were added from two samples from the probably also represents an extinct species. This early Pleistocene Nishiyama Formation (c. 1.3 Ma, study presents the geological–geographical distribu- fide Sato et al., 1987), central Japan (Fig. 1). tions and the speciation–extinction history of this For comparison with the carapace morphology of species, and discusses its palaeo-biogeographical this species, we also examined specimens of Loxocor- significance for changes in species diversity of niculum mutsuense Ishizaki, 1971, and of an unde- the ostracod fauna of the Japan Sea as related scribed species of Loxoconcha, both in the family to environmental fluctuations since the late Loxoconchidae Sars, 1925, from a sample of the early Caenozoic. Pleistocene Kaidate Formation (c. 0.9 Ma, fide Kato The aims of this study are (1) the description of a et al., 1995), central Japan (Fig. 1), previously studied new ostracod species of genus Loxoconcha from the by Ozawa & Kamiya (2005b). The carapace morphol- Japan Sea and its palaeo-biogeographical signifi- ogy, and the number, morphology and distributional cance, (2) a phylogentic examination of this species in pattern of pore systems of these specimens were Japanese loxoconchid ostracods based on rarely used examined in detail with a scanning electron micro- characters, and (3) the presentation of an evolution- scope (JEOL-JSM-5310). ary hypothesis on sexual dimorphism and paedomor- Our data on the number and distributional pat- phosis in valve characters. terns of pore systems of Loxocorniculum mutsuense

© 2008 The Linnean Society of London, Zoological Journal of the Linnean Society, 2008, 153, 239–251 LOXOCONCHA SPECIES AND SEXUAL DIMORPHISM 241 from A-2 juvenile to adult stages, and the phyloge- Type locality: Sakuramachi, Oyabe City, Toyama netic relationships among the 17 species in the genus Prefecture, central Japan (36°41.1′N, 136°52.1′E) of Loxoconcha from Japan, analysed based on the Ozawa & Kamiya (2001) from the early Pleistocene differentiation of the distributional pattern of pore Omma Formation, central Japan (Fig. 1). systems (DDP analysis of Kamiya, 1997), were obtained from Ishii et al. (2005) and Dr T. Ishii Diagnosis: Carapace subrhomboidal in lateral view, (unpubl. data). and medium-sized. Dorsal and ventral margins All illustrated specimens are deposited in the slightly curved, slightly concave in ventral margin at National Science Museum, Japan (NSM). Type speci- one-third of carapace length. Valve surface covered mens are deposited in the collections of the NSM and with round fossae in anterior and ventral areas, and

