A complete molted exoskeleton of the asteropygine phacopid trilobite Greenops widderensisLieberman and Kloc, 1997 from the Middle Devonian (Givetian) Widder Formation in southwestern Ontario, Canada that has suffered predatory trauma provides insights into the sequence of regeneration of segments. The molt configuration is such that it is possible to interpret the molting technique used by the trilobite. Predatory trauma affected four areas of the exoskeleton. The pygidium shows loss of the spinose margin on one side and damage to a single spine on the other; one genal spine has been broken and partially regrown; and the posterior of the glabella has been removed. It is thought that the first three traumas occurred during life, as these areas affected show signs of exoskeletal regeneration. The fourth trauma probably occurred to the exuvium. Analysis of the degree of regeneration of the pygidial pleurae indicates that there was an anteroposterior polarity to the regeneration. Other examples in the literature suggest that this regeneration polarity pattern may have been widespread in trilobites. It is suggested that, as in modern arthropods and annelids, this sequential regeneration was under the control of segmentation polarity genes.