Introduction.- Ecological Evolutionary Units.- Patterns of Morphological Change in Fossil Lineages.- Why Atrypides.- Climate and Environment in the Silurian and Devonian.- Ecological Interactions.- Research Hypotheses.- Materials and Methods.- Geometric Morphometrics.- Data Set.- Results.- Temporal Variation.- Spatial Variation.- Mean Morphological Shape.- Encrustation Versus Morphological Shape.-Discussion.- Morphology.- Climate Setting in the Silurian and Devonian.- Atrypide Distribution.-Atrypide Diversity.- Temporal Variation.- Spatial Variation- Ecological Causes-Environmental Effect.- Conclusion.