THE PHYLOGENY OF CHASMATASPIDIDA (ARTHROPODA; CHELICERATA)

Chasmataspidids were aquatic euchelicerates with a known fossil record from the Middle Ordovician to the Middle Devonian. The placement of Chasmataspidida in chelicerate phylogeny has fluctuated over the course of their scientific history, being considered at various times either a polyphyletic grouping of species with xiphosuran and eurypterid affinities, a paraphyletic grade at the base of Eurypterida, or a monophyletic clade within Eurypterida. Current phylogenies resolve chasmataspidids as a clade sister to the group comprising eurypterids and arachnids; however, sampling of chasmataspidids within these analyses is limited and their internal relationships are poorly resolved, making it difficult to ascertain the chasmataspidid ground pattern morphology and test macroevolutionary and biogeographic trends within the group.

We present a new phylogenetic analysis focusing specifically on chasmataspidids in order to resolve their internal relations and verify their position within euchelicerate phylogeny. The matrix comprises a combination of characters from previous phylogenies that incorporated chasmataspidids as part of broader analyses of chelicerates and novel characters focused on chasmataspidid-specific anatomy not specifically tested for in those phylogenies. The resulting phylogeny includes all fourteen currently described chasmataspidid species and will provide a framework for future phylogenetic research to fully integrate chasmataspidids into broad questions about chelicerate evolution during the early Paleozoic, including the timing of early euchelicerate group appearances.