**4. Sexual dimorphism in the inner carapace, with Paedomorphosis**

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.

#### **4.1. Hingement morphology and dimorphism**

The genus *Loxocorniculum* of the family Loxoconchidae was established [65] based primarily on modern *Loxocorniculum fischeri* from the Caribbean Sea, and is characterised by a hornlike protuberance on the postero-dorsal corner of the carapace. However, except for the 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].

64 Sexual Dimorphism

furca in the male [37].

and Abe (1993).

Palaeozoic, are reviewed below.

**4.1. Hingement morphology and dimorphism** 

observations of its ecological behaviour under experimental conditions [51], this species pushes off from sea-bottom sediments just before swimming in water, especially using the furca (Figure 13). Based on observations of video recordings, the male tends to swim around much more actively than the female; thus explaining the function of the relatively large

**Figure 13.** Schematic profile of 'push-off' behaviour just before swimming for modern *Vargula hilgendorfi* (Myodocopida), indicating the location of its furca by black arrows, modified from Vannier

including mating behaviour and reproduction modes, since the Palaeozoic.

**4. Sexual dimorphism in the inner carapace, with Paedomorphosis** 

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

The genus *Loxocorniculum* of the family Loxoconchidae was established [65] based primarily on modern *Loxocorniculum fischeri* from the Caribbean Sea, and is characterised by a hornlike protuberance on the postero-dorsal corner of the carapace. However, except for the

A further four examples of sexually dimorphic appendages and eyes are found on *Vargula hilgendorfi*, as follows: (a) the existence or absence of two suckers on the first appendage (antennule), (b) different numbers of bristles on the first appendage, (c) different sizes of the basal part of the second appendage (antenna), (d) different sizes of compound eyes [37, 51]. The probable function of (a) is support by the male form of the female carapace during mating behaviour. However, the functions of the other sexually dimorphic characteristics (b)–(d) remain unclear. The Myodocopida first appeared during the early Palaeozoic (Ordovician), and still inhabit many marine environments [7]. Therefore, these other sexually dimorphic characteristics are interesting examples of myodocopid ostracod morphology, and indicate the evolutionary processes associated with their ecology, The ostracod genus *Loxoconcha* (family Loxoconchidae) is widely distributed in shallow marine environments from tropical to subarctic regions [52–54]. This is one of the most diversified ostracod genera, which comprises ca. 600 species [7]. This genus is common in and around Japan [55–59], and about 40 living and fossil species have been described [60]. Thus, *Loxoconcha* is one of the most important Japanese ostracod genera.

A new fossil species *Loxoconcha kamiyai* from Pleistocene strata from the eastern coast of the Sea of Japan (Figure 14) was described, and its carapace morphology examined [16]. Palaeobiogeography of *L. kamiyai* was discussed, and its phylogenetic relationship to related

**Figure 14.** SEM images of two *Loxoconcha* species (Podocopida) from Quaternary deposits of central Japan, modified from Ozawa (2010). Arrows indicate anterior.

loxoconchid species was assessed, based on the pore distribution pattern (a type of ostracod sensory organ; Figure 15). The number, distribution, and differentiation of pores on the ostracod carapace surface during ontogeny have been studied to determine phylogenetic relationships among species [61]. The reconstruction of ostracod phylogeny based on pore analyses was first proposed by [21] for 14 species of the genus *Cythere*. His work was followed by [26] [62–64] for species of other genera. This method of phylogenetic reconstruction, proposed by [21], was termed 'differentiation of distributional pattern of pore (DDP) analysis' [61]. The pore distribution in *L. kamiyai* was examined by this method [16], and the results were compared to the pore data of 17 other *Loxoconcha* species (Figure 16).

The History of Sexual Dimorphism in Ostracoda (Arthropoda, Crustacea) Since the Palaeozoic 67

Sado Island in central Japan (age from [70] [71]). The same hingement character is found in *L. mutsuense* of various geological ages in fossil specimens from many areas (Figure 20), such as Pleistocene strata around 1.5 and 0.9 Ma in central Japan [16], and also in modern specimens from shallow marine environments off the northeastern and southwestern

**Figure 16.** Results of DDP analysis for eighteen *Loxoconcha* species (Podocopida), modified from Ishii *et al.* (2005) and Ozawa and Ishii (2008). Numbers indicate total numbers of pores for each lineage and

Japanese coast [66] [72].

stage. Trees drawn by hand.

