The first Silurian horseshoe crab reveals details of the xiphosuran ground plan

Abstract

Horseshoe crabs are an ancient lineage with an evolutionary history stretching back 450 million years and are generally considered to be examples of 'living fossils' exhibiting slow rates of evolution. Despite this reputation, relatively little is known of the early evolution of the group, with only two species described from the Ordovician and a subsequent 80-million-year gap in their fossil record until xiphosurids appear in the Late Devonian. Furthermore, all described Ordovician species are assigned to a single genus, with their close phylogenetic relatedness rendering it unclear whether their morphology is representative of the horseshoe crab ground pattern or an independently derived condition. Here, a new species of horseshoe crab is described from the Silurian of Indiana, USA. The new species bridges the temporal gap in the xiphosuran fossil record and has an overall morphology similar to that of the Ordovician taxa. These new data provide critical information on the ancestral morphology of horseshoe crabs, showing that xiphosurids evolved from forms with a fused thoracetron exhibiting axial segment boundaries, and demonstrate the persistence of basal Xiphosura into the Silurian. Laser-stimulated fluorescence is also shown to be an effective method for studying and imaging arthropod fossils exhibiting challenging preservation.

Keywords: Ciurcalimulus; Silurian; Xiphosura; Xiphosurida; ground plan; horseshoe crab; laser stimulated fluorescence.

Figure 1.

Ciurcalimulus discobolus gen. et sp. nov., holotype (YPM IP 548961). (a) Imaged dry under polarized light, showing specimen relief. (b) Imaged immersed in ethanol under polarized light, providing greater contrast of the carbonized cuticle. (c) Interpretive drawing of specimen. Carbonized cuticle is shown in brown, damaged or incomplete margins by dashed lines. (d) Imaged under ultraviolet light, exhibiting fluorescence of both the carbonized cuticle and to a lesser extent the exfoliated regions of the fossil, affording the clearest view of the segment boundaries in the thoracetron axis. (e) Imaged under 447 nm blue laser, with carbonized cuticle and exfoliated regions again both fluorescing. The outline of the fossil is most readily apparent here as is the morphology of the cardiac lobe. (f) Imaged under 532 nm green laser, clearly showing the location of carbonized cuticle via strong fluorescence. Scale bars = 5 mm.

Figure 2.

Ciurcalimulus discobolus gen. et sp. nov., holotype (YPM IP 548961). Details of the specimen photographed under normal light, with the magnified regions indicated. Scale bars of magnifications = 1 mm, 5 mm on overview image.

Figure 3.

Simplified phylogeny of Xiphosura showing the phylogenetic position of Ciurcalimulus discobolus. For full details of the phylogeny, including branch supports, see electronic supplementary material, figure S1. Stages are shown within each Period, with names and ages as per the International Commission on Stratigraphy.