The structure of the sternum of all major Recent scorpion groups is analyzed in detail. Based on this analysis, two fundamental sternum types are identified, described and illustrated, type 1 and type 2. These sternum types are distinguished by criteria based on external and internal structural features. The sternum types described herein are offered as a replacement for the various characterizations used throughout the last 140 years which emphasize only gross overall shape and proportions. Phylogenetic and taxonomic ramifications of these new sternal types are discussed. The Carboniferous fossil scorpion Palaeopisthacanthus schucherti Petrunkevitch is assigned to sternum type 1. The type 1 sternum is also assigned to the three primitive Recent scorpion groups, the pseudochactids, the buthoids, and the chaerilids. Sternum type 2 is defined for the remaining scorpion groups, those complying with Type C orthobothriotaxy. Based on these assignments, sternum type 1 is considered primitive. Within these two basic sternal types a hypothesis of horizontal and vertical compression is offered as a cause-effect for the unique sterna of the buthoid and bothriurid scorpions.
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