**Figure 15.** (1) Distribution pattern of pores in adult left valve of *Loxoconcha kamiyai* (Podocopida), modified from Ozawa and Ishii (2008). Position of one missing pore of this species is determined by comparison with the distribution pattern of pores of *Loxoconcha mutsuense* (= *Loxocorniculum mutsuense*; Podocopida) in Ishii *et al.* (2005). Arrows indicate anterior. (2) Close-up view of SEM images of pores in antero-dorsal marginal area on left valve of *Loxoconcha kamiyai* (Podocopida) from Quaternary deposits of central Japan, modified from Ozawa (2010).

On the basis of the DDP results for its adult and A-1 juvenile stages, *L. kamiyai* was determined to be the species most closely related to *Loxoconcha mutsuense* (= *Loxocorniculum mutsuense*). Both species have the same total number of pores on the carapace at the A-1 juvenile stage (75 pores per valve; Figure 16). The difference in total number of pores in the adult stage is only one between these two species, missing on the central area in *L. kamiyai* (Figure 15). These results strongly suggest its closest phylogenetic affinity to another species, *L. mutsuense*, in the same family [16].

Both *Loxoconcha kamiyai* and *Loxoconcha mutsuense* show a unique and remarkable sexual dimorphism in the adult stage, especially in the anterior element of the hingement (Figures 17 and 18). On the right valve, the anterior hingement element of the adult male is commonly smaller and rounder than that of the adult female. Its shape is very similar to the small, round anterior element of its A-1 juvenile stage [16]. The anterior element of the female hingement is larger and more rectangular than that of either the male or the A-1 juvenile (Figures 17 and 18).

These morphological characteristics of *Loxoconcha kamiyai* are seen in specimens from diverse geological ages and geographical regions (Figure 19); in fossil specimens from Pliocene sediments (4–3 Ma) in the Nagano and Niigata Prefectures of central Japan (age from [67–69]) and Pleistocene strata around 1.5 and 0.9 Ma from the Noto Peninsula and Sado Island in central Japan (age from [70] [71]). The same hingement character is found in *L. mutsuense* of various geological ages in fossil specimens from many areas (Figure 20), such as Pleistocene strata around 1.5 and 0.9 Ma in central Japan [16], and also in modern specimens from shallow marine environments off the northeastern and southwestern Japanese coast [66] [72].

66 Sexual Dimorphism

analyses was first proposed by [21] for 14 species of the genus *Cythere*. His work was followed by [26] [62–64] for species of other genera. This method of phylogenetic reconstruction, proposed by [21], was termed 'differentiation of distributional pattern of pore (DDP) analysis' [61]. The pore distribution in *L. kamiyai* was examined by this method [16], and the results

**Figure 15.** (1) Distribution pattern of pores in adult left valve of *Loxoconcha kamiyai* (Podocopida), modified from Ozawa and Ishii (2008). Position of one missing pore of this species is determined by comparison with the distribution pattern of pores of *Loxoconcha mutsuense* (= *Loxocorniculum mutsuense*; Podocopida) in Ishii *et al.* (2005). Arrows indicate anterior. (2) Close-up view of SEM images of pores in antero-dorsal marginal area on left valve of *Loxoconcha kamiyai* (Podocopida) from Quaternary deposits

On the basis of the DDP results for its adult and A-1 juvenile stages, *L. kamiyai* was determined to be the species most closely related to *Loxoconcha mutsuense* (= *Loxocorniculum mutsuense*). Both species have the same total number of pores on the carapace at the A-1 juvenile stage (75 pores per valve; Figure 16). The difference in total number of pores in the adult stage is only one between these two species, missing on the central area in *L. kamiyai* (Figure 15). These results strongly suggest its closest phylogenetic affinity to another

Both *Loxoconcha kamiyai* and *Loxoconcha mutsuense* show a unique and remarkable sexual dimorphism in the adult stage, especially in the anterior element of the hingement (Figures 17 and 18). On the right valve, the anterior hingement element of the adult male is commonly smaller and rounder than that of the adult female. Its shape is very similar to the small, round anterior element of its A-1 juvenile stage [16]. The anterior element of the female hingement is larger and more rectangular than that of either the male or the A-1

These morphological characteristics of *Loxoconcha kamiyai* are seen in specimens from diverse geological ages and geographical regions (Figure 19); in fossil specimens from Pliocene sediments (4–3 Ma) in the Nagano and Niigata Prefectures of central Japan (age from [67–69]) and Pleistocene strata around 1.5 and 0.9 Ma from the Noto Peninsula and

of central Japan, modified from Ozawa (2010).

species, *L. mutsuense*, in the same family [16].

juvenile (Figures 17 and 18).

were compared to the pore data of 17 other *Loxoconcha* species (Figure 16).