identified by numbers with the prefix MPC. The type numerous fine pits in marginal areas, but sparse Downloaded from https://academic.oup.com/zoolinnean/article/153/2/239/2606412 by guest on 25 March 2021 locality of the new species and sample numbers are and smooth surface areas observed at median part, in the same as both the location of the Sakuramachi and around adductor muscle field. Round fossae area in the early Pleistocene Omma Formation arranged in concentric rows running subparallel to (Fig. 1) and the numbers in Ozawa & Kamiya (2001), antero-ventral, mid-ventral and postero-ventral respectively. margins. Two weak radial ridges in mid-anterior and antero-ventral areas extending anteriorly. Three weak crests in postero-dorsal corner, mid-posterior area and postero-ventral corner. SYSTEMATIC DESCRIPTIONS ORDER PODOCOPIDA SARS, 1866 Description: Valves subrhomboidal in lateral view, SUPERFAMILY CYTHEROIDEA BAIRD, 1850 and medium-sized (Fig. 2). Dorsal and ventral FAMILY LOXOCONCHIDAE SARS, 1925 margins slightly curved, slightly concave in ventral SUBFAMILY LOXOCONCHINAE SARS, 1925 margin at one-third of carapace length. Greatest GENUS LLOXOCONCHA SARS, 1866 length near mid-height, and greatest height near LOXOCONCHA KAMIYAI SP. NOV. mid-length. Anterior margin obliquely rounded. (FIG.2) Posterior margin truncated obliquely in upper half Loxoconcha sp. 1 Ozawa, 1996: 112, pl. 6, fig. 10. and lower half, making blunt angle slightly above mid-height. Valve surface covered with round fossae in anterior Etymology: In honour of Dr Takahiro Kamiya and ventral areas, and numerous fine pits in mar- (Kanazawa University, Japan), an expert in the phy- ginal areas (Fig. 2A–D). Round fossae arranged in logeny, speciation and copulatory behaviour of Loxo- concentric rows running subparallel to antero- concha from the western Pacific region. ventral, mid-ventral and postero-ventral margins. Sparse ornamentation and smooth surface areas at median part, in and around adductor muscle field. Types: Holotype, female, right valve, MPC-03671, Two weak radial ridges in mid-anterior and an- from sample C505 (Fig. 2A). Paratypes, female, right tero-ventral areas extending anteriorly. Three weak valve, MPC-03672, from sample C304 (Fig. 2E); male, crests found in postero-dorsal corner, mid-posterior right valve, MPC-03673, from sample C402 (Fig. 2C); area and postero-ventral corner, respectively. Eye female, left valve, MPC-03674, from sample C501 tubercle round and not distinct. Carapace surface (Fig. 2F); female, left valve, MPC-03675, from sample covered with a total of 85 pore systems in the adult, C506 (Fig. 2B); male, left valve, MPC-03676, from comprising 68 normal pore systems having sieve sample C506 (Fig. 2D, J); male right valve, MPC- plates 0.01–0.02 mm in diameter, and 17 marginal 03677, from sample C304 (Fig. 2G). pore systems c. 0.005 mm in diameter; a total of 75 pore systems are found in the A-1 juvenile (Fig. 3; Table 1). Table 1. Total numbers of pore systems of the two species Hingement: gongylodont; right valve with single for the last three moult stages tooth surrounded by horseshoe-shaped depression in anterior element; numerous fine teeth in median Species name/stage Adult A-1 A-2 element; few small teeth and one large horseshoe- shaped tooth in posterior element, with large socket Loxoconcha kamiyai 85 75 ? opening ventrally (Fig. 2E–G). Hinge line slightly Loxocorniculum mustuense*867555curved. Muscle scars: four adductor scars; two separate frontal scars; two subrounded mandible scars. *Data reported by Ishii et al. (2005). Vestibules present along anterior and posterior

© 2008 The Linnean Society of London, Zoological Journal of the Linnean Society, 2008, 153, 239–251 242 H. OZAWA and T. ISHII Downloaded from https://academic.oup.com/zoolinnean/article/153/2/239/2606412 by guest on 25 March 2021

Figure 2. Loxoconcha kamiyai sp. nov. A, female, right valve from outside (holotype, MPC-03671). B, female, left valve from outside (paratype, MPC-03675). C, male, right valve from outside (paratype, MPC-03673). D, male, left valve from outside (paratype, MPC-03676). E, female, right valve from inside (paratype, MPC-03672). F, female, left valve from inside (paratype, MPC-03674). G, male, right valve from inside (paratype, MPC-03677). H, A-1 juvenile, left valve (MPC-03678). I, A-1 juvenile, right valve from inside (MPC-03679). J, close-up view of normal pore systems with sieve plates around eye spot of male on left valve (paratype, MPC-03676). K, close-up view of four marginal pore systems and two normal pore systems in postero-ventral marginal area of female on complete carapace (MPC-03680). All specimens from the Pleistocene Omma Formation. ᭣

Occurrence: Pliocene and Pleistocene, on the Japan Downloaded from https://academic.oup.com/zoolinnean/article/153/2/239/2606412 by guest on 25 March 2021 Sea coast, Japan (Fig. 1); early and late Pliocene

etb Ogikubo Formation, late Pliocene Yabuta, Sasaoka and Junicho formations, and early Pleistocene Omma, Nishiyama, Haizume, Daishaka, Kawachi, Kaidate and Shichiba formations. wr

