 Principal evolutionary trends from primitive morphologies, such as exemplified by "Eoredlichia", include the origin of new types of eyes, improvement of enrollment and articulation mechanisms, increased size of pygidium (micropygy to isopygy), and development of extreme spinosity in certain groups.Changes also included narrowing of the thorax and increasing or decreasing numbers of thoracic segments.Specific changes to the cephalon are also noted; variable glabella size and shape, position of eyes and facial sutures, and hypostome specialization.