Euproops danae (Belinuridae) cluster confirms deep origin of gregarious behaviour in xiphosurids

Xiphosurids are archetypal marine arthropods that have received considerable research interest from both biologists and palaeontologists. Mating clusters of extant horseshoe crabs have informed research on clustering in other fossil arthropods, such as trilobites. Furthermore, the evolutionary stasis exhibited by horseshoe crabs suggests that fossil xiphosurids may also have mated in clusters. However, no body fossil evidence has been presented to support this idea. Here, we document a cluster of seven Euproops danae (Meek et Worthen, 1865) specimens preserved on the same bedding plane of Pennsylvanian-aged Farrington Group siltstone. These specimens record potential reproductive strategies of belinurids that are comparable to extant counterparts. This gregarious behaviour may have decreased the impact of predation and/or contributed to genetic diversity. The cluster ultimately highlights a further example of xiphosurid evolutionary stasis.