wr ms Remarks: This species is similar to Loxocorniculum mutsuense defined by Ishizaki (1971: pl. 5, fig. 11; pl. 6, figs 3, 6, 7; pl. 7, fig. 5) from Recent sediments in Aomori Bay, north-eastern Japan, in general external carapace ornamentation and lateral outline. However, it differs from Loxocorniculum mutsuense in the lack 0.1mm of one normal pore system in the median area (Fig. 3; normal pore etb: eye tubercle Table 1), its rounded outline in lateral view and the marginal pore ms: muscle scar area missing normal pore wr: weak ridge nature of the valve surface. Reticulations covering (in comparison the whole carapace surface and two radial ridges in with L. mutsuense) the mid-anterior and antero-ventral parts are weaker, and are not distinct in the present species. Three Figure 3. Distributional pattern of pore systems in adult radial ridges in the posterior area are weaker in this left valve of Loxoconcha kamiyai sp. nov. Position of species. A horn-like protuberance in the postero- one missing pore system of this species is determined by dorsal corner is not developed in the present species. comparison with the distributional pattern of pore systems Sparse ornamentations and smooth surface areas at of Loxocorniculum mutsuense Ishizaki, 1971 (Ishii et al., the median part, in and around the adductor muscle 2005). field, are observed in the present species, but not in Loxocorniculum mutsuense. The present species is also similar to Loxoconcha margins, relatively deep along antero-ventral margin pentoekensis Kingma, 1948, figured by Zhao & (Fig. 2E–G). Marginal infoldment moderately broad, Whatley (1989: pl. 2, figs 9, 10), from Pliocene and especially in anterior half. Pleistocene strata in eastern Java, Indonesia, in Sexual dimorphism distinct (Fig. 2A–F). Carapace general external carapace ornamentation. However, it of male more slender in lateral view. Dorsal margin differs from Loxoconcha pentoekensis by its slightly slightly rounded in female, in lateral view. The ante- curved dorsal outline in lateral view and the nature of rior element in the hingement of female (0.04 mm in the valve surface. Two weak radial ridges in the width) is twice as wide as that of male (0.02 mm in mid-anterior and antero-ventral parts and three weak width). radial crests in the posterior area are found in the present species. A large ear-like protuberance in the Dimensions (mm): Holotype, L = 0.65, H = 0.43 postero-dorsal corner is not developed. Sparse orna- (female, right valve, MPC-03671). Paratypes, mentations and smooth surface areas at the median L = 0.63, H = 0.41 (female, right valve, MPC-03672); part are found. L = 0.68, H = 0.39 (male, right valve, MPC-03673); Loxoconcha kamiyai has some variability in (1) L = 0.61, H = 0.42 (female, left valve, MPC-03674); development of rounded fossae in the anterior mar- L = 0.61, H = 0.40 (female, left valve, MPC-03675); ginal and posterior marginal areas, (2) development L = 0.73, H = 0.40 (male, left valve, MPC-03676); of two antero-radial ridges and three weak postero- L = 0.68, H = 0.40 (male, right valve, MPC-03677). crests, (3) areas of sparse ornamentation and smooth

© 2008 The Linnean Society of London, Zoological Journal of the Linnean Society, 2008, 153, 239–251 244 H. OZAWA and T. ISHII surface parts in the median area, and (4) areas covered by ﬁne pits and fossae in anterior, ventral Adult and posterior margins.

w etb DISCUSSION x PHYLOGENETIC RELATIONSHIP BASED ON THE DISTRIBUTION OF PORE SYSTEMS z The number, distribution and differentiation of pore

