SCORPIONS AS EURYPTERID ANALOGS

Eurypterids are extinct, chelicerate arthropods that have been allied with the xiphosurans (particularly Limulus) and with arachnids (scorpions). The choice of which of these two modern groups to use as an eurypterid analog for paleobiological interpretations depends on the question being addressed. For studying eurypterid molting, scorpions are the logical choice as analog. Eurypterids and scorpions share striking morphologic and structural similarities despite their different habitats (aquatic vs. terrestrial): both the eurypterid opisthosoma and scorpion abdomen are differentiated into two portions, a broad, 7-segmented anterior portion, and a narrower 5-segment posterior portion that terminates in a telson (in eurypterids) or sting (in scorpions). Major differences between eurypterids and scorpions include the position of the mouth, and adaptations of scorpions to a terrestrial habitat (e.g., a lighter exoskeleton and spiracles in place of book gills). Despite these physiological differences, the similarities in basic body plan suggest that both groups faced similar functional challenges during ecdysis. Our study of comparative scorpion and eurypterid taphonomy reveals similar patterns of disarticulation between the two groups in support of this view. We conclude that body plan is the critical factor to consider in arthropod molting and that scorpions are the best extant group to use for insight to this aspect of eurypterid paleobiology.