Ontogenetic change of shell ornamentation of two Pistocythereis ostracods from Japan with soft-parts description

Abstract

Pistocythereis bradyi and Pistocythereis bradyformis are trachyleberidid ostracods that have been abundant in the inner bays of East Asia since the Pliocene. In this study, we described the appendages of the two Pistocythereis species and investigated the changes in shell traits throughout ontogeny in both species. Although the morphological characters seen on the inside of the shell, such as hinge, ocular sinus, marginal spine and duplicature, are common to both species, there were inter- and intraspecific variations in the external surface ornamentation. The variation in traits visible on the outside of the shell is thought to be due to functional requirements, such as protecting fragile setae.