systems on the carapace during ostracod ontogeny y Downloaded from https://academic.oup.com/zoolinnean/article/153/2/239/2606412 by guest on 25 March 2021 were studied to determine the phylogenetic relationships among ostracod species. The reconstruction of 0.05 mm ostracod phylogeny by analysing pore systems was v first proposed by Tsukagoshi (1990) for the 14 species Figure 4. Patterns of normal pore systems below the eye in the genus Cythere O. F. Müller, 1785. This work tubercle by PBE analysis of Ishii et al. (2005) for the adult was followed by Irizuki (1993), who studied 21 species left valve of Loxoconcha kamiyai sp. nov., based on in eight genera of hemicytherinae ostracods, and Ishii Figure 3. Pore systems with italic letters (v–z) are the et al. (2005), who investigated 17 species in the genus same as those of Ishii et al. (2005), respectively. etb: eye Loxoconcha. Kamiya (1997) named the phylogenetic tubercle. reconstruction method proposed by Tsukagoshi (1990) ‘differentiation of distributional pattern of pore systems (DDP) analysis’. The distribution of pore reflects phylogenetically related groups within systems in Loxoconcha kamiyai was examined with the genus. PBE analysis in this study reveals the this method and the results were compared with the L-shaped pore pattern of Ishii et al. (2005) below the pore system data for other Loxoconcha species pub- eye-tubercle in Loxoconcha kamiyai (see v, w, x, y and lished by Ishii et al. (2005). z in Fig. 4); it differs from their ‘upside-down L-shape’ On the basis of the DDP results for its adult and pattern. We therefore consider that this species A-1 juvenile stages, Loxoconcha kamiyai is judged to belongs to their Group A, which also includes be the species most closely related to Loxocorniculum Loxocorniculum mutsuense (Fig. 5). This result for mutsuense, in the family Loxoconchidae. Both species Loxoconcha kamiyai corresponds well with the phy- have the same total number of normal and radial pore logenetic relationships of the species in the genus systems on the carapace at the A-1 juvenile stage (75 Loxoconcha from Japan proposed by Ishii et al. pore systems per valve, Table 1). The difference in (2005). total number of pore systems in the adult stage is just The genus Loxocorniculum was established by one normal pore system between the two species Benson & Coleman (1963) primarily on the basis of (Table 1), which is missing on the central area in modern specimens of Loxocorniculum fischeri (Brady, Loxoconcha kamiyai (Fig. 3). Furthermore, only these 1869) from the Caribbean Sea near Panama. It two species have seven marginal pore systems at the is characterized by a horn-like protuberance on postero-ventral to mid-posterior margins at the adult the postero-dorsal corner of the carapace. However, and A-1 juvenile stages; the other 16 species, includ- except for the horn-like protuberance, the appearance ing Loxoconcha japonica Ishizaki, 1968, have just five of the carapace of the species in this genus, inclu- marginal pore systems in the postero-marginal areas ding Loxocorniculum mutsuense Ishizaki, 1971 from at the adult and A-1 juvenile stages (T. Ishii, unpubl. Japan, is very similar to that of the genus Loxoconcha data). This character is unique among the species of as noted by Ishii et al. (2005). The phylogenetic Loxoconcha from Japan (T. Ishii, unpubl. data), and independence of Loxocorniculum in Japan as a genus strongly suggests a very close phylogenetic position distinct from Loxoconcha has been debated. There- between these two species within the genus Loxocon- fore, we tentatively include Loxocorniculum mut- cha. This assessment is also supported by similar suense, proposed as a new, extant species from Japan surface ornamentations, such as the existence of two by Ishizaki (1971), in the genus Loxoconcha following radial ridges in the anterior area. the opinion of Ishii et al. (2005). Loxoconcha species from Japan can be divided into two groups (A and B) on the basis of the distributional patterns of their pore systems (Ishii et al., 2005), MODE OF LIFE AND MICROHABITAT especially the pore pattern below the eye tubercle Loxoconcha kamiyai has a round carapace outline in (PBE analysis of Ishii et al., 2005). This division lateral view and a ‘rugby-ball’ shape in posterior

A-4 A-3 A-2 A-1 Adult Group A 130˚E 140˚E 70 82 Loxoconcha harimensis Holocene Pleistocene L. hattorii Pliocene L. kattoi Japan Sea 72 83 L. tosaensis L. epeterseni a: Hayashi (1988) L. kitanipponica b: this study a 42 54 L. zamia b1: Daishaka Fm. L. prolaeta b2: Sasaoka Fm. b1 40˚N 83 b3: Shichiba, Kaidate, b2 L. japonica 40˚N Kawachi Fms. 73 b4: Nishiyama, 31 83 Downloaded from https://academic.oup.com/zoolinnean/article/153/2/239/2606412 by guest on 25 March 2021 L. lilljeborgii Haizume Fms. b3 b5: Junicho Fm. 74 87 b4 L. optima c: Ozawa (1996) c 85 b5 Loxoconcha kamiyai d Pacific 43 55 75 Ocean 86 e Loxocorniculum mutsuense h g f i d: Irizuki et al. (1998) Group B e: Frydl (1982) 42 55 74 89 j L. subkotoraforma f: Ikeya et al. (1985) 28˚N g: Nakao et al. (2001) 43 56 76 89 k L. pulchra l Ryukyu h: Yajima (1987) Islands i: Yasuhara et al. (2002) 31 44 57 83 100 L. uranouchiensis j: Ishizaki (1983) 30˚N l l k: Nakao (1993; cf.) 59 83 104 124˚E 26˚N l: Ruan & Hao (1988) L. sp. l 45 ll 60 87 112 128˚E 140˚E L. kosugii Ryukyu Islands

