In trilobites as in many others extinct organisms, our understanding of the mechanisms of evolution is based on morphological and ontogenetic features. Data from ontogenetic development are essential to provide an insight into the origins of evolutionary changes. In phacopid trilobites, detailed studies of ontogenetic series have been achieved using quantitative methods. A comparison of ontogenetic trajectories of closely related species has been conducted to understand how the both morphological disparity and developmental patterning of different ontogenetic stages were structured and to determine how the environmental or developmental constraints influenced morphological changes in trilobites. Study of such morphological disparity in developmental evolution requires knowledge of morphological features and timing and rate of development among closely related species. At a macroevolutionary scale, such quantitative studies should allow us (1) to determine the impact of individual ontogenies on morphological diversification and (2) to identify the evolutionary patterns in Phacopidae during 100 Ma of existence.