Numerous exuviae of three Dechenella species (D. givetensis, D. ziegleri and D. calxensis) from the Middle Devonian (Givetian) of NE France (Ardenne Massif) provide the opportunity to identify the evolutionary modifications of ontogeny of the three Dechenella species and to elaborate a conceptual framework of developmental shape changes. First we used biometric and morphometric approaches to characterize shape modifications. Then we computed ontogenetic trajectories by multivariate regression of geometric shape variables on centroid size in order to compare them. Finally, we compared parallelism between trajectories and rates of development relative to size. These analyses demonstrate a significant difference in the cranidial developmental trajectories of D. givetensis and D. ziegleri indicating an allometric repatterning. However, pygidia of these species share the same allometric pattern with a distinct developmental rate suggesting that heterochrony could be a partial explanation for the body shape evolution. Pygidial ontogeny of D. calxensis corresponds to an allometric repatterning with respect to both other species. This work illustrates the complexity of evolutionary modifications of ontogeny constituting an important process in morphological novelties.