doi:10.1017/S1477201907002131 Printed in the United Kingdom C © The Natural History Museum Phylogeny of the basal swimming eurypterids (Chelicerata;

SYNOPSIS The phylogeny of a broad selection of taxa at the base of the monophyletic Eurypterina (swimming eurypterids) is analysed. The results suggest that Onychopterella is the most basal of these forms, in agreement with its early stratigraphic occurrence. One step up fromOnychopterella is a split between the superfamily Eurypteroidea and a clade comprising the four other clades in Euryp-terina: Mixopteroidea, Waeringopteroidea, Adelophthalmoidea and Pterygotoidea. Eurypteroidea is composed of two major clades; the Eurypteridae consists of most species of Eurypterus and North American representatives of Erieopterus. The Dolichopteridae, numerically less species-rich, but more morphologically diverse, consists of Dolichopterus, Ruedemannipterus, Buffalopterus, Strobil-opterus, Syntomopterus and most likely ‘Eurypterus ’ minor. Polyphyly of Eurypterus and paraphyly of Onychopterella differ from present taxonomic assignments. Not surprisingly, the fossil record of the clade is shown to be very poor as expected for animals lacking a mineralised exoskeleton. This is reflected in a low Relative Completeness Index (RCI) value of −41%