Figure 5. Results of DDP analysis for 17 loxoconchid Figure 6. Geographical and geological distribution of species and Loxoconcha kamiyai sp. nov., modified from Loxocorniculum mutsuense Ishizaki, 1971, based on data Ishii et al. (2005). Numbers indicate total numbers of pore from this and previous studies. Fm.: Formation, cf.: con- systems for each lineage and stage. Trees drawn by hand. ferrable species. view with a convex ventral area (Fig. 2A–D, K). Pliocene Ogikubo Formation at the Japan Sea side These morphological characters are common in of central Japan (Fig. 1; c. 3.5 Ma, fide Nagamori, phytal-dwelling ostracods, including Loxoconcha Furukawa & Hayatsu, 2003). The oldest record of species, in relation to their microhabitats and life Loxocorniculum mutsuense is from the Pliocene style – e.g. Loxoconcha japonica Ishizaki, 1968 and Ananai Formation on the Pacific side of south- Loxocorniculum mutsuense live on the leaf surfaces of western Japan (Fig. 6; Ishizaki, 1983; c.3Ma,fide marine plants such as Zostera (Kamiya, 1988). This Iwai et al., 2006). Therefore, it is still difficult to clearly differs from the bottom-dwelling species that specify which is the ancestral species on the sole basis inhabit the surface of the sand bottom, which show an of their fossil records. The genus Loxoconcha has elongate outline in lateral view and a triangular southern origins, and shows high species diversity in shape in posterior view with a flat ventral plane, e.g. areas affected by the modern warm Kuroshio Current Loxoconcha uranouchiensis Ishizaki, 1968 (Kamiya, along the western Pacific coasts and in East and 1988). Although it became extinct during the Pleis- Southeast Asia. So Loxocorniculum mutsuense, first tocene, Loxoconcha kamiyai is therefore inferred to appearing along Pacific coasts during the Pliocene, have been a phytal-dwelling species on the basis of its might be the probable ancestral species of Loxoconcha carapace shape. kamiyai, distributed only along Japan Sea coasts. According to its geographical and geological occurrences (Fig. 1), Loxoconcha kamiyai is judged to be a GEOGRAPHICAL AND GEOLOGICAL DISTRIBUTION OF formerly endemic species restricted to the Pliocene– TWO SPECIES AND THEIR ORIGIN AND Pleistocene of the Japan Sea. Therefore, its origin EXTINCTION–SURVIVAL appears to be along the coasts of the Japan Sea, and Based on the oldest fossil record, Loxoconcha kamiyai this species would have been extinct for the last and Loxocorniculum mutsuense first appeared in 0.5 Myr, based on the youngest fossil record from the Japan during the Pliocene, around 3 Myr ago. The Shichiba Formation in central Japan (Fig. 1; age fide oldest record of Loxoconcha kamiyai is from the Kato et al., 1995). The period of extinction of this

130˚E 140˚E Recent occurrence f: Frydl (1982) a: Ishizaki (1971) g: Zhou (1995) b: Ikeya & Itoh (1991) h: Okubo (1980) c: Kamiya et al. (2001) i: Schornikov & Chavtur (2001) d: Irizuki (1989) j: Lee et al. (2000) e: Kamiya (1988) k: Zhao & Wang (1988)

i 40˚N

a Downloaded from https://academic.oup.com/zoolinnean/article/153/2/239/2606412 by guest on 25 March 2021 Bo-Hai 40˚N k Sea Japan Sea

