**5. Summary and future work**

72 Sexual Dimorphism

lophodont types.

anterior.

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

**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

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*)


4. To elucidate the functions of sexually dimorphic characteristics in extinct groups; *e*.*g*., the brood pouch of Palaeocopida in the Palaeozoic, fossil eggs or juvenile carapaces in the inner part of the adult carapaces from Palaeozoic sediments must be identified. For Palaeocopida, we must attempt to find fossils resting their appendages, despite their rarity. Furthermore, detailed observation of the ecological behaviour of living species will facilitate understanding of the actual functions of sexually dimorphic morphology in both living and extinct species [31]. Due to the excellent ostracod fossil record from Palaeozoic to Cenozoic, the living ostracod sexual dimorphism data can be applied to extinct species. Combined studies of the ecology and functional morphology of both living and fossil ostracod species will clarify the history of the evolutionary ecology and reproductive modes of organisms during the last ca. 500 million years.

The History of Sexual Dimorphism in Ostracoda (Arthropoda, Crustacea) Since the Palaeozoic 75

[2] Kamiya T. Different sex-ratios in two Recent species of *Loxoconcha* (Ostracoda).

[3] Martens K. Preface: To mate or not to mate !. In: Martens K. (ed.) Sex and Parthenogenesis: Evolutionary Ecology of Reproductive Modes in Non-Marine

[4] Benson RH., Berdan JM., van den Bold WA., Hanai T., Hessland I., Howe HV., Kesling RV., Levinson SA., Reyment RA., Moore RC., Scott HW., Shaver RH., Sohn IG., Stover LE., Swain FM., Sylvester−Bradley PC. Treatise of Invertebrate Palaeontology, Part Q Arthropoda 3, Crustacea, Ostracoda. New York: Geological Society of America,

[5] Benson RH. The ontogeny of an ostracodologist. The Paleontological Society Papers,

[6] Ozawa H. Chapter 4.1.8: Ostracoda. In: Tanimura Y., Tuji A. (eds.) Microfossils: Their microscopic world explored. A Book Series from the National Museum of Nature and

[7] Horne DJ. Key events in the ecological radiation of the Ostracoda. The Paleontological

[8] Abe K. Population structure of *Keijella bisanensis* (Okubo) (Ostracoda, Crustacea)—An inquiry into how far the population structure will be preserved in the fossil record. Journal of the Faculty of Science, University of Tokyo, Section II, 1983, 20: 443–488. [9] Kamiya T. Heterochronic dimorphism of *Loxoconcha uranouchiensis* (Ostracoda) and its

[10] Horne DJ., Cohen A., Martens K. Taxonomy, morphology and biology of Quaternary and living Ostracoda. In: Holmes JA., Chivas A. (eds.) The Ostracoda: Applications in Quaternary Research, AGU Geophysical Monograph Series 131. Washington DC.:

[11] Clarkson ENK. Invertebrate Palaeontology and Evolution (4th Edition). Oxford:

[12] Callomon JH. Sexual dimorphism in Jurassic ammonites. Transactions of the Leicester

[13] Makowski H. Problem of sexual dimorphism in ammonites. Palaeontologia Polonica,

[14] Maeda H. Dimorphism of Late Cretaceous false-puzosine ammonites, *Yokoyamaceras* Wright and Matsumoto, 1954 and *Neopuzosua* Matsumoto, 1954. Transactions and

Proceedings of Palaeontological Society of Japan, New Series, 1993, 170: 186–211. [15] Yajima M., Kamiya T. Chapter 1: Reproduction (Sex ratio & Sexual dimorphism). In: Ikeya N., Tanabe K. (eds.) Life-History of Paleontology, Paleontological Science Series 3.

[16] Ozawa H., Ishii T. Taxonomy and sexual dimorphism of a new species of *Loxoconcha* (Podocopida: Ostracoda) from the Pleistocene of the Japan Sea. Zoological Journal of the

Science 13. Hadano: Tokai University Press; 2012, p. 142–151. (in Japanese)

Senckenbergiana Lethaia, 1998, 68: 337–345.

Boulder: University of Kansas Press; 1961.

Society Papers, 2003, 9: 181–202.

2003, 9: 1–8.

Ostracods. Leiden: Backhuys Publishers; 1998, p. xv–xvii.

implication for speciation. Paleobiology, 1992, 18: 221–236.

America Geophysical Union; 2002, p. 5–36.

Literary and Philosophical Society, 1963, 57: 21–56.

Tokyo: Asakura Publishing; 2001, p. 1–8. (in Japanese)

Linnean Society, 2008, 153: 239–251.

Blackwell Science; 1998.

1963, 12: 1–92.
