THE CAMBRIAN ORIGIN OF CHELICERATES – INSIGHTS FROM EXTERNAL AND INTERNAL ANATOMY

Chelicerates represent one of the most successful groups of euarthropods, and most of them play a critical role in modern terrestrial ecosystems as obligate predators. Despite their substantial extant diversity and ecological significance, the deep evolutionary origin of chelicerates remains contentious. Several groups have been traditionally regarded as chelicerate ancestors, but recent discoveries of soft-bodied euarthropods from sites of exceptional preservation in early and middle Cambrian marine deposits around the world suggest that modern chelicerates evolved from a highly diverse ancestral lineage that includes non-biomineralized representatives. Major groups of Cambrian euarthropod implicated in chelicerate origins include the megacheirans, which are typified by the presence of raptorial first appendages, the sanctacaridis, habeliidans, and more recently the mollisoniids. However, none of these taxa convincingly show the critical defining character observed in all modern chelicerates, that is the specialization of the deutocerebral appendage pair as jacknife or pincer-like chelicerae.

New data on exceptionally preserved fossils from the mid-Cambrian of North America cast new light on early chelicerate evolution and the origin of their archetypical body plan. Paleoneuroanatomical data from the Burgess Shale (Canada) species Mollisonia symmetrica illustrates the ancestral organization of the central nervous system in the chelicerate stem-lineage before the evolution of the features that typify modern representatives, such as eye doublets, a fused synganglion and elongate posterior connectives. Critically, the discovery of a new taxon from the Wheeler Formation (USA) unequivocally demonstrates the presence of several chelicerate synapomorphies, including a pair of robust pincer-like chelicerae, a fused prosoma with five pairs of biramous cephalic appendages with pediform exopods, and an opisthosoma with flattened lamellae similar to book gills. The results of a comprehensive phylogenetic analysis resolve the new taxon as a derived member of the chelicerate stem group, and clarify the interrelationships among disparate Cambrian euarthropods including trilobites, megacheirans, sanctacaridids, habeliidans and mollisoniids as members of this morphologically disparate lineage.