**Figure 16.** Results of DDP analysis for eighteen *Loxoconcha* species (Podocopida), modified from Ishii *et al.* (2005) and Ozawa and Ishii (2008). Numbers indicate total numbers of pores for each lineage and stage. Trees drawn by hand.

The History of Sexual Dimorphism in Ostracoda (Arthropoda, Crustacea) Since the Palaeozoic 69

**Figure 19.** Geographical and geological occurrences of *Loxoconcha kamiyai* (Podocopida) based on data

**Figure 20.** Geographical and geological occurrences of *Loxoconcha mutsuense* (= *Loxocorniculum* 

distributional data from southwestern Japan of Irizuki (2004).

*mutsuense*; Podocopida) based on data from previous studies, modified from Ozawa (2010), adding with

This hingement sexual dimorphism in modern specimens of *Loxoconcha mutsuense* (= *Loxocorniculum mutsuense*) from the coast of northeastern Japan was mentioned only briefly

from previous studies, modified from Ozawa (2010).

**Figure 17.** Comparison of lateral view (adult female, A-1 juvenile, adult male) of inner right valve of *Loxoconcha kamiyai* (Podocopida) from Quaternary deposits of central Japan, modified from Ozawa and Ishii (2008). (1): SEM images of lateral view from inside; arrows indicate locations of anterior hingement element, (2): SEM images of close-up view of anterior hingement element, (3): Sketch of anterior hingement element (= 2). Upper row: adult female, middle row: A-1 juvenile, lower row: adult male. White arrows indicate anterior.

**Figure 18.** Comparison of SEM images of lateral view (adult female, A-1 juvenile, adult male) for right valve of *Loxoconcha mutsuense* (= *Loxocorniculum mutsuense*; Podocopida) from Quaternary deposits of central Japan, modified from Ozawa and Ishii (2008). (1): External lateral view, (2): Internal lateral view; arrows indicate locations of anterior hingement element, (3): Close-up view of anterior hingement element. Upper row: adult female, middle row: A-1 juvenile, lower row: adult male. White arrows indicate anterior.

White arrows indicate anterior.

indicate anterior.

**Figure 17.** Comparison of lateral view (adult female, A-1 juvenile, adult male) of inner right valve of *Loxoconcha kamiyai* (Podocopida) from Quaternary deposits of central Japan, modified from Ozawa and Ishii (2008). (1): SEM images of lateral view from inside; arrows indicate locations of anterior hingement element, (2): SEM images of close-up view of anterior hingement element, (3): Sketch of anterior hingement element (= 2). Upper row: adult female, middle row: A-1 juvenile, lower row: adult male.

**Figure 18.** Comparison of SEM images of lateral view (adult female, A-1 juvenile, adult male) for right valve of *Loxoconcha mutsuense* (= *Loxocorniculum mutsuense*; Podocopida) from Quaternary deposits of central Japan, modified from Ozawa and Ishii (2008). (1): External lateral view, (2): Internal lateral view; arrows indicate locations of anterior hingement element, (3): Close-up view of anterior hingement element. Upper row: adult female, middle row: A-1 juvenile, lower row: adult male. White arrows

**Figure 19.** Geographical and geological occurrences of *Loxoconcha kamiyai* (Podocopida) based on data from previous studies, modified from Ozawa (2010).

**Figure 20.** Geographical and geological occurrences of *Loxoconcha mutsuense* (= *Loxocorniculum mutsuense*; Podocopida) based on data from previous studies, modified from Ozawa (2010), adding with distributional data from southwestern Japan of Irizuki (2004).

