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Selected specimens from the Jurassic ammonoid family Arietitidae were investigated using morphometric methods of transverse and longitudinal conch section analysis. The family Arietitidae is characterized by similarities in the conch geometry, but variation can be demonstrated by means of differences in conch morphology. Our study focuses on a specimen of the arietitid <i>Pararnioceras</i> sp., which revealed striking changes in conch morphology due to a syn vivo growth through a parasitic serpulid. Changes in its ontogenetic development are compared with specimens without epizoans. The ecological interpretation of the morphometric data allows the conclusion that the host possessed the ability to counteract the parasitic conch abnormalities by adapting its housing growth, thus ensuring survival.