The phylogeny and systematics of Xiphosura.

Xiphosurans are aquatic chelicerates with a fossil record extending into the Early Ordovician and known from a total of 88 described species, four of which are extant. Known for their apparent morphological conservatism, for which they have gained notoriety as supposed 'living fossils', recent analyses have demonstrated xiphosurans to have an ecologically diverse evolutionary history, with several groups moving into non-marine environments and developing morphologies markedly different from those of the modern species. The combination of their long evolutionary and complex ecological history along with their paradoxical patterns of morphological stasis in some clades and experimentation among others has resulted in Xiphosura being of particular interest for macroevolutionary study. Phylogenetic analyses have shown the current taxonomic framework for Xiphosura-set out in the in 1955-to be outdated and in need of revision, with several common genera such as Dunbar, 1923 and Størmer, 1952 acting as wastebasket taxa. Here, an expanded xiphosuran phylogeny is presented, comprising 58 xiphosuran species as part of a 158 taxon chelicerate matrix coded for 259 characters. Analysing the matrix under both Bayesian inference and parsimony optimisation criteria retrieves a concordant tree topology that forms the basis of a genus-level systematic revision of xiphosuran taxonomy. The genera Meek, 1867, König, 1820, , , and are demonstrated to be non-monophyletic and the previously synonymized genera Raymond, 1944 and Cockerell, 1905 are shown to be valid. In addition, nine new genera ( gen. nov. gen. nov. and gen. nov. in Belinurina; gen. nov. in Paleolimulidae; gen. nov. and gen. nov. in Austrolimulidae; and gen. nov., gen. nov., and gen. nov. in Limulidae) are erected to accommodate xiphosuran species not encompassed by existing genera. One new species, gen. et sp. nov., is also described. Three putative xiphosuran genera- Raymond, 1944, Chlupáč, 1963, and Chlupáč, 1963-are determined to be non-xiphosuran arthropods and as such are removed from Xiphosura. The priority of König, 1820 over Pictet, 1846 is also confirmed. This work is critical for facilitating the study of the xiphosuran fossil record and is the first step in resolving longstanding questions regarding the geographic distribution of the modern horseshoe crab species and whether they truly represent 'living fossils'. Understanding the long evolutionary history of Xiphosura is vital for interpreting how the modern species may respond to environmental change and in guiding conservation efforts.