This hingement sexual dimorphism in modern specimens of *Loxoconcha mutsuense* (= *Loxocorniculum mutsuense*) from the coast of northeastern Japan was mentioned only briefly

in [66]. Ishizaki referred to a "hinge structure delicate in male but stronger (bold) in female with prominent tooth within anterior socket of right valve" ([66], p. 90) in the systematic description of this species. However, he did not show clear illustrations of these dimorphic characteristics for comparison. *L. mutsuense* from the coast of southwestern Japan was redescribed in a carapace sketch from an internal view of the female right valve [72]. Okubo's illustration (Fig. 17b in [72], p. 425) clearly shows the large anterior hingement tooth on the female. However, he also did not refer to this characteristic or the morphology of the male's hingement in the text. Considering the findings of [16], Ozawa and Ishii concluded that this sexually dimorphic morphology is a stable characteristic within each species, and not a geographical or geological variation or a deformity within a single species.

The History of Sexual Dimorphism in Ostracoda (Arthropoda, Crustacea) Since the Palaeozoic 71

processes of the ecology and reproductive modes of shallow marine podocopids during

**Figure 21.** Schematic profile for location of anterior hingement element and inferred location of copulatory oragn stretching out from inner carapace for *Loxoconcha kamiyai* and *Loxoconcha mutsuense* (= *Loxocorniculum mutsuense*; Podocopida) in lateral view for left valve of male, based on observations for mating behaviour of other living *Loxoconcha* species *(L. japonica* and *L. uranouchiensis*) in Kamiya (1988b).

**4.2. Example of the inner carapace with implications for the historical origin of** 

A type of sexual dimorphism with paedomorphosis in the inner marginal area of ostracod carapaces has been reported in the freshwater podocopid *Vestalenula cornelia*, of the family Darwinulidae [17], although this dimorphism was not found on the hingement (Figures 6 and 22). According to [17], the sexual dimorphism in *V. cornelia* is found along the ventral edge of the valve (Figure 22). The male has two internal tooth-shaped structures on the left valve, whereas the female has a single internal tooth on the left valve. Furthermore, the female has a keel-shaped structure on the right valve, which is absent from the male form (Figure 22). It is interesting that the A-1 juvenile has a similar arrangement to that of the adult male, in terms of carapace length–height and lateral outline [17]. Therefore, the male

form of this species also exhibits paedomorphic morphology.

the Cenozoic.

**the ostracod male** 

Using the female's hingement as a standard, the male morphology in these two species can be explained as a type of heterochrony of [73] [74]; *i*.*e*., paedomorphosis [16]. Paedomorphic examples of podocopid hingements have been found in two species in 11 pairs from five families—the Cytheridae, Leptocytheridae, Hemicytheridae, Cytheruridae, and Loxoconchidae—within the superfamily Cytheridea from the Miocene to the present [19, 75, 76]. This remarkable morphological difference in the anterior hingement element, between the sexes and the A-1 juvenile stage within the same species from one family, was first reported by [16].

This is likely why few publications include clear illustrations of ostracod hingements of male and female forms together with the A-1 juvenile stage, especially for ostracod taxa with a complex, rather than simple, hingement morphology; *e*.*g*., adont and lophodont hingement types. We know of only one example of a detailed comparison of the number of teeth per gongylodont hingement in adult male and female forms with the A-1 juvenile of *Loxoconcha uranouchiensis* [9]. Further examples of the sexual dimorphism and paedomorphosis of the complicated hingement type will likely be found in other species or families of podocopid ostracods if hingements of male, female, and A-1 juveniles are precisely examined using SEM.

With regard to copulatory behaviour in *Loxoconcha kamiyai* and *L. mutsuense*, hingement sexual dimorphism does not appear to be directly related to functional morphology [16]. The anterior hingement element is located on the inner area of the carapace at the anterodorsal margin. This is farthest from the copulatory organ, which stretches out from the postero-ventral area between the two valves during mating (Figure 21) [16] [77]. Therefore, as yet there are no reasonable interpretations of the actual function of this kind of sexually dimorphic morphology.

Therefore, the mating and reproductive behaviours of the living species *Loxoconcha mutsuense* must be observed in detail by video recording, because the other species, *L. kamiyai*, has been extinct since the middle Pleistocene [16]. Such detailed observations will for the first time allow clarification of the actual function and significance of this dimorphism in their life-history. The significance for this dimorphism in the life-history or mating behaviour of the two species will facilitate elucidation of the evolutionary processes of the ecology and reproductive modes of shallow marine podocopids during the Cenozoic.

