The phylogenetic position of the extinct arachnid order Phalangiotarbida Haase, 1890, with reference to the fauna from the Writhlington Geological Nature Reserve (Somerset, UK)

Abstract

Study of abundant phalangiotarbid (Arachnida: Phalangiotarbida) material – provisionally assigned here to Bornatarbus mayasii (Haupt in Nindel 1955) – from the Upper Carboniferous of Writhlington, UK, has revealed new information about some previously equivocal characters. The present authors report a trifurcate apotele, possible spiracles on sternite 5, and confirm the presence of 10 opisthosomal tergites plus a dorsal anal operculum. The affinities of phalangiotarbids are obscure, with most authors favouring affinities with Opiliones (harvestmen) and/or Acari (mites and ticks). Phalangiotarbida is scored for characters used in previous studies of arachnid relationships. A cladistic analysis based on 63 characters using 13 terminal arachnid taxa (plus a hypothetical outgroup), resolves Phalangiotarbida as sister group to (Palpigradi + Tetrapulmonata): the taxon Megoperculata sensu Shultz (1990). Even under cladistic analysis, the position of the Phalangiotarbida remains hard to resolve, but a prosomal sternite with distinct sclerites potentially groups them with the Megoperculata.