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Horseshoe crabs (class Xiphosura) are a long-lived clade of aquatic chelicerate arthropods with a fossil record spanning approximately 480 million years. Though Xiphosura are often noted for their morphological stability, further investigation of evolutionary rate and paleoecological trends have revealed a remarkably dynamic clade, with both temporal and phylogenetic variability in evolutionary trends. Additionally, heterochrony has been revealed to be a strong driver behind xiphosuran evolution and the exploration of non-marine niches. Using combined geometric morphometric and evolutionary rate techniques, we further highlight the incongruency of the fossil record of xiphosurans with their designation as a “living fossil” or stabilomorph clade. Here, we compare the results of a geometric morphometric analysis with a discrete character evolutionary rate calculation performed using the R package Claddis. Both analyses incorporated 55 xiphosuran species, ranging temporally from the Ordovician Lunataspis aurora to all four modern species. Morphometric data was collected as 2Dlandmarks and semi-landmarks, with variable numbers of points due to varying levels of preservation amongst fossil specimens. These data were then used to produce a PCA for the visualization of morphospace. Both studies support a dynamic evolutionary history for Xiphosura. The discrete character analysis revealed peaks in discrete character evolution in the heterochronic non-marine clades, as well as an overall declining trend in evolutionary rate. Similarly, the clades with higher evolutionary rates occupy a wider portion of morphospace compared with the more morphologically stable clades.
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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 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.
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- Award ID(s):
- 1943082
- PAR ID:
- 10218773
- Date Published:
- Journal Name:
- PeerJ
- Volume:
- 8
- ISSN:
- 2167-8359
- Page Range / eLocation ID:
- e10431
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.7717/peerj.10431
