A chelicera-bearing arthropod reveals the Cambrian origin of chelicerates

Chelicerata is a megadiverse (over 120,000 species) arthropod clade that includes familiar taxa of profound ecological and economic importance, such as scorpions, spiders and mites¹. Extant chelicerates share a unique anatomical character, the chelicerae-feeding first appendages terminated by a simple pincer-like chela². The fossil record of these primarily predatory animals spans almost 500 million years³, suggesting a likely yet undocumented origin during the Cambrian Explosion. Artiopods^4-6, megacheirans^4,7-9, habeliids^10-13 and mollisoniids^14,15 have been considered Cambrian stem- or crown-group chelicerates, but they all lack unequivocal chelicerae, leaving the emergence of chelicerae-bearing arthropods unclear. Here we describe Megachelicerax cousteaui gen. et sp. nov., a large soft-bodied arthropod from the middle Cambrian of Utah featuring massive three-segmented chelicerae, along with five pairs of pseudobiramous prosomal limbs with non-foliaceous exopodal rami, and plate-like lamellae-bearing opisthosomal appendages. Bayesian and parsimony phylogenetic analyses resolve Megachelicerax as a stem-group chelicerate bridging Cambrian habeliids and post-Cambrian chelicerae-bearing synziphosurines. This finding provides unequivocal evidence of large predatory chelicerates in the Cambrian, illuminates their body plan's origin, and confirms habeliids, mollisoniids and probably megacheirans as members of total-group Chelicerata.