Reconciling morphology with phylogenomics in Chelicerata (Arthropoda)

Abstract Whereas traditional morphology-based analyses have most often recovered horseshoe crabs as early branching chelicerates, nearly all phylogenomic analyses instead place this lineage as nested within Arachnida. Although this repositioning is not yet widely accepted, it resolves several important problems: (i) the placement of sea spiders (Pycnogonida) among fossil taxa; (ii) the questionable status of Mollisonia and Sanctacaris as chelicerates (herein rejected); (iii) the absence of any convincing crown group chelicerates from the Cambrian, whereas large-bodied and well armoured taxa are abundant in Ordovician deposits. The earliest chelicerates are hypothesized to have been minute-bodied inhabitants of interstices, hence their absence from the Cambrian fossil record. Tagmosis is also questioned as a useful concept for Chelicerata because the two main defining features of tagmata, somitic consolidation and the evolution of specialized functions in different body regions, are uncoupled in this lineage. A prosomal head is potentially a synapomorphy that unites only a portion of arachnids, including horseshoe crabs.