The study on the origin and evolution of sigillid ostracods characteristic of submarine caves

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Sigillids are primitive podocopid ostracods having unequal valves and many small adductor muscle scars. The author found four troglobitic sigillid genera, Kasella Tabuki and Hanai 1999 and three new genera of Genus A, B and C, from coral reefs in the Pacific. In this study, the author taxonomically examined the carapace features of all the sigillid genera in detail and tried to demonstrate the phylogenetic affinities among the genera and the origin and evolution of the troglobitic sigillids.Sigillids are classified into two taxonomical groups for the presence of ornamentation: the non-ornate sigillids composed of Cardobairdia, Saipanetta and Genus A, and the ornate sigillids of Kasella, Genus B and Genus C. Cardobairdia is close to the ornate sigillids in having the 'original hingement' in the right valve. Saipanetta and Genus A differ from the other genera in having the 'two-layered hingement'. The ornate Genus B is closer to Cardobairdia in lateral outline and shape of adductor scar. Genus C shares many key carapace features not only with Kasella, but also with Genus B. While Kasella has many 'advanced carapace features', it shows the features possibly resulted from so-called reversion such as a posteroventral spine on the right valve appeared also in Cardobairdia and a primitive hingement structure similar to that of healdioideans.Only the non-troglobitic sigillids, Cardobairdia and Saipanetta, have the fossil records since the Jurassic. This fact suggests that the troglobitic sigillids derived from the non-troglobitic sigillids. The Genus A possibly derived from Saipanetta or its relatives. Genus B is thought to have originated from Cardobairdia or its relatives, Genus C from Genus B or its relatives and Kasella from Genus C or its relatives.