The ctenonotellid ostracode species Brezelina palmata (Krause, 1889) and the tetradellid species Ogmoopsis bocki (Öpik, 1935) are represented by seven and five molting stages, respectively. Both species display an association of velar and size dimorphism in the three last instars. The analysis of the juvenile morphology of O. bocki provides evidence for a close relationship with the earliest known palaeocope ostracode species Nanopsis nanella (Moberg and Segerberg, 1906). The population age structure of B. palmata in synchronous beds at two localities reveals different types of thanatocoenoses, supporting the hypothesis of a deepening gradient along the Baltic-Ladoga Klint.