Functional morphology of the shell in platycope ostracodes ‐ a study of arrested evolution

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Abstract
Shell morphology in relation to soft parts is described in the modern species Cytherella abyssorum G. O. Sars, based on microtome sections, serial peels and thin sections. Functionally important features of the cytherellids include the extensive development of the intervalvar cuticle (ligament) along a strongly curved line, the consequently very narrow ventral slit between valves in opened carapaces and a special mode of egg care which is associated (domatial) type of sexual shell dimorphism. Platycope ostracods represent a pronouncedly conservative evolutionary lineage; their essential characters are fully developed already in the earliest known true platycopes from the Silurian Period. The cause of the slow evolutionary change is suggested to have been the virtual impossibility to change their specific morphological organization to fit some other mode of life or to diversify widely within the given morphological framework. The same obviously applies to the lingulaccan brachiopods and probably also other bradytelic groups.