b c j d Yellow Sea k e h g f g Pacific Ocean

120˚E 130˚E 140˚E 30˚N

Figure 7. Recent geographical occurrences of Loxocorniculum mutsuense Ishizaki, 1971, based on data from previous studies. species is the same as those of 23 now extinct, of Loxoconcha kamiyai. Therefore, the origin of Loxo- formerly endemic species in the Japan Sea in corniculum mutsuense appears to be the Pacific coast three families, the Hemicytheridae, Cytheruridae and in or around Japan. Eucytheridae, as reported by Ozawa & Kamiya Thereafter, this species expanded its range, prob- (2005b), Ozawa (2006) and H. Ozawa (unpubl. data). ably by floating on the leaves of marine plants, and These 23 species would have become extinct because became widely distributed on the coasts of the Pacific of low-salinity water at the Japan Sea surface during Ocean, Japan Sea, Yellow Sea and Bo-Hai Sea coasts glacial and corresponding low sea-level periods since until today (Figs 6, 7). Loxocorniculum mutsuense is the early Pleistocene related to glacio-eustatic sea- found in both open-marine and inner-bay environ- level changes. These species probably only inhabited ments (Ishizaki, 1971; Zhou, 1995), where it was able open-marine environments in shallow areas, and to survive because of its wide salinity tolerance. This could not have lived in low-salinity areas such as the ecological tolerance may have allowed this species to brackish inner bay (Ozawa & Kamiya, 2005b). have a wider distribution in East Asia than Loxocon- As for these 23 species, the palaeo-occurrence of cha kamiyai. Loxoconcha kamiyai is also inferred to have been restricted to shallow, open, marine environments on SEXUAL DIMORPHISM IN HINGEMENT the basis of its fossil occurrence with shallow, open- These two species in the family Loxoconchidae, Loxo- marine ostracods from Pleistocene strata (Ozawa, concha kamiyai and Loxocorniculum mutsuense, 1996). It would also have become extinct during the commonly show a unique and remarkable sexual Pleistocene because of its narrow ecological niche and dimorphism in the adult stage, especially in the ante- salinity tolerance. rior element of the hingement (Figs 8, 9). On the right Loxocorniculum mutsuense first appeared along the valve, the anterior hingement element of the adult Pacific coast around 3 Myr ago and migrated into the male is commonly smaller and rounder than that of Japan Sea around 2 Myr ago in view of its oldest the adult female. Its shape is very similar to the fossil record on the Japan Sea coast from the Pliocene small, round anterior element of its A-1 juvenile Sasaoka Formation in northern Japan (Fig. 5; age fide stage. The anterior element of the female hingement Yamada, Irizuki & Tanaka, 2002). Its first appearance is larger and more rectangular than that of either the in the Japan Sea is more than 1 Myr later than that male or the A-1 juvenile stage (Figs 8, 9).

© 2008 The Linnean Society of London, Zoological Journal of the Linnean Society, 2008, 153, 239–251 LOXOCONCHA SPECIES AND SEXUAL DIMORPHISM 247 Downloaded from https://academic.oup.com/zoolinnean/article/153/2/239/2606412 by guest on 25 March 2021

Figure 8. Comparison of lateral view (adult female, A-1 juvenile, adult male) of inner right valve of Loxoconcha kamiyai sp. nov. A, lateral view from inside. B, close-up view of anterior hingement element. C, sketch of anterior hingement element (= B). Upper row: adult female; middle row: A-1 juvenile; lower row: adult male. All specimens from the early Pleistocene Kaidate Formation, central Japan.

Figure 9. Comparison of lateral view (adult female, A-1 juvenile, adult male) for right valve of Loxocorniculum mutsuense Ishizaki, 1971. A, external lateral view. B, internal lateral view. C, close-up view of anterior hingement element. Upper row: adult female; middle row: A-1 juvenile; lower row: adult male. All specimens from the early Pleistocene Kaidate Formation, central Japan.

These morphological characters of Loxoconcha We know of only one example for comparison with the kamiyai are seen in specimens from different geologi- number of teeth per one gongylodont hingement in cal ages and separate geographical areas, i.e. in fossil the adult male, female and A-1 juvenile: Loxoconcha specimens from the Omma Formation near the Noto uranouchiensis (Kamiya, 1992). It may be expected Peninsula (Figs 1, 2; 1.7–1.4 Ma; age ﬁde Takata, that sexual dimorphism and paedomorphosis of the 2000) and the Kaidate Formation on Sado Island complicated-type hingement will be found in other (Figs 1, 8; 0.9 Ma, ﬁde Kato et al., 1995). The same species or families of Cytheroidea and in podocopid character is found in Loxocorniculum mutsuense from ostracods if hingements of males, females and A-1 different geological ages and in fossil specimens from juveniles are examined by SEM. areas such as the Omma Formation near the Noto It is still unclear why only two species of this