70 Sexual Dimorphism

reported by [16].

precisely examined using SEM.

sexually dimorphic morphology.

in [66]. Ishizaki referred to a "hinge structure delicate in male but stronger (bold) in female with prominent tooth within anterior socket of right valve" ([66], p. 90) in the systematic description of this species. However, he did not show clear illustrations of these dimorphic characteristics for comparison. *L. mutsuense* from the coast of southwestern Japan was redescribed in a carapace sketch from an internal view of the female right valve [72]. Okubo's illustration (Fig. 17b in [72], p. 425) clearly shows the large anterior hingement tooth on the female. However, he also did not refer to this characteristic or the morphology of the male's hingement in the text. Considering the findings of [16], Ozawa and Ishii concluded that this sexually dimorphic morphology is a stable characteristic within each species, and not a

Using the female's hingement as a standard, the male morphology in these two species can be explained as a type of heterochrony of [73] [74]; *i*.*e*., paedomorphosis [16]. Paedomorphic examples of podocopid hingements have been found in two species in 11 pairs from five families—the Cytheridae, Leptocytheridae, Hemicytheridae, Cytheruridae, and Loxoconchidae—within the superfamily Cytheridea from the Miocene to the present [19, 75, 76]. This remarkable morphological difference in the anterior hingement element, between the sexes and the A-1 juvenile stage within the same species from one family, was first

This is likely why few publications include clear illustrations of ostracod hingements of male and female forms together with the A-1 juvenile stage, especially for ostracod taxa with a complex, rather than simple, hingement morphology; *e*.*g*., adont and lophodont hingement types. We know of only one example of a detailed comparison of the number of teeth per gongylodont hingement in adult male and female forms with the A-1 juvenile of *Loxoconcha uranouchiensis* [9]. Further examples of the sexual dimorphism and paedomorphosis of the complicated hingement type will likely be found in other species or families of podocopid ostracods if hingements of male, female, and A-1 juveniles are

With regard to copulatory behaviour in *Loxoconcha kamiyai* and *L. mutsuense*, hingement sexual dimorphism does not appear to be directly related to functional morphology [16]. The anterior hingement element is located on the inner area of the carapace at the anterodorsal margin. This is farthest from the copulatory organ, which stretches out from the postero-ventral area between the two valves during mating (Figure 21) [16] [77]. Therefore, as yet there are no reasonable interpretations of the actual function of this kind of

Therefore, the mating and reproductive behaviours of the living species *Loxoconcha mutsuense* must be observed in detail by video recording, because the other species, *L. kamiyai*, has been extinct since the middle Pleistocene [16]. Such detailed observations will for the first time allow clarification of the actual function and significance of this dimorphism in their life-history. The significance for this dimorphism in the life-history or mating behaviour of the two species will facilitate elucidation of the evolutionary

geographical or geological variation or a deformity within a single species.

**Figure 21.** Schematic profile for location of anterior hingement element and inferred location of copulatory oragn stretching out from inner carapace for *Loxoconcha kamiyai* and *Loxoconcha mutsuense* (= *Loxocorniculum mutsuense*; Podocopida) in lateral view for left valve of male, based on observations for mating behaviour of other living *Loxoconcha* species *(L. japonica* and *L. uranouchiensis*) in Kamiya (1988b).

### **4.2. Example of the inner carapace with implications for the historical origin of the ostracod male**

A type of sexual dimorphism with paedomorphosis in the inner marginal area of ostracod carapaces has been reported in the freshwater podocopid *Vestalenula cornelia*, of the family Darwinulidae [17], although this dimorphism was not found on the hingement (Figures 6 and 22). According to [17], the sexual dimorphism in *V. cornelia* is found along the ventral edge of the valve (Figure 22). The male has two internal tooth-shaped structures on the left valve, whereas the female has a single internal tooth on the left valve. Furthermore, the female has a keel-shaped structure on the right valve, which is absent from the male form (Figure 22). It is interesting that the A-1 juvenile has a similar arrangement to that of the adult male, in terms of carapace length–height and lateral outline [17]. Therefore, the male form of this species also exhibits paedomorphic morphology.

A speculative hypothesis to explain this interesting observation of paedomorphosis was proposed by [16], who suggested that the adult male forms of marine podocopid ostracods may have originated from adult female forms by paedomorphosis in ancient times; *i*.*e*., the early Palaeozoic. The gongylodont hingement, characteristic of the family Loxoconchidae, is generally considered to be one of the most complex-shaped and derived hingements among all podocopid ostracod families since the late Cretaceous [7] [52]. Thus, the most derived hingement in loxoconchid ostracods would have by chance exhibited atavistic features. These may have been common in ancient and primitive ancestors of marine ostracods, although most podocopid species had already lost these characteristics by the early Cenozoic. Identification of this kind of sexual dimorphism in more complicated hingement shapes may be easier than in simpler and more primitive hingements, such as the adont or lophodont types.

