**2. General features of ostracods for taxonomy, ecology, and morphology**

Ostracods are a class of small crustaceans (Figures 1 and 2), the adult form of which is typically ca. 1.0 mm in length. They are not generally well known to many people except for researchers of fossils or living crustaceans due to their small size and lack of commercial significance compared to other crustaceans; *e*.*g*., shrimps and crabs.

Class Ostracoda consists of the following six orders: Podocopida, Platycopida, Palaeocopida, Leperditicopida, Myodocopida, and Halocypridina [10]. This chapter focuses mainly on ostracod species belonging to the order Podocopida, which consists of more than 20,000 named living and fossil species distributed globally, because Podocopida is the most diversified taxonomic group in class Ostracoda (Figure 3) [10]. The well-known bioluminescent organism 'the sea firefly' (called 'umi-hotaru' in Japanese) is a kind of ostracod but belongs to another order, Myodocopida.

Ostracods occur in most aquatic environments on earth (Figure 3), such as the deep sea to a depth of several thousands of meters, through to shallow seas on the continental shelf [5]. They also live in rock pools in intertidal zones, brackish water areas at river mouths, lagoons and estuaries, freshwater lakes, ponds, irrigated rice fields, and temporary puddles. Most species are benthic throughout their lives, and crawl on or through the surface sediment and among aquatic plants [10]. A number of interstitial species, which live between sediment particles, are also distributed globally [18] [19].

The most distinctive feature of ostracods is the calcareous bivalved carapace (or shell), consisting of two valves that completely envelop the soft body and all appendages (Figures 1 and 2) [10]. Various types of appendage are protruded between opened valves for locomotion, feeding, and reproduction. The two valves (termed right and left) are connected by a hingement running along the dorsal margin (Figure 2). The word ostracod (or 'ostracode') is derived from Greek word 'ostrakon', which means 'a shell'. This carapace (or shell) has various morphological characteristics (Figures 1 and 2) that allow detailed taxonomic and phylogenetic studies to be performed on both living and fossil specimens [20–22].

Like other crustaceans such as decapods, ostracods grow by moulting (ecdysis; Figure 4). For example, in Podocopida, there are usually eight moult stages between egg and adult [20]. Species with strongly calcified carapaces, such as most marine species, are relatively easily fossilised. Ostracods are abundant in sediments globally, beginning in the early Ordovician (ca. 490 Ma) [7]. The proteinaceous (= 'chitinous') soft body and appendages are rarely fossilised due to a lack of mineralised parts, with rare exceptions, such as preservation of the soft parts of Silurian fossils [23]. Therefore, an ostracod fossil typically consists only of the hard-calcified carapace; however this is sufficient for identification of both modern and fossil specimens to the species level based on various carapace morphological characteristics.

In particular, the surface ornamentation (reticulation, fossae, muri, eye tubercle, ridges and so on), hingement type, muscle scar morphology (Figures 1 and 2), and pore shape and numbers are useful for ostracod taxonomy [24]. With living specimens, the morphology of male copulatory organs and other appendages are also used for species identification, similar to identification techniques used for other crustaceans, such as decapods [24–26]. Fossil ostracods are commonly utilised by palaeontologists as important palaeoenvironmental and stratigraphic (geological age) indices, and have long been used in oil and gas exploration [4] [27–29].

The History of Sexual Dimorphism in Ostracoda (Arthropoda, Crustacea) Since the Palaeozoic 59

**Figure 7.** Carapace outlines of female and male of *Beyrichia salteriana* and *Bolbibollia labrosa* in Family Beyrichiidae (Palaeocopida) from Silurian deposits of Sweden and Canada respectively, modifed from

inferred that species of this family inhabited Palaeozoic shallow-marine shelf-water environments [7]. Thus, a reasonable interpretation of pouch function in this family is needed to clarify the detailed evolutionary processes of the ecology and reproductive modes

Carapace shape and size sexual dimorphism is common in ostracods, and males can be larger or smaller than females [4]. One example from the Mesozoic is a non-marine species of the podocopid genus *Cypridea* [15] [32]. This genus occurred only during the Jurassic and Cretaceous, and is not found in Cenozoic deposits. The shape and size of *Cypridea subvaldensis* (Figure 8) from Cretaceous sediments in northeastern China was studied, and the differences between the two forms were recognised to represent sexual dimorphism

Benson *et al.* (1961). Arrows indicate anterior.

*3.1.2. Mesozoic example in an extinct species* 

within a single species [32].

of Palaeocopida in Palaeozoic shallow-marine water areas.