Peninsula (Fig. 6; 1.7–1.4 Ma) (H. Ozawa, unpubl. species-group having seven radial pore systems at the Downloaded from https://academic.oup.com/zoolinnean/article/153/2/239/2606412 by guest on 25 March 2021 data) and the Kaidate Formation on Sado Island posterior margin in the genus Loxoconcha show sexual (Figs 6, 9; 0.9 Ma), and also on modern specimens dimorphism and paedomorphosis in the anterior from Aomori Bay in northern Japan (Fig. 7, a; Ish- element of the hingement. The sexual dimorphism of izaki, 1971) and the Seto Inland Sea, south-western the hingement appears not to be directly related to its Japan (Fig. 7, h; Okubo, 1980). functional morphology for copulatory behaviour in Ishizaki (1971) only briefly mentioned this sexual these phytal species. The anterior element of the dimorphism in modern specimens of Loxocorniculum hingement is located on the inner area of the carapace mutsuense from Aomori Bay. He referred to a ‘hinge at the anterodorsal margin; this is furthest from the structure delicate in male but stronger (bold) in copulatory organ that stretches out from the postero- female; with prominent tooth within anterior socket ventral area when the ostracods copulate. of right valve’ (p. 90) in his systematic description of A kind of sexual dimorphism and paedomorphosis that species. However, he did not show clear illustra- of the inner marginal area of the ostracod carapace tions of these dimorphic characters for comparison. has also been reported in a freshwater podocopid, Furthermore, Okubo (1980) redescribed Loxocor- Vestalenula cornelia Smith, Kamiya & Horne, 2006 niculum mutsuense from the Seto Inland Sea, with a in the Family Darwinulidae (Smith et al., 2006), carapace sketch from an internal view of the female although in this case it is not on the hingement. right valve. His illustration (Okubo, 1980: 425, According to that study, the sexual dimorphism in fig. 17b) shows the large anterior tooth of the hinge- Vestalenula cornelia is found along the ventral edge of ment on the female of this species. However, he did the valve. The male has two internal tooth-shaped not refer to this character or the morphology of the structures on the left valve, while the female has a male’s hingement in the text of his description. single internal tooth on the left valve. Furthermore, We therefore conclude that this dimorphic morphol- the female has a keel-shaped structure on the right ogy is a stable character within each species, and not valve, which is lacking in the male. It is interesting a geographical or geological variation within one that the A-1 juvenile of this species has a similar species. arrangement to that of the male with a similar cara- Taking the female’s hingement as a standard, the pace length–height and lateral outline (Smith et al., male’s morphological characters in these loxoconchid 2006). Thus, this male also exhibits paedomorphic species can be explained as a kind of heterochrony, morphology. i.e. paedomorphosis. These paedomorphic examples We propose a speculative hypothesis for this inter- of podocopid hingements have been found in two esting problem on the basis of these two paedomor- different species in 11 pairs of five families – phic examples. We suggest that the adult males of the Cytheridae, Leptocytheridae, Hemicytheridae, marine podocopid ostracods may have originated from Cytheruridae and Loxoconchidae within the super- the adult female by paedomorphosis in ancient times, family Cytheroidea – from the Miocene to the pre- i.e. the early Palaeozoic. The gongylodont hingement, sent (Tsukagoshi, 1994; Tsukagoshi & Kamiya, 1996). characteristic of the family Loxoconchidae, is gener- However, this is the first report of the remarkable ally considered to be one of the most complicated and morphological difference in the anterior hingement evolved hingements among all families of podocopid element between the male and female together with ostracods since the late Cretaceous (e.g. Horne, 2003). the A-1 juvenile within the same species in other Thus, this most evolved hingement in loxoconchid families of ostracods. ostracods in the late Cenozoic would have by chance Perhaps this is because few publications show clear exhibited atavistic features. These may have been illustrations of the hingements of males and females common in the ancient and primitive ancestors of together with A-1 juveniles, especially for ostracod marine ostracods, although most podocopid ostracods taxa having hingements of complicated rather than had already lost these characters by the early Ceno- simple morphology, e.g. adont and lophodont types. zoic. This kind of sexual dimorphism in ostracod

© 2008 The Linnean Society of London, Zoological Journal of the Linnean Society, 2008, 153, 239–251 LOXOCONCHA SPECIES AND SEXUAL DIMORPHISM 249 hingements may be much easier to ﬁnd in more could be an important character for evaluating the complicated hingements than in simpler and more history of sexual dimorphism in ostracods since primitive hingements such as the adont or lophodont the Palaeozoic. types. Non-marine ostracods are considered to have originated and diversiﬁed from marine ostracods several ACKNOWLEDGEMENTS times during the Palaeozoic (e.g. Horne, 2003). There- We wish to thank Dr Yoshihiro Tanimura (National fore, the sexual dimorphism and paedomorphosis Science Museum, Japan) for assistance in preparing in the hingement of a marine species, Loxoconcha the manuscript and the registration of fossil speci- kamiyai, and in structures on the internal ventral mens. We extend our thanks to Professor Takahiro

