AFFINITIES OF TRIGONOTARBIDA Although not comprehensive in its coverage of arachnids the present cladistic analysis supports a sistergroup relationship between trigonotarbids and ricinuleids to the exclusion of spiders, whip spiders, and palpigrades. This is a consistent result (irrespective of analytical parameters), and lends credence to the hypothesis that ricinuleids could be the sister group of trigonotarbids. This arrangement reflects the putative synapomorphies elucidated previously, which include laterally subdivided tergites, a prosoma−opisthosoma locking ridge, terminal palpal claw, and elements of the mouth (Dunlop, 1996c; Dunlop et al., 2009). The implication would be that (Trigonotarbida + Ricinulei) belong together in the Pantetrapulmonata sensu Shultz (2007), and that these two orders resolve as sister group to the Tetrapulmonata (i.e. spiders, whip spiders, and whip scorpions). Caveats are that our principal goal was not to test the position of Trigonotarbida, and that older hypotheses for the affinities of ricinuleids, such as with harvestmen (Opiliones) or mites (Acari) or one of the mite subgroups (e.g. Parasitiformes), are not reflected in our choice of outgroup. A more detailed analysis of both living and fossil arachnids is needed to investigate the position of Trigonotarbida further, but the presence of two pairs of lungs and ‘clasp-knife’ chelicerae strongly suggest placement close to or amongst the tetrapulmonate arachnids.

Published as part of Jones, Fiona M., Dunlop, Jason A., Friedman, Matt & Garwood, Russell J., 2014, Trigonotarbus johnsoni Pocock, 1911, revealed by X-ray computed tomography, with a cladistic analysis of the extinct trigonotarbid arachnids, pp. 49-70 in Zoological Journal of the Linnean Society 172 (1) on page 60, DOI: 10.1111/zoj.12167, http://zenodo.org/record/5313645