The phylogeny and systematics of Xiphosura

Abstract

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shown the current taxonomic framework for Xiphosura—set out in the Treatise of Invertebrate Paleontology in 1955—to be outdated and in need of revision, with several common genera such as Paleolimulus Dunbar, 1923 and Limulitella Størmer, 1952 acting as wastebasket taxa.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 Euproops Meek, 1867, Belinurus König, 1820, Paleolimulus, Limulitella, and Limulus are demonstrated to be non-monophyletic and the previously synonymized genera Koenigiella Raymond, 1944 and Prestwichianella Cockerell, 1905 are shown to be valid. In addition, nine new genera (Andersoniella gen. nov., Macrobelinurus gen. nov., and Parabelinurus gen. nov. in Belinurina; Norilimulus gen. nov. in Paleolimulidae; Batracholimulus gen. nov. and Boeotiaspis gen. nov. in Austrolimulidae; and Allolimulus gen. nov., Keuperlimulus gen. nov., and Volanalimulus gen. nov. in Limulidae) are erected to accommodate xiphosuran species not encompassed by existing genera. One new species, Volanalimulus madagascarensis gen. et sp. nov., is also described. Three putative xiphosuran genera—Elleria Raymond, 1944, Archeolimulus Chlupáč, 1963, and Drabovaspis Chlupáč, 1963—are determined to be non-xiphosuran arthropods and as such are removed from Xiphosura. The priority of Belinurus König, 1820 over Bellinurus 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.

A revision ofProlimulus woodwardiFritsch, 1899 with comparison to other highly paedomorphic belinurids

“…Comparing Figs. 16 and 17 , the key differences are the distribution of Belinurus , Koenigiella Lamsdell, 2020b and Prestwichianella Lamsdell, 2020b in morphospace. Belinurus has a constrained distribution, while Koenigiella and Prestwichianella have extensive distributions across PC1 and PC2 respectively.…”

“…Belinurus has a constrained distribution, while Koenigiella and Prestwichianella have extensive distributions across PC1 and PC2 respectively. Notably, Euproops danae —the only representative of Euproops sensu Lamsdell (2020b) —has the largest spread in morphospace.…”

“… Lamsdell ( 2020b , p. 17) suggested that Prolimulus “strong[s] affinity to Alanops and Pringlia , and there could be an argument for synonymizing Prolimulus with one of these genera”. We disagree with this suggestion based on our observations here.…”

“…Comparing the distribution of genera Bicknell & Pates (2020) with Lamsdell (2020b) allows the taxonomic framework based on phylogenetic topology to be examined and scrutinized. The position of Macrobelinurus Lamsdell, 2020b and Andersoniella Lamsdell, 2020b specimens in morphospace separate from the other clusters strongly supports the validity of these genera.…”

“…We had initially used generic assignment of Bicknell & Pates (2020) for this analysis. However, during the course of peer review, Lamsdell (2020b) presented a revision of Xiphosurida and proposed that Belinurina consisted of 14 genera. To compare, contrast, and explore the distribution of these newly erected groups with the more conservative perspective of Bicknell & Pates (2020) , we presented the distribution of genera suggested in Bicknell & Pates (2020) and Lamsdell (2020b) .…”