margin of a freshwater species, Vestalenula cornelia, Kamiya, Dr Tomomi Sato (Kanazawa University), Downloaded from https://academic.oup.com/zoolinnean/article/153/2/239/2606412 by guest on 25 March 2021 may be an interesting perspective for discussing the Emeritus Professor Noriyuki Ikeya, Drs Akira origin of male ostracods and the history of their Tsukagoshi, Shinnosuke Yamada (Shizuoka Univer- sexual dimorphism together with paedomorphosis for sity), Bao-Chun Zhou (Shanghai Museum of Natural geologically long periods (i.e. since the Palaeozoic). It History), Hiroyuki Takata (Shimane University) and will be necessary to collect data regarding sexually the late Tsuyoshi Matsuzaka for collecting and supply dimorphic characters on the carapaces of many ostra- of ostracod samples, providing useful suggestions, cod taxa with heterochronic morphology since the and aiding with access to specimens and literature. early Palaeozoic, by examining the excellent fossil We also thank Dr Robin J. Smith (Lake Biwa records from marine and non-marine environments Museum) for providing valuable comments. Construc- worldwide. tive reviews by Dr Renate Matzke-Karasz (Ludwig- Maximilians-University) and editing by Dr Peter J. CONCLUSIONS Hayward (University of Wales Swansea) helped to improve the manuscript. This study was partly sup- 1. A new ostracod species, Loxoconcha kamiyai sp. ported by a Grant-in-Aid for JSPS Research Fellow nov. in the Family Loxoconchidae, is described for Young Scientists (06372 to H. Ozawa). from the early Pleistocene Omma Formation at the Japan Sea coast in central Japan. Based on its geological and geographical distributions, this REFERENCES species is considered to have once been endemic to Benson RH, Coleman GLII . 1963. Recent marine ostra- the Japan Sea, where it would have evolved until codes from the eastern Gulf of Mexico. University of Kansas the Pliocene. This species would have become Paleontological Contributions, Arthropoda Article 2: 1–52. extinct within this marginal sea during glacial Frydl PM. 1982. Holocene ostracods in the southern Boso maximum periods of the Pleistocene, probably due Peninsula. University Museum, University of Tokyo Bulletin to its narrower salinity tolerances and geographi- 20: 61–140. cal distributions than those of extant species Hanai T, Ikeya N, Ishizaki K, Sekiguchi Y, Yajima M. inhabiting the euryhaline environments in other 1977. Checklist of Ostracoda from Japan and its adjacent seas. seas. University Museum, University of Tokyo Bulletin 12: 2. The distributional patterns of pore systems on the 1–119. valve in this species at the adult and juvenile Hayashi K. 1988. Pliocene-Pleistocene palaeoenvironment stages are examined. These results strongly and fossil ostracod fauna from southwestern Hokkaido, suggest its closest phylogenetic affinities to a living Japan. In: Hanai T, Ikeya N, Ishizaki K, eds. Evolutionary species, Loxocorniculum mutsuense in the biology of Ostracoda–its fundamentals and applications. Loxoconchidae. Tokyo: Kodansha, Amsterdam: Elsevier, 557–568. Horne DJ. 2003. Key events in the ecological radiation of the 3. These two loxoconchid species show a unique Ostracoda. In: Park LE, Smith AJ, eds. Bridging the gap: sexual dimorphism in the adult stage on the ante- trends in the ostracode biological and geological sciences. rior element of the hingement. Taking the female The Paleontological Society Papers 9: 181–202. hingement morphology as a standard, the male Ikeya N, Cronin TM. 1993. Quantitative analysis of hingement can be explained in terms of a kind of Ostracoda and water masses around Japan: application to heterochrony, i.e. paedomorphosis. Sexual dimor- Pliocene and Pleistocene paleoceanography. Micropalaeon- phism on hingement with paedomorphosis occurs tology 39: 263–281. in only one phylogenetic group of the genus Ikeya N, Itoh H. 1991. Recent Ostracoda from the Sendai Loxoconcha, which is distinguished by the ontoge- Bay region, Paciﬁc coast of northeastern Japan. Report of netic distributional patterns of pore systems. This Faculty of Science, Shizuoka University 25: 93–141. unique morphological feature may represent relict Ikeya N, Okubo I, Kitazato H, Ueda H. 1985. Excursion 4, primitive characters of ancient ostracods, and Shizuoka (Pleistocene and living Ostracoda, shallow

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