The History of Sexual Dimorphism in Ostracoda (Arthropoda, Crustacea) Since the Palaeozoic 73

and in structures on the internal ventral margin of a freshwater species (*Vestalenula cornelia*) may provide insight into the origin of the ostracod male and the post-Palaeozoic history of ostracod sexual dimorphism with paedomorphosis [16]. Therefore, more data regarding the sexually dimorphic characteristics of ostracod carapaces (or appendages as much as possible) of many taxonomic groups, accompanied by heterochronic morphology since the early Palaeozoic, should be collected. Additionally, the excellent marine and non-marine ostracod fossil records since the Ordovician that are extant worldwide should be further

1. Many ostracod species have the ability to reproduce sexually, and are relatively easily fossilised because due to their highly calcified carapaces. Ostracods are abundant in sediments ranging from the Palaeozoic Ordovician (since ca. 490 million years ago) to the Cenozoic Holocene, in modern deposits. Considering these unique characteristics, ostracods represent useful tools for investigation of the history of sexual dimorphism of organisms on earth since the Ordovician. Many examples of ostracod sexual dimorphism, in terms of both shape and size, are evident on carapaces and appendages

2. Two podocopid species of the family Loxoconchidae (*e*.*g*., *Loxoconch kamiyai*) show a unique sexual dimorphism in the adult stage on the anterior hingement element. Pore distribution patterns on their carapaces strongly suggest close phylogenetic affinities for these two species. Taking the female hingement morphology as a standard, male hingement can be explained in terms of a type of heterochrony; *i*.*e*., paedomorphosis. Sexual dimorphism on the hingement accompanied by paedomorphosis occurs in only one phylogenetic group in this family*,* which is distinguished by the ontogenetic pore pattern distribution. This unique morphological feature may represent relict primitive characteristics of ancient ostracods, and could be important for evaluation of the history of sexual dimorphism and the origin of sex in ostracods since the early Palaeozoic. To clarify the long-term history of evolutionary processes in terms of their ecology and reproductive modes since the Ordovician, more data on sexual dimorphism of ostracod carapaces with appendages of many taxa that exhibit heterochronic morphology should

3. Many examples of ostracod sexual dimorphism are found in various taxonomic groups, including both living and extinct species. However, the actual functions of most of the dimorphic characteristics remain unclear, even for many living species, although many hypotheses have been put forward. Many of these sexually dimorphic characteristics are likely strongly related to the ecology of reproductive modes, such as mating behaviour and brood care. To clarify the actual functions in living species, ostracod behaviour, especially mating and brood care under breeding conditions, should be observed using video recordings. The lack of such observations is the primary reason why the ecological behaviour of most living ostracod species is

researched.

**5. Summary and future work** 

from the Palaeozoic to Recent.

be collected.

unclear.

**Figure 22.** Comparison of SEM images of internal lateral view (adult female, adult male, A-1 juvenile) for right and left valves of *Vestalenula cornelia* (Podocopida) from modern springs in Yaku-shima Island of southwestern Japan, modified from Smith *et al.* (2006). Left column: adult female, central column: adult male, right column: A-1 juvenile, upper row: right valve, lower row: left valve. Arrows indicate anterior.

Non-marine ostracods are considered to have originated and diversified from marine ostracods multiple times, mainly during the Palaeozoic and Mesozoic [7]. Therefore, sexual dimorphism with paedomorphosis in the hingement of a marine species (*Loxoconcha kamiyai*) and in structures on the internal ventral margin of a freshwater species (*Vestalenula cornelia*) may provide insight into the origin of the ostracod male and the post-Palaeozoic history of ostracod sexual dimorphism with paedomorphosis [16]. Therefore, more data regarding the sexually dimorphic characteristics of ostracod carapaces (or appendages as much as possible) of many taxonomic groups, accompanied by heterochronic morphology since the early Palaeozoic, should be collected. Additionally, the excellent marine and non-marine ostracod fossil records since the Ordovician that are extant worldwide should be further researched.
