A large number of three-dimensionally preserved, pyritized specimens of different instars of the rhomboentomozoid ostracod Franklinella (Franklinella) lysogorensis n. sp. were recovered from a mudstone of the Givetian/Frasnian transition beds at Wzdół in the Holy Cross Mountains, Poland. Seven growth stages are identified. Sexual dimorphism is manifested by differences of ribbing pattern in adults. Within the superfamily Entomozoacea, the presence of two types of sexual dimorphism is suggested: ornamental in rhomboentomozoids and domiciliar in entomoprimitiids. Formerly poorly known morphological features of the rhomboentomozoids, including the anteroventral indentation and a contact list ornamented